Food Chemistry 202 (2016) 165–175

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Food Chemistry journal homepage: www.elsevier.com/locate/foodchem

Review

Health benefits of the potato affected by domestic cooking: A review Jinhu Tian, Jianchu Chen, Xingqian Ye ⇑, Shiguo Chen ⇑ Zhejiang University, Department of Food Science and Nutrition, Zhejiang Key Laboratory for Agro-Food Processing, Fuli Institute of Food Science, Zhejiang R & D Center for Food Technology and Equipment, Hangzhou 310058, China

a r t i c l e

i n f o

Article history: Received 26 September 2015 Received in revised form 26 January 2016 Accepted 