Food & Function View Article Online
Published on 19 May 2015. Downloaded by UNIVERSITY OF OTAGO on 14/07/2015 11:07:21.
REVIEW
Cite this: Food Funct., 2015, 6, 1752
View Journal | View Issue
Acrylamide: inhibition of formation in processed food and mitigation of toxicity in cells, animals, and humans Mendel Friedman Potentially toxic acrylamide is largely derived from the heat-inducing reactions between the amino group of the amino acid asparagine and carbonyl groups of glucose and fructose in plant-derived foods including cereals, coffees, almonds, olives, potatoes, and sweet potatoes. This review surveys and consolidates the following dietary aspects of acrylamide: distribution in food, exposure and consumption by diverse populations, reduction of the content in different food categories, and mitigation of adverse in vivo effects. Methods to reduce acrylamide levels include selecting commercial food with a low acrylamide content, selecting cereal and potato varieties with low levels of asparagine and reducing sugars, selecting processing conditions that minimize acrylamide formation, adding food-compatible compounds and
Received 26th March 2015, Accepted 1st May 2015 DOI: 10.1039/c5fo00320b www.rsc.org/foodfunction
plant extracts to food formulations before processing that inhibit acrylamide formation during processing of cereal products, coffees, teas, olives, almonds, and potato products, and reducing multiorgan toxicity (antifertility, carcinogenicity, neurotoxicity, teratogenicity). The herein described observations and recommendations are of scientific interest for food chemistry, pharmacology, and toxicology, but also have the potential to benefit nutrition, food safety, and human health.
Introduction Heat-induced amino-carbonyl and related interactions between food ingredients encompass those changes commonly termed browning reactions.1,2 These include reactions of amino acids, peptides, and proteins with reducing sugars glucose and fructose and ascorbic acid (vitamin C). These socalled Maillard browning reactions can result in the destruction of amino acids and decrease in digestibility and nutritional quality. Numerous food processing methods that transform raw into edible food benefit nutrition and health.3 Reports that the reactive α, β-unsaturated (conjugated) compound called acrylamide (CH2vCH–CONH2) is formed during the processing of foods under conditions that also induce the formation of Maillard browning products stimulated interest in its chemistry, biochemistry, and safety. To contribute to this effort, we previously reviewed the chemistry, pharmacology, toxicology of acrylamide, and possible ways to reduce its formation in food and in vivo toxicity.4,5 With Prof. Mottram of the University of Reading, UK, we organized and edited the
Western Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, 800 Buchanan St., Albany, CA 94710, USA. E-mail: [email protected]; Fax: +1-510-559-6162; Tel: +1-510-559-5615
1752 | Food Funct., 2015, 6, 1752–1772
Proceedings of two symposia on acrylamide.6,7 Noteworthy reviews by other investigators include those by Mills et al.8 and Capuano and Fogliano.9 This integrated review presents, updates and interprets new information on extensive worldwide efforts to develop insights into acrylamide formation in model systems and in food as well as reports on dietary exposure, reduction in processed cereal products (bread, bread rolls, cookies), roasted products (coffees, teas, almonds, barley), olives, and white and sweet potato products (French fries, chips, crisps). Also covered are approaches to mitigate reported carcinogenicity, developmental toxicity including teratogenicity, fertility, and neurotoxicity of acrylamide in cells, animals, and humans. The cited results will hopefully facilitate the development of improved industrial processes and domestic cooking conditions to decrease the acrylamide burden of the diet and its toxicity after consumption.
Adverse effects of dietary acrylamide on humans A key objective of studies on the exposure of different population subgroups (infants, teenagers, adults) to diet-based acrylamide is whether such self-assessment of consumption of different food categories can be used to relate to or predict
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 19 May 2015. Downloaded by UNIVERSITY OF OTAGO on 14/07/2015 11:07:21.
Food & Function
adverse human effects.10 This is a challenging problem with mixed results, as indicated by the following selected studies. Kütting et al.11 detected haemoglobin adducts of acrylamide in 999 of 1008 human blood samples from smokers and non-smokers. They suggest that (a) smoking can be regarded as the main source of overall acrylamide intake in people with occupational exposure to acrylamide and that food intake is of environmental importance; (b) data on neurotoxicity and reproductive toxicity due to acrylamide are not likely to occur in the general population except in very high consumers; (c) carcinogenicity and mutagenicity are possible health risks for the general population; (d) precautionary measures should be implemented such as minimizing the acrylamide content of the diet, to avoid unnecessary increase in the overall tumour risk; and (e) such measures include the use of appropriate technology in food processing without adversely affecting colour, flavour and nutritional value, monitoring the acrylamide levels of widely distributed processed food, and minimizing home-made acrylamide formation by guidance to consumers. The results of studies on the dietary exposure to acrylamide of the general French population in relation to international health-based guidance values show that the mean acrylamide concentration (in g per kg fresh weight) of about 20 food groups ranged from 2 for fruit to 954 for potato chips/crisps, with the highest values for potato chips (954), biscuits (729), and French fries (724). The main contributors to dietary exposure, accounting for both content and food and consumption patterns, (in %) were French fries (44.8) and coffee (27.7) for adults and French fries (60.8), sweetened biscuits (18.8), and potato chips (4.09) for children. Because the results may indicate a health concern for young children, the authors suggest the need to continue efforts to reduce dietary exposure to acrylamide.12 A Polish study found that the mean acrylamide content in baby foods depended on the food product, ranging from 2 to 516 μg kg−1.13 The exposure of infants (in g per kg body weight per day) aged 6–12 months at the minimum level ranged from 0.41 to 0.62, at the average level, from 2.10 to 4.32, and at the maximum level, from 7.47 to 12.35. This maximum value was significantly higher than the exposure estimated for the entire Polish population. Because the World Health Organization (WHO) estimated that the lifetime exposure to acrylamide of 0.14 g per kg body weight may increase the risk of one additional cancer per 10 000 people, the authors suggest that (a) the observed high estimated exposure of infants to acrylamide may be associated with a potential cancer risk, with the caveat that the exposure to acrylamide from baby food is of short duration compared to the lifetime exposure; and (b) the reduction of acrylamide content of baby food should be a high priority for risk management. An estimate of the dietary exposure of Polish teenagers (n = 261) to acrylamide during a 7 day daily food record found that (a) for girls the estimated dietary intake of acrylamide (in μg per kg body weight per day) was 0.09 (50th percentile), 0.32 (75th percentile), and 1.04 (95th percentile), with the corres-
This journal is © The Royal Society of Chemistry 2015
Review
ponding values for boys of 0.13, 0.41, and 1.18, respectively; and (b) the main sources of acrylamide were French fries, potato crisps, corn flakes, bread, and salty sticks.14 These results suggest that need to discourage teenagers from consuming food with high acrylamide content (such as French fries, potato crisps).
Chemistry of acrylamide formation and inhibition Many studies have strikingly shown the complexity of the chemical basis of acrylamide formation and indicated possible approaches to reduce its generation. Here, we highlight some of these studies in both model systems and cookies. Such studies can provide insights into potential practical approaches to limit the acrylamide content in different food categories. We also examine specific methods that have been proposed to inhibit acrylamide formation in cereal products, coffee, olives, and potatoes. Insights into acrylamide formation and inhibition in model systems Studies in a heated glucose/asparagine model system show that the B6 vitamer pyridoxamine inhibits acrylamide formation in the temperature range 120 to 180 °C, apparently by scavenging intermediate dicarbonyl compounds such as glyoxal and methylglyoxal formed during degradation of glucose and other events in the Maillard reactions.15 The inhibition was higher than with the same concentration of vitamin C and was accompanied by a reduction in the degree of browning.16 The authors isolated and characterized the structures of the acrylamide-pyridoxamine adducts as direct Michael addition products. These results imply that pyridoxamine has the potential to mitigate acrylamide formation in lipid-rich foods, where oxidation products might contribute to acrylamide formation. Arribas-Lorenzo and Morales15 also measured the formation of 3-propionamide, a transient intermediate in acrylamide formation during thermal degradation of asparagine initiated by reducing carbohydrates or aldehydes. Their results suggest the possibility of an alternate mechanism of acrylamide formation in food low in asparagine.17,18 The decarboxylation of asparagine seems to be a key step in its conversion to acrylamide in the presence of alkadienals, suggesting that the inhibition of acrylamide formation by alkadienals should be mainly directed to the inhibiting the formation of the alkdienals.19 Role of antioxidants in acrylamide formation and inhibition The protective effect of antioxidants against acrylamide formation might be due to their ability to inhibit the formation of lipid oxidation products that contribute to acrylamide formation.20 By contrast, a related study suggests that the addition of antioxidants with a carbonyl group, such as curcumin, to foods might increase the formation of acrylamide
Food Funct., 2015, 6, 1752–1772 | 1753
View Article Online
Published on 19 May 2015. Downloaded by UNIVERSITY OF OTAGO on 14/07/2015 11:07:21.
Review
from asparagine during long-term heating if the free sugar concentration is low and the asparagine concentration is high.21 Cai et al.22 offer a mechanistic rationalization for the apparent increase in acrylamide formation and inhibition of its elimination in a model asparagine/glucose Maillard reaction system ( pH, 6.8) caused by high levels of added chlorogenic acid. The promotion of acrylamide formation seems to occur via increased hydroxymethylfurfural (HMF) formation and decreasing the activation energy for conversion of 3-aminopropionamide to acrylamide.9 By contrast, the quinone derivative of chlorogenic acid decreased acrylamide formation. Studies on the inhibition of acrylamide include those by Cheng et al.,23,24 who found that several antioxidative flavones and isoflavones (apigenin, luteolin, naringenin, tricin, daidzein, daidzin, genistein, genistin) inhibited acrylamide formation up to 52.1% in a model system, presumably by reacting with key Maillard intermediates such as 3-aminopropionamide. The rate of inhibition correlated with the change of trolox equivalent antioxidant capacity of the flavonoids measured by the DPPH assay. It seems that depending on concentration, natural antioxidants can both favour or prevent acrylamide formation in model systems. Below, we describe the use of antioxidants to inhibit acrylamide formation of food. Oxidation of asparagine at physiological conditions generates acrylamide Because acrylamide and glycidamide haemoglobin adducts were significantly increased in mice treated with compounds that induce oxidative stress, Tareke et al.25 investigated the possible formation of acrylamide from asparagine at 37 °C. The incubation of asparagine at physiological conditions (37 °C, pH 7.4) with hydrogen peroxide resulted in a timeand hydrogen-peroxide-concentration-dependent formation of acrylamide, suggesting that endogenous acrylamide might be formed in pathological conditions that are associated with long-term oxidative stress. Formation of N(ε)-(carboxymethyl)-L-lysine (CML) in cookies CML, an advanced glycation end product (AGE) formed by reaction of glyoxal with the ε-NH2 group of lysine, is concurrently formed at much higher rates than acrylamide during the baking of dough into cookies.26,27 The rates of formation of the two compounds do not seem to be the same, however, as indicated by the fact the highest level of acrylamide was obtained in cookies baked at 205 °C for 11 min or 155 °C for 21 min, whereas the highest level of CML was obtained in cookies baked at 230 °C for 1.5 min. No acrylamide was detected under the latter baking condition. The concentration of both compounds decreased during prolonged baking, presumably as a result of heat-induced degradation. It seems that cookies might contain two new ingredients, acrylamide and CML formed under different time-temperature conditions. CML, is reported to cause adverse effects on the kidney and liver functions of rats.28 In addition to producing acrylamide and CML, heat and high pH can induce the formation of other
1754 | Food Funct., 2015, 6, 1752–1772
Food & Function
potentially toxic compounds, including furan and hydroxymethylfurfural,9,29,30 the structurally related bioactive compound lysinoalanine,31 and bioactive D-amino acids.32
The reduction of acrylamide formation in processed foods Asparaginases inhibit acrylamide formation in food The exceptional ability of the enzyme asparaginase to inhibit acrylamide formation in food merits a separate discussion. Blast cells lack the ability to synthesize the amino acid L-asparagine and rely on other sources of L-asparagine for protein synthesis. The microbial enzyme asparaginase (EC 3.5.1.1) that hydrolyses asparagine to aspartic acid and ammonia is therefore used clinically in combination chemotherapy protocols to treat acute lymphoblastic leukaemia. The lack of asparagine inhibits growth of the leukemic cells.33,34 Because asparagine is also a key precursor in the formation of acrylamide, several studies have examined the potential of asparaginases from microbial sources to reduce the acrylamide content of processed foods, reviewed by Anese et al.35 and Zuo et al.36 These include the following selected observations. An investigation of acrylamide formation in fried potatoes in relation to blanching (75 °C, 10 min) and asparaginase treatments (10 000 U L−1, 40 °C, 20 min) before final frying (175 °C, 3 min) showed that the enzyme reduced the acrylamide content of 2075 μg kg−1 by 30%, suggesting that the blanching heat treatment prior to frying might cause swelling of the potato starch that improves the diffusion of asparagine towards the asparaginase solution surrounding the strips and that the physical structure of the food affects efficacy of the enzyme.37,38 The effect of the addition of 100 and 50 U asparaginase per kg of wheat-based dough before deep-frying at 180 °C and 200 °C for 4, 6, and 8 min resulted in the near complete conversion of the asparagine content to aspartic acid and an up to 90% reduction of the acrylamide level of the fried dough pastries (rosquillas) widely consumed in Spain.39 The enzyme also hydrolysed 37% of the glutamine in the dough to glutamic acid, an undesirable side reaction. Maillard reaction and browning parameters were not affected, suggesting the value of the treatment for acrylamide reduction. In another study on asparaginase in potato products, the acrylamide content (in mg kg−1) of fried peeled potato samples (160 °C, 10 min, electric oven) was reduced from 2.53 to 0.46 (82% reduction) after the samples were immersed in a solution of L-asparaginase (8000 U L−1) for 30 min before frying.40 The application of asparaginase to doughs used to bake biscuits, crisp bread, French fries, and sliced potato chips using recipes that resemble industrial processes resulted in the reduction of acrylamide content in the final product of 34–92%.41 Treatment of the potato slices with asparaginase after blanching lowered the acrylamide content of French fries by 60–85% and that in potato chips by up to 60%. A 1 min
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 19 May 2015. Downloaded by UNIVERSITY OF OTAGO on 14/07/2015 11:07:21.
Food & Function
enzyme treatment was as effective as a 20 min soak, suggesting the value of the method for large-scale industrial use. An L-asparaginase isolated from a microbial source exhibited low glutaminase activity, inhibited poly-acrylamide formation in 10% acrylamide solution, and reduced acrylamide formation by 80% in fried potatoes (15 min, 100 mL oil, 175 °C) soaked in the enzyme solution (30 U mL−1).42 A newly isolated fungal L-asparaginase with low glutaminase activity decreased the acrylamide content in biscuits and bread by 81.6% and 94.2%, respectively, following treatment with 10 000 U kg−1 flour for 30 min at 45 °C.33 The enzyme also showed antiproliferative activity against leukaemia cells, suggesting its value in both the food industry and medicine. The Food and Drug Administration (FDA) recognized asparaginase as safe (generally regarded as safe, GRAS), suggesting that it may be safely used to reduce acrylamide in food.43 An ideal asparaginase formulation for food use would be one that could act rapidly, be stable to proteolysis and degradation under food-processing conditions, and not induce allergic effects after consumption. Acrylamide reduction in cereal products Biscuits, breads, bread rolls, and cookies consumed worldwide are prepared from barley, corn, rye, wheat, and other seeds that are ground into flours which are then transformed into dough. Exposure of the dough to baking conditions results in the formation of final cereal-based products that contain heatinduced acrylamide. Here, we will describe, in chronological order, selected efforts to reduce the acrylamide content of cereal products (reviewed by Claus et al.44) by varying baking conditions and by food-compatible dough additives. Breads and biscuits. A study of baking parameters for wheat bread found that (a) more than 99% of the acrylamide was present in the crust; (b) added asparagine increased the acrylamide content from 80 to up to 6000 μg kg−1; (c) increases in temperature to >200 °C and time of baking resulted in increased acrylamide content in crust dry matter from 10 to 1900 μg kg−1; and (d) a significant correlation was observed between colour and acrylamide content in the crust, suggesting that low-asparagine wheat flour will produce lowacrylamide bread.45 Another baking study showed that acrylamide concentration and browning intensity of gingerbread increased with baking time and correlated with each other and that a reduction of >60% in acrylamide content was achieved by using sodium hydrogencarbonate instead of ammonium salt in the dough formulation.46 Pilot studies showed that added L-cysteine, increased fermentation time, reduced temperature, prolonged heat treatment, and the use of deck ovens minimized the acrylamide content of wheat bread and bread rolls.44 It has also been shown that fortification of bakery dough with calcium carbonate, amino acids, and yeast as well as lower pH, reduces the acrylamide content of breads and biscuits,47 that a combined process that uses conventional baking and vacuum postbaking mitigates acrylamide and hydroxymethylfurfural for-
This journal is © The Royal Society of Chemistry 2015
Review
mation in biscuits as compared to conventional baking, suggesting that the new process might provide a higher level of safety for baby biscuits,48 that added sodium chloride lowered the acrylamide content of biscuits baked at 180 °C and 190 °C but not at 200 °C,49 and that acrylamide formation in biscuits can be predicted by a high-throughput mass spectrometry fingerprinting technique, suggesting that the model could be used for rapid screening of biscuits to assess changes in formulations and processing conditions.50 A reduction in acrylamide up to 36% was obtained by adding 0.04 M calcium chloride solution to the wheat flour recipe, whereas a 23% reduction was observed using 0.04 M sodium chloride in a model system of dough pieces in sealed pressure tubes.51 In a study using different bread flours it was shown that the addition of glycine to rye and wheat flours produced more acrylamide than the corresponding bread from wheat, whereas rye flour produced more HMF than wheat flour, suggesting that the type of flour might affect acrylamide levels of baked products.52 This study also reported that regardless of breadflour formulation, the more severe the toasting of the bread, the higher the acrylamide and HMF levels and antioxidant capacity, and that adding asparaginase reduced acrylamide formation by up to 88%. Another comparison of bread flours showed that less acrylamide was formed per unit of asparagine in heated rye flour samples than in samples from wheat flour, suggesting that the reputation of rye as a high acrylamide risk cereal may be undeserved.53 The acrylamide content of mixed rye bread, which ranged from 578 to 1280 μg kg−1, was reduced by about 27% by added lactic acid bacteria (LAB) with low amylolytic and high proteolytic activity, presumably by lowering the pH of the baking formulation.54 In a study on corn it was shown that addition of citric acid to corn extrudates prevented acrylamide formation during extrusion cooking, but has a detrimental effect on texture properties.55 The same study also found that the acrylamide content of the extrudates produced with 22% feed moisture decreased by 61% in the carbon dioxide injection method compared to conventional extrusion. The corresponding reduction with 24% feed moisture was 82%. The authors conclude that this study is a first example of using an extrusion cooker as a process reactor to investigate the effects of different parameters on acrylamide formation. Barley, another grain used in bread production, has also been investigated for its acrylamide content. It was found that the acrylamide concentration of roasted barley decreased from about 160 μg kg−1 to about 100 μg kg−1 during storage at room temperature for 500 days, possibly as a result of acrylamide evaporation or reaction with barley ingredients.56 Cookies. A number of studies have investigated acrylamide formation in the production of cookies. Indeed, a direct correlation was observed between the concentration of undesirable acrylamide and desirable antioxidant activity (determined by electron paramagnetic resonance spectroscopy, EPR) in a
Food Funct., 2015, 6, 1752–1772 | 1755
View Article Online
Published on 19 May 2015. Downloaded by UNIVERSITY OF OTAGO on 14/07/2015 11:07:21.
Review
series of laboratory-prepared and commercial cookies.57 Increasing the baking time from 10 to 30 min resulted in an increase in acrylamide content from about 10 to 300 μg kg−1 and a parallel increase in the antioxidative activity. Compared to sucrose, the use of fructose in the dough formulation increased both acrylamide and antioxidant activity of the final products. It seems that suppressing the Maillard reaction might result in both reduction of acrylamide content and in an increase of beneficial antioxidants. A related study that describes the influence of recipes and baking conditions on the relationship between acrylamide formation and antioxidant levels of wheat-based biscuits found that wholemeal biscuits contain low acrylamide and high antioxidant levels and that asparaginase reduced the acrylamide content of the biscuits by about 50%.58 A study on the mitigation of acrylamide formation in cookies by five antioxidants (bamboo leaves, sodium erythorbate, tea polyphenols, vitamin E, and tert-butyl hydroquinone) found that bamboo leaves (0.2 g kg−1) and vitamin E (0.1 g kg−1) effectively achieved balance between acrylamide reduction (up to 71.2%) and sensory acceptance, suggesting that the use of antioxidants could be an effective in reducing acrylamide formation.59 A study on the effects of baking conditions of cookies on the formation of acrylamide, Nε-(carboxymethyl)-lysine (CML) and antioxidative activity found that: (a) the highest level of acrylamide (328.9 μg kg−1) was present in cookies baked at 155 °C for 21 min and at 205 °C for 11 min (329.3 μg kg−1); (b) the highest level of CML (118.0 μg kg−1) was present in cookies baked at 230 °C for 1.5 min; and (c) the antioxidative activity increased under the more severe baking conditions, suggesting that optimizing the baking temperature was not enough for making high-quality cookies.26 Replacing 15% of wheat flour with the soybean-derived food ingredient okara resulted in cookies that had a 60% higher content of acrylamide, a 100% higher content of HMF, and a 400% increase in CML as compared to the control cookies.60 This study also reported that soybean-containing commercial bakery products had higher concentrations of acrylamide and CML than corresponding controls, suggesting that adding soybean to wheat flour to enhance nutritional quality could adversely affect the safety of the baked products. The addition of cinnamon, clove, coriander, cumin, and turmeric as powders or as aqueous extracts reduced acrylamide formation in cookie models, and also in potato starch-based models.61 A 4% aqueous clove extract caused a maximum 50.9% reduction of acrylamide in the cookies and a 2% proanthocyanidin extract from grape seeds induced a maximum 62.2% reduction in the potato model. The other additives induced similar reductions in acrylamide levels. Because the tested compounds are part of the human diet and are also reported to exhibit antimicrobial properties against foodborne pathogens,62 the described cookies might possess multifunctional health benefits. An amaranth protein isolate, but not amaranth flour, has been shown to decrease the acrylamide content of cookies by
1756 | Food Funct., 2015, 6, 1752–1772
Food & Function
89%, fried tortillas chips by 51%, and baked tortilla chips by 62%, suggesting that adding the high quality protein may have dual benefits: a reduction of acrylamide formation and an improvement in the nutritional quality of the foods.63 The authors suggest that acrylamide reduction may have resulted from the competition between asparagine and protein amino acid residues for the carbonyl compounds involved in acrylamide formation, a mechanism that merits further study. Although the acrylamide content of cookies increased with temperature for both conventional and steam-assisted ovens at the same baking time, steam-assisted baking resulted in lower acrylamide concentration at the 165 °C baking temperature and lower surface colour for all temperatures (165, 189, and 195 °C), suggesting that steam-assisted baking might provide healthier cookies for consumers while maintaining physical quality.64 A kinetic study revealed that the browning index may be considered as a reliable indicator of the acrylamide concentration in cookies baked at different temperatures. Radio frequency (RF) heaters are used to remove moisture from baked products without changing their sensory properties. RF post drying of partially baked cookies resulted in an acrylamide content of 74.6 μg kg−1 as compared to 107.3 μg kg−1 for control cookies, suggesting that baking processes could be re-designed to lower the acrylamide content of cookies.65 In summary, the cited studies indicate that the possibility exists to significantly reduce the acrylamide levels of baked products by modifying baking conditions and by adding safe plant-derived compounds to the baking formations that either compete with asparagine during the Maillard reaction or chemically modify acrylamide during the baking process. The possible significance of the resulting changes for nutrition and food safety is largely unknown. Another approach is to develop new wheat and barley varieties that have low content of acrylamide precursors, as described by Halford et al.66 Acrylamide reduction in roasted coffee, almonds, and teas The roasting of coffee (Coffea arabica or Coffea robusta) beans induces the formation of undesirable compounds such as acrylamide,67–70 furan,71 5-hydroxymethylfurfural and Nε-(carboxymethyl)-L-lysine,72 as well as beneficial antioxidants such as chlorogenic acid.73 The acrylamide in roasted coffee is largely extracted in the brew and stable during consumption.74 The following brief overview in chronological order is limited to a description of efforts designed to reduce the acrylamide content in roasted coffees, almonds, and teas. The acrylamide level during roasting has been observed to reach a peak level and then decrease, suggesting that both formation and reduction occur during the roast process.74 The same authors also observed that the reduction of the acrylamide content in coffee during storage follows second-order reaction kinetics with an activation energy of 73 kJ mol−1, suggesting that old roasted coffees might have lower levels of acrylamide than those determined immediately after production.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 19 May 2015. Downloaded by UNIVERSITY OF OTAGO on 14/07/2015 11:07:21.
Food & Function
Supercritical carbon dioxide with 9.5% ethanol extraction (200 °C, 200 bar) of coffee did not affect the caffeine content but extracted up to 79% of the acrylamide content, suggesting that the process offers a clean, efficient, and environmentally acceptable method of removing acrylamide from coffees and possibly other foods.75 Vacuum-roasted coffee (200 °C, 0.15 kPa) with a medium roast degree had about 50% less acrylamide than conventionally roasted coffee, suggesting that the low pressure during the vacuum process prevented acrylamide from being accumulated.76 The colours and sensory properties of the coffees prepared by the two methods were similar. Because about 10 billion canned coffees are consumed annually in Japan, Narita and Inouye77 investigated the effect of additives on the acrylamide levels of heat-treated (sterilized) coffee during the manufacturing process. They found that adding L-cysteine or dithiothreitol resulted in a 95% decrease in the acrylamide content of two types of coffee heated at 121 °C for 6 min; adding lysine or arginine had no effect; the cysteine also induced a nearly quantitative decrease in the acrylamide content of roasted barley tea. A likely explanation for the observed effects is that the SH groups of cysteine and dithiothreitol react with the double bond of acrylamide to form inactive adducts.5 It is also relevant to note that the D-cysteine adduct of acrylamide can serve as an internal standard for the analysis of the acrylamide contents of coffee, French fries, and potato chips,78 and that the amino acid 32 D-cysteine causes adverse effects in mice. Mixing of instant coffee (20%, w/v) with sucrose (0–10%, w/v) and baker’s yeast (Saccharomyces cerevisiae, 1–2%, w/v) in a closed glass vessel followed by fermentation at 30 °C for 48 h caused a reduction of the acrylamide level by about 70% and of the hydroxymethylfurfural level by up to 99.2%,79 suggesting that yeast fermentation is a promising method for reducing the levels of both compounds in instant coffee. The authors conclude that the fermentation process can be easily adapted into the instant coffee industrial production line. Related studies on the effect of roasting reported that: (a) roasting of almonds induces the formation of acrylamide up to about 1600 μg kg−1 and that controlling the roasting temperature is a critical factor for limiting the acrylamide content;80 (b) roasting of almonds below 146 °C resulted in acrylamide levels below 200 ppb at all roasting times and storage of the almonds at 60 °C may further reduce the acrylamide content of the almonds;81 (c) the considerable amounts of acrylamide formed during roasting of teas at 120 °C, that is reduced at a roasting temperature of 180 °C, presumably as result of heatinduced degradation;82,83 and (d) roasting does not seem to produce acrylamide in hazelnuts, macadamia nuts, pistachios, or walnuts.84 Reduction of acrylamide in olives Processed olives have the potential to contribute a significant fraction to the total acrylamide burden of the diet. High temperature sterilization and lye treatment of olives are presumably used to kill harmful microorganisms but, in turn, these treat-
This journal is © The Royal Society of Chemistry 2015
Review
ments can induce the formation of acrylamide. We will briefly examine in approximately chronological order some of the factors that favour acrylamide formation and possible approaches that have been tried to reduce the acrylamide content of processed olives. A study of the amounts of acrylamide in different Spanish brands of black ripe olives and the effects of industrial processing (sterilization) that involves exposure to heat and alkali on acrylamide formation found that: (a) the acrylamide levels ranged from 176 to 1578 μg kg−1 of pulp or a nine-fold variation from lowest to highest value; (b) the lowest levels were found in ripe olives using the traditional preservation in brine that involves penetration of lye to reach the stone and two water washings; (c) olives processed using one lye treatment and one water washing had the highest amounts; (d) no correlation was apparent between acrylamide content and the small amounts of amino acids or sugars of the olives before sterilization, suggesting that the acrylamide might not be formed by the classical route from asparagine and glucose; and (e) the effect of storage for six months was minor.85 The above suggestion of an alternate mechanism is reinforced by comparison of acrylamide formation in olive water from untreated green olives and thermally and lye-treated green olives, in which was found that acrylamide formation might be associated with a peptide or protein fraction by an unknown mechanism.86 An analysis of the acrylamide content of several Spanish foods including olives found that unprocessed bulk olives contained no acrylamide and that the acrylamide content of bottled and canned black olives ranged from 124 to 316 μg kg−1 and of bottled and canned green olives from 0 to 93 μg kg−1),87 suggesting that the level of maturation might affect acrylamide content. The cited values are much lower than those shown in the FDA data base.88 A detailed study of the factors that influence processinginduced acrylamide formation of California-style ripe olives found that: (a) black olives contained higher amounts (in μg kg−1) of acrylamide (409.7) as compared to green ripe olives (44.0), Greek olives (
Published on 19 May 2015. Downloaded by UNIVERSITY OF OTAGO on 14/07/2015 11:07:21.
REVIEW
Cite this: Food Funct., 2015, 6, 1752
View Journal | View Issue
Acrylamide: inhibition of formation in processed food and mitigation of toxicity in cells, animals, and humans Mendel Friedman Potentially toxic acrylamide is largely derived from the heat-inducing reactions between the amino group of the amino acid asparagine and carbonyl groups of glucose and fructose in plant-derived foods including cereals, coffees, almonds, olives, potatoes, and sweet potatoes. This review surveys and consolidates the following dietary aspects of acrylamide: distribution in food, exposure and consumption by diverse populations, reduction of the content in different food categories, and mitigation of adverse in vivo effects. Methods to reduce acrylamide levels include selecting commercial food with a low acrylamide content, selecting cereal and potato varieties with low levels of asparagine and reducing sugars, selecting processing conditions that minimize acrylamide formation, adding food-compatible compounds and
Received 26th March 2015, Accepted 1st May 2015 DOI: 10.1039/c5fo00320b www.rsc.org/foodfunction
plant extracts to food formulations before processing that inhibit acrylamide formation during processing of cereal products, coffees, teas, olives, almonds, and potato products, and reducing multiorgan toxicity (antifertility, carcinogenicity, neurotoxicity, teratogenicity). The herein described observations and recommendations are of scientific interest for food chemistry, pharmacology, and toxicology, but also have the potential to benefit nutrition, food safety, and human health.
Introduction Heat-induced amino-carbonyl and related interactions between food ingredients encompass those changes commonly termed browning reactions.1,2 These include reactions of amino acids, peptides, and proteins with reducing sugars glucose and fructose and ascorbic acid (vitamin C). These socalled Maillard browning reactions can result in the destruction of amino acids and decrease in digestibility and nutritional quality. Numerous food processing methods that transform raw into edible food benefit nutrition and health.3 Reports that the reactive α, β-unsaturated (conjugated) compound called acrylamide (CH2vCH–CONH2) is formed during the processing of foods under conditions that also induce the formation of Maillard browning products stimulated interest in its chemistry, biochemistry, and safety. To contribute to this effort, we previously reviewed the chemistry, pharmacology, toxicology of acrylamide, and possible ways to reduce its formation in food and in vivo toxicity.4,5 With Prof. Mottram of the University of Reading, UK, we organized and edited the
Western Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, 800 Buchanan St., Albany, CA 94710, USA. E-mail: [email protected]; Fax: +1-510-559-6162; Tel: +1-510-559-5615
1752 | Food Funct., 2015, 6, 1752–1772
Proceedings of two symposia on acrylamide.6,7 Noteworthy reviews by other investigators include those by Mills et al.8 and Capuano and Fogliano.9 This integrated review presents, updates and interprets new information on extensive worldwide efforts to develop insights into acrylamide formation in model systems and in food as well as reports on dietary exposure, reduction in processed cereal products (bread, bread rolls, cookies), roasted products (coffees, teas, almonds, barley), olives, and white and sweet potato products (French fries, chips, crisps). Also covered are approaches to mitigate reported carcinogenicity, developmental toxicity including teratogenicity, fertility, and neurotoxicity of acrylamide in cells, animals, and humans. The cited results will hopefully facilitate the development of improved industrial processes and domestic cooking conditions to decrease the acrylamide burden of the diet and its toxicity after consumption.
Adverse effects of dietary acrylamide on humans A key objective of studies on the exposure of different population subgroups (infants, teenagers, adults) to diet-based acrylamide is whether such self-assessment of consumption of different food categories can be used to relate to or predict
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 19 May 2015. Downloaded by UNIVERSITY OF OTAGO on 14/07/2015 11:07:21.
Food & Function
adverse human effects.10 This is a challenging problem with mixed results, as indicated by the following selected studies. Kütting et al.11 detected haemoglobin adducts of acrylamide in 999 of 1008 human blood samples from smokers and non-smokers. They suggest that (a) smoking can be regarded as the main source of overall acrylamide intake in people with occupational exposure to acrylamide and that food intake is of environmental importance; (b) data on neurotoxicity and reproductive toxicity due to acrylamide are not likely to occur in the general population except in very high consumers; (c) carcinogenicity and mutagenicity are possible health risks for the general population; (d) precautionary measures should be implemented such as minimizing the acrylamide content of the diet, to avoid unnecessary increase in the overall tumour risk; and (e) such measures include the use of appropriate technology in food processing without adversely affecting colour, flavour and nutritional value, monitoring the acrylamide levels of widely distributed processed food, and minimizing home-made acrylamide formation by guidance to consumers. The results of studies on the dietary exposure to acrylamide of the general French population in relation to international health-based guidance values show that the mean acrylamide concentration (in g per kg fresh weight) of about 20 food groups ranged from 2 for fruit to 954 for potato chips/crisps, with the highest values for potato chips (954), biscuits (729), and French fries (724). The main contributors to dietary exposure, accounting for both content and food and consumption patterns, (in %) were French fries (44.8) and coffee (27.7) for adults and French fries (60.8), sweetened biscuits (18.8), and potato chips (4.09) for children. Because the results may indicate a health concern for young children, the authors suggest the need to continue efforts to reduce dietary exposure to acrylamide.12 A Polish study found that the mean acrylamide content in baby foods depended on the food product, ranging from 2 to 516 μg kg−1.13 The exposure of infants (in g per kg body weight per day) aged 6–12 months at the minimum level ranged from 0.41 to 0.62, at the average level, from 2.10 to 4.32, and at the maximum level, from 7.47 to 12.35. This maximum value was significantly higher than the exposure estimated for the entire Polish population. Because the World Health Organization (WHO) estimated that the lifetime exposure to acrylamide of 0.14 g per kg body weight may increase the risk of one additional cancer per 10 000 people, the authors suggest that (a) the observed high estimated exposure of infants to acrylamide may be associated with a potential cancer risk, with the caveat that the exposure to acrylamide from baby food is of short duration compared to the lifetime exposure; and (b) the reduction of acrylamide content of baby food should be a high priority for risk management. An estimate of the dietary exposure of Polish teenagers (n = 261) to acrylamide during a 7 day daily food record found that (a) for girls the estimated dietary intake of acrylamide (in μg per kg body weight per day) was 0.09 (50th percentile), 0.32 (75th percentile), and 1.04 (95th percentile), with the corres-
This journal is © The Royal Society of Chemistry 2015
Review
ponding values for boys of 0.13, 0.41, and 1.18, respectively; and (b) the main sources of acrylamide were French fries, potato crisps, corn flakes, bread, and salty sticks.14 These results suggest that need to discourage teenagers from consuming food with high acrylamide content (such as French fries, potato crisps).
Chemistry of acrylamide formation and inhibition Many studies have strikingly shown the complexity of the chemical basis of acrylamide formation and indicated possible approaches to reduce its generation. Here, we highlight some of these studies in both model systems and cookies. Such studies can provide insights into potential practical approaches to limit the acrylamide content in different food categories. We also examine specific methods that have been proposed to inhibit acrylamide formation in cereal products, coffee, olives, and potatoes. Insights into acrylamide formation and inhibition in model systems Studies in a heated glucose/asparagine model system show that the B6 vitamer pyridoxamine inhibits acrylamide formation in the temperature range 120 to 180 °C, apparently by scavenging intermediate dicarbonyl compounds such as glyoxal and methylglyoxal formed during degradation of glucose and other events in the Maillard reactions.15 The inhibition was higher than with the same concentration of vitamin C and was accompanied by a reduction in the degree of browning.16 The authors isolated and characterized the structures of the acrylamide-pyridoxamine adducts as direct Michael addition products. These results imply that pyridoxamine has the potential to mitigate acrylamide formation in lipid-rich foods, where oxidation products might contribute to acrylamide formation. Arribas-Lorenzo and Morales15 also measured the formation of 3-propionamide, a transient intermediate in acrylamide formation during thermal degradation of asparagine initiated by reducing carbohydrates or aldehydes. Their results suggest the possibility of an alternate mechanism of acrylamide formation in food low in asparagine.17,18 The decarboxylation of asparagine seems to be a key step in its conversion to acrylamide in the presence of alkadienals, suggesting that the inhibition of acrylamide formation by alkadienals should be mainly directed to the inhibiting the formation of the alkdienals.19 Role of antioxidants in acrylamide formation and inhibition The protective effect of antioxidants against acrylamide formation might be due to their ability to inhibit the formation of lipid oxidation products that contribute to acrylamide formation.20 By contrast, a related study suggests that the addition of antioxidants with a carbonyl group, such as curcumin, to foods might increase the formation of acrylamide
Food Funct., 2015, 6, 1752–1772 | 1753
View Article Online
Published on 19 May 2015. Downloaded by UNIVERSITY OF OTAGO on 14/07/2015 11:07:21.
Review
from asparagine during long-term heating if the free sugar concentration is low and the asparagine concentration is high.21 Cai et al.22 offer a mechanistic rationalization for the apparent increase in acrylamide formation and inhibition of its elimination in a model asparagine/glucose Maillard reaction system ( pH, 6.8) caused by high levels of added chlorogenic acid. The promotion of acrylamide formation seems to occur via increased hydroxymethylfurfural (HMF) formation and decreasing the activation energy for conversion of 3-aminopropionamide to acrylamide.9 By contrast, the quinone derivative of chlorogenic acid decreased acrylamide formation. Studies on the inhibition of acrylamide include those by Cheng et al.,23,24 who found that several antioxidative flavones and isoflavones (apigenin, luteolin, naringenin, tricin, daidzein, daidzin, genistein, genistin) inhibited acrylamide formation up to 52.1% in a model system, presumably by reacting with key Maillard intermediates such as 3-aminopropionamide. The rate of inhibition correlated with the change of trolox equivalent antioxidant capacity of the flavonoids measured by the DPPH assay. It seems that depending on concentration, natural antioxidants can both favour or prevent acrylamide formation in model systems. Below, we describe the use of antioxidants to inhibit acrylamide formation of food. Oxidation of asparagine at physiological conditions generates acrylamide Because acrylamide and glycidamide haemoglobin adducts were significantly increased in mice treated with compounds that induce oxidative stress, Tareke et al.25 investigated the possible formation of acrylamide from asparagine at 37 °C. The incubation of asparagine at physiological conditions (37 °C, pH 7.4) with hydrogen peroxide resulted in a timeand hydrogen-peroxide-concentration-dependent formation of acrylamide, suggesting that endogenous acrylamide might be formed in pathological conditions that are associated with long-term oxidative stress. Formation of N(ε)-(carboxymethyl)-L-lysine (CML) in cookies CML, an advanced glycation end product (AGE) formed by reaction of glyoxal with the ε-NH2 group of lysine, is concurrently formed at much higher rates than acrylamide during the baking of dough into cookies.26,27 The rates of formation of the two compounds do not seem to be the same, however, as indicated by the fact the highest level of acrylamide was obtained in cookies baked at 205 °C for 11 min or 155 °C for 21 min, whereas the highest level of CML was obtained in cookies baked at 230 °C for 1.5 min. No acrylamide was detected under the latter baking condition. The concentration of both compounds decreased during prolonged baking, presumably as a result of heat-induced degradation. It seems that cookies might contain two new ingredients, acrylamide and CML formed under different time-temperature conditions. CML, is reported to cause adverse effects on the kidney and liver functions of rats.28 In addition to producing acrylamide and CML, heat and high pH can induce the formation of other
1754 | Food Funct., 2015, 6, 1752–1772
Food & Function
potentially toxic compounds, including furan and hydroxymethylfurfural,9,29,30 the structurally related bioactive compound lysinoalanine,31 and bioactive D-amino acids.32
The reduction of acrylamide formation in processed foods Asparaginases inhibit acrylamide formation in food The exceptional ability of the enzyme asparaginase to inhibit acrylamide formation in food merits a separate discussion. Blast cells lack the ability to synthesize the amino acid L-asparagine and rely on other sources of L-asparagine for protein synthesis. The microbial enzyme asparaginase (EC 3.5.1.1) that hydrolyses asparagine to aspartic acid and ammonia is therefore used clinically in combination chemotherapy protocols to treat acute lymphoblastic leukaemia. The lack of asparagine inhibits growth of the leukemic cells.33,34 Because asparagine is also a key precursor in the formation of acrylamide, several studies have examined the potential of asparaginases from microbial sources to reduce the acrylamide content of processed foods, reviewed by Anese et al.35 and Zuo et al.36 These include the following selected observations. An investigation of acrylamide formation in fried potatoes in relation to blanching (75 °C, 10 min) and asparaginase treatments (10 000 U L−1, 40 °C, 20 min) before final frying (175 °C, 3 min) showed that the enzyme reduced the acrylamide content of 2075 μg kg−1 by 30%, suggesting that the blanching heat treatment prior to frying might cause swelling of the potato starch that improves the diffusion of asparagine towards the asparaginase solution surrounding the strips and that the physical structure of the food affects efficacy of the enzyme.37,38 The effect of the addition of 100 and 50 U asparaginase per kg of wheat-based dough before deep-frying at 180 °C and 200 °C for 4, 6, and 8 min resulted in the near complete conversion of the asparagine content to aspartic acid and an up to 90% reduction of the acrylamide level of the fried dough pastries (rosquillas) widely consumed in Spain.39 The enzyme also hydrolysed 37% of the glutamine in the dough to glutamic acid, an undesirable side reaction. Maillard reaction and browning parameters were not affected, suggesting the value of the treatment for acrylamide reduction. In another study on asparaginase in potato products, the acrylamide content (in mg kg−1) of fried peeled potato samples (160 °C, 10 min, electric oven) was reduced from 2.53 to 0.46 (82% reduction) after the samples were immersed in a solution of L-asparaginase (8000 U L−1) for 30 min before frying.40 The application of asparaginase to doughs used to bake biscuits, crisp bread, French fries, and sliced potato chips using recipes that resemble industrial processes resulted in the reduction of acrylamide content in the final product of 34–92%.41 Treatment of the potato slices with asparaginase after blanching lowered the acrylamide content of French fries by 60–85% and that in potato chips by up to 60%. A 1 min
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 19 May 2015. Downloaded by UNIVERSITY OF OTAGO on 14/07/2015 11:07:21.
Food & Function
enzyme treatment was as effective as a 20 min soak, suggesting the value of the method for large-scale industrial use. An L-asparaginase isolated from a microbial source exhibited low glutaminase activity, inhibited poly-acrylamide formation in 10% acrylamide solution, and reduced acrylamide formation by 80% in fried potatoes (15 min, 100 mL oil, 175 °C) soaked in the enzyme solution (30 U mL−1).42 A newly isolated fungal L-asparaginase with low glutaminase activity decreased the acrylamide content in biscuits and bread by 81.6% and 94.2%, respectively, following treatment with 10 000 U kg−1 flour for 30 min at 45 °C.33 The enzyme also showed antiproliferative activity against leukaemia cells, suggesting its value in both the food industry and medicine. The Food and Drug Administration (FDA) recognized asparaginase as safe (generally regarded as safe, GRAS), suggesting that it may be safely used to reduce acrylamide in food.43 An ideal asparaginase formulation for food use would be one that could act rapidly, be stable to proteolysis and degradation under food-processing conditions, and not induce allergic effects after consumption. Acrylamide reduction in cereal products Biscuits, breads, bread rolls, and cookies consumed worldwide are prepared from barley, corn, rye, wheat, and other seeds that are ground into flours which are then transformed into dough. Exposure of the dough to baking conditions results in the formation of final cereal-based products that contain heatinduced acrylamide. Here, we will describe, in chronological order, selected efforts to reduce the acrylamide content of cereal products (reviewed by Claus et al.44) by varying baking conditions and by food-compatible dough additives. Breads and biscuits. A study of baking parameters for wheat bread found that (a) more than 99% of the acrylamide was present in the crust; (b) added asparagine increased the acrylamide content from 80 to up to 6000 μg kg−1; (c) increases in temperature to >200 °C and time of baking resulted in increased acrylamide content in crust dry matter from 10 to 1900 μg kg−1; and (d) a significant correlation was observed between colour and acrylamide content in the crust, suggesting that low-asparagine wheat flour will produce lowacrylamide bread.45 Another baking study showed that acrylamide concentration and browning intensity of gingerbread increased with baking time and correlated with each other and that a reduction of >60% in acrylamide content was achieved by using sodium hydrogencarbonate instead of ammonium salt in the dough formulation.46 Pilot studies showed that added L-cysteine, increased fermentation time, reduced temperature, prolonged heat treatment, and the use of deck ovens minimized the acrylamide content of wheat bread and bread rolls.44 It has also been shown that fortification of bakery dough with calcium carbonate, amino acids, and yeast as well as lower pH, reduces the acrylamide content of breads and biscuits,47 that a combined process that uses conventional baking and vacuum postbaking mitigates acrylamide and hydroxymethylfurfural for-
This journal is © The Royal Society of Chemistry 2015
Review
mation in biscuits as compared to conventional baking, suggesting that the new process might provide a higher level of safety for baby biscuits,48 that added sodium chloride lowered the acrylamide content of biscuits baked at 180 °C and 190 °C but not at 200 °C,49 and that acrylamide formation in biscuits can be predicted by a high-throughput mass spectrometry fingerprinting technique, suggesting that the model could be used for rapid screening of biscuits to assess changes in formulations and processing conditions.50 A reduction in acrylamide up to 36% was obtained by adding 0.04 M calcium chloride solution to the wheat flour recipe, whereas a 23% reduction was observed using 0.04 M sodium chloride in a model system of dough pieces in sealed pressure tubes.51 In a study using different bread flours it was shown that the addition of glycine to rye and wheat flours produced more acrylamide than the corresponding bread from wheat, whereas rye flour produced more HMF than wheat flour, suggesting that the type of flour might affect acrylamide levels of baked products.52 This study also reported that regardless of breadflour formulation, the more severe the toasting of the bread, the higher the acrylamide and HMF levels and antioxidant capacity, and that adding asparaginase reduced acrylamide formation by up to 88%. Another comparison of bread flours showed that less acrylamide was formed per unit of asparagine in heated rye flour samples than in samples from wheat flour, suggesting that the reputation of rye as a high acrylamide risk cereal may be undeserved.53 The acrylamide content of mixed rye bread, which ranged from 578 to 1280 μg kg−1, was reduced by about 27% by added lactic acid bacteria (LAB) with low amylolytic and high proteolytic activity, presumably by lowering the pH of the baking formulation.54 In a study on corn it was shown that addition of citric acid to corn extrudates prevented acrylamide formation during extrusion cooking, but has a detrimental effect on texture properties.55 The same study also found that the acrylamide content of the extrudates produced with 22% feed moisture decreased by 61% in the carbon dioxide injection method compared to conventional extrusion. The corresponding reduction with 24% feed moisture was 82%. The authors conclude that this study is a first example of using an extrusion cooker as a process reactor to investigate the effects of different parameters on acrylamide formation. Barley, another grain used in bread production, has also been investigated for its acrylamide content. It was found that the acrylamide concentration of roasted barley decreased from about 160 μg kg−1 to about 100 μg kg−1 during storage at room temperature for 500 days, possibly as a result of acrylamide evaporation or reaction with barley ingredients.56 Cookies. A number of studies have investigated acrylamide formation in the production of cookies. Indeed, a direct correlation was observed between the concentration of undesirable acrylamide and desirable antioxidant activity (determined by electron paramagnetic resonance spectroscopy, EPR) in a
Food Funct., 2015, 6, 1752–1772 | 1755
View Article Online
Published on 19 May 2015. Downloaded by UNIVERSITY OF OTAGO on 14/07/2015 11:07:21.
Review
series of laboratory-prepared and commercial cookies.57 Increasing the baking time from 10 to 30 min resulted in an increase in acrylamide content from about 10 to 300 μg kg−1 and a parallel increase in the antioxidative activity. Compared to sucrose, the use of fructose in the dough formulation increased both acrylamide and antioxidant activity of the final products. It seems that suppressing the Maillard reaction might result in both reduction of acrylamide content and in an increase of beneficial antioxidants. A related study that describes the influence of recipes and baking conditions on the relationship between acrylamide formation and antioxidant levels of wheat-based biscuits found that wholemeal biscuits contain low acrylamide and high antioxidant levels and that asparaginase reduced the acrylamide content of the biscuits by about 50%.58 A study on the mitigation of acrylamide formation in cookies by five antioxidants (bamboo leaves, sodium erythorbate, tea polyphenols, vitamin E, and tert-butyl hydroquinone) found that bamboo leaves (0.2 g kg−1) and vitamin E (0.1 g kg−1) effectively achieved balance between acrylamide reduction (up to 71.2%) and sensory acceptance, suggesting that the use of antioxidants could be an effective in reducing acrylamide formation.59 A study on the effects of baking conditions of cookies on the formation of acrylamide, Nε-(carboxymethyl)-lysine (CML) and antioxidative activity found that: (a) the highest level of acrylamide (328.9 μg kg−1) was present in cookies baked at 155 °C for 21 min and at 205 °C for 11 min (329.3 μg kg−1); (b) the highest level of CML (118.0 μg kg−1) was present in cookies baked at 230 °C for 1.5 min; and (c) the antioxidative activity increased under the more severe baking conditions, suggesting that optimizing the baking temperature was not enough for making high-quality cookies.26 Replacing 15% of wheat flour with the soybean-derived food ingredient okara resulted in cookies that had a 60% higher content of acrylamide, a 100% higher content of HMF, and a 400% increase in CML as compared to the control cookies.60 This study also reported that soybean-containing commercial bakery products had higher concentrations of acrylamide and CML than corresponding controls, suggesting that adding soybean to wheat flour to enhance nutritional quality could adversely affect the safety of the baked products. The addition of cinnamon, clove, coriander, cumin, and turmeric as powders or as aqueous extracts reduced acrylamide formation in cookie models, and also in potato starch-based models.61 A 4% aqueous clove extract caused a maximum 50.9% reduction of acrylamide in the cookies and a 2% proanthocyanidin extract from grape seeds induced a maximum 62.2% reduction in the potato model. The other additives induced similar reductions in acrylamide levels. Because the tested compounds are part of the human diet and are also reported to exhibit antimicrobial properties against foodborne pathogens,62 the described cookies might possess multifunctional health benefits. An amaranth protein isolate, but not amaranth flour, has been shown to decrease the acrylamide content of cookies by
1756 | Food Funct., 2015, 6, 1752–1772
Food & Function
89%, fried tortillas chips by 51%, and baked tortilla chips by 62%, suggesting that adding the high quality protein may have dual benefits: a reduction of acrylamide formation and an improvement in the nutritional quality of the foods.63 The authors suggest that acrylamide reduction may have resulted from the competition between asparagine and protein amino acid residues for the carbonyl compounds involved in acrylamide formation, a mechanism that merits further study. Although the acrylamide content of cookies increased with temperature for both conventional and steam-assisted ovens at the same baking time, steam-assisted baking resulted in lower acrylamide concentration at the 165 °C baking temperature and lower surface colour for all temperatures (165, 189, and 195 °C), suggesting that steam-assisted baking might provide healthier cookies for consumers while maintaining physical quality.64 A kinetic study revealed that the browning index may be considered as a reliable indicator of the acrylamide concentration in cookies baked at different temperatures. Radio frequency (RF) heaters are used to remove moisture from baked products without changing their sensory properties. RF post drying of partially baked cookies resulted in an acrylamide content of 74.6 μg kg−1 as compared to 107.3 μg kg−1 for control cookies, suggesting that baking processes could be re-designed to lower the acrylamide content of cookies.65 In summary, the cited studies indicate that the possibility exists to significantly reduce the acrylamide levels of baked products by modifying baking conditions and by adding safe plant-derived compounds to the baking formations that either compete with asparagine during the Maillard reaction or chemically modify acrylamide during the baking process. The possible significance of the resulting changes for nutrition and food safety is largely unknown. Another approach is to develop new wheat and barley varieties that have low content of acrylamide precursors, as described by Halford et al.66 Acrylamide reduction in roasted coffee, almonds, and teas The roasting of coffee (Coffea arabica or Coffea robusta) beans induces the formation of undesirable compounds such as acrylamide,67–70 furan,71 5-hydroxymethylfurfural and Nε-(carboxymethyl)-L-lysine,72 as well as beneficial antioxidants such as chlorogenic acid.73 The acrylamide in roasted coffee is largely extracted in the brew and stable during consumption.74 The following brief overview in chronological order is limited to a description of efforts designed to reduce the acrylamide content in roasted coffees, almonds, and teas. The acrylamide level during roasting has been observed to reach a peak level and then decrease, suggesting that both formation and reduction occur during the roast process.74 The same authors also observed that the reduction of the acrylamide content in coffee during storage follows second-order reaction kinetics with an activation energy of 73 kJ mol−1, suggesting that old roasted coffees might have lower levels of acrylamide than those determined immediately after production.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 19 May 2015. Downloaded by UNIVERSITY OF OTAGO on 14/07/2015 11:07:21.
Food & Function
Supercritical carbon dioxide with 9.5% ethanol extraction (200 °C, 200 bar) of coffee did not affect the caffeine content but extracted up to 79% of the acrylamide content, suggesting that the process offers a clean, efficient, and environmentally acceptable method of removing acrylamide from coffees and possibly other foods.75 Vacuum-roasted coffee (200 °C, 0.15 kPa) with a medium roast degree had about 50% less acrylamide than conventionally roasted coffee, suggesting that the low pressure during the vacuum process prevented acrylamide from being accumulated.76 The colours and sensory properties of the coffees prepared by the two methods were similar. Because about 10 billion canned coffees are consumed annually in Japan, Narita and Inouye77 investigated the effect of additives on the acrylamide levels of heat-treated (sterilized) coffee during the manufacturing process. They found that adding L-cysteine or dithiothreitol resulted in a 95% decrease in the acrylamide content of two types of coffee heated at 121 °C for 6 min; adding lysine or arginine had no effect; the cysteine also induced a nearly quantitative decrease in the acrylamide content of roasted barley tea. A likely explanation for the observed effects is that the SH groups of cysteine and dithiothreitol react with the double bond of acrylamide to form inactive adducts.5 It is also relevant to note that the D-cysteine adduct of acrylamide can serve as an internal standard for the analysis of the acrylamide contents of coffee, French fries, and potato chips,78 and that the amino acid 32 D-cysteine causes adverse effects in mice. Mixing of instant coffee (20%, w/v) with sucrose (0–10%, w/v) and baker’s yeast (Saccharomyces cerevisiae, 1–2%, w/v) in a closed glass vessel followed by fermentation at 30 °C for 48 h caused a reduction of the acrylamide level by about 70% and of the hydroxymethylfurfural level by up to 99.2%,79 suggesting that yeast fermentation is a promising method for reducing the levels of both compounds in instant coffee. The authors conclude that the fermentation process can be easily adapted into the instant coffee industrial production line. Related studies on the effect of roasting reported that: (a) roasting of almonds induces the formation of acrylamide up to about 1600 μg kg−1 and that controlling the roasting temperature is a critical factor for limiting the acrylamide content;80 (b) roasting of almonds below 146 °C resulted in acrylamide levels below 200 ppb at all roasting times and storage of the almonds at 60 °C may further reduce the acrylamide content of the almonds;81 (c) the considerable amounts of acrylamide formed during roasting of teas at 120 °C, that is reduced at a roasting temperature of 180 °C, presumably as result of heatinduced degradation;82,83 and (d) roasting does not seem to produce acrylamide in hazelnuts, macadamia nuts, pistachios, or walnuts.84 Reduction of acrylamide in olives Processed olives have the potential to contribute a significant fraction to the total acrylamide burden of the diet. High temperature sterilization and lye treatment of olives are presumably used to kill harmful microorganisms but, in turn, these treat-
This journal is © The Royal Society of Chemistry 2015
Review
ments can induce the formation of acrylamide. We will briefly examine in approximately chronological order some of the factors that favour acrylamide formation and possible approaches that have been tried to reduce the acrylamide content of processed olives. A study of the amounts of acrylamide in different Spanish brands of black ripe olives and the effects of industrial processing (sterilization) that involves exposure to heat and alkali on acrylamide formation found that: (a) the acrylamide levels ranged from 176 to 1578 μg kg−1 of pulp or a nine-fold variation from lowest to highest value; (b) the lowest levels were found in ripe olives using the traditional preservation in brine that involves penetration of lye to reach the stone and two water washings; (c) olives processed using one lye treatment and one water washing had the highest amounts; (d) no correlation was apparent between acrylamide content and the small amounts of amino acids or sugars of the olives before sterilization, suggesting that the acrylamide might not be formed by the classical route from asparagine and glucose; and (e) the effect of storage for six months was minor.85 The above suggestion of an alternate mechanism is reinforced by comparison of acrylamide formation in olive water from untreated green olives and thermally and lye-treated green olives, in which was found that acrylamide formation might be associated with a peptide or protein fraction by an unknown mechanism.86 An analysis of the acrylamide content of several Spanish foods including olives found that unprocessed bulk olives contained no acrylamide and that the acrylamide content of bottled and canned black olives ranged from 124 to 316 μg kg−1 and of bottled and canned green olives from 0 to 93 μg kg−1),87 suggesting that the level of maturation might affect acrylamide content. The cited values are much lower than those shown in the FDA data base.88 A detailed study of the factors that influence processinginduced acrylamide formation of California-style ripe olives found that: (a) black olives contained higher amounts (in μg kg−1) of acrylamide (409.7) as compared to green ripe olives (44.0), Greek olives (
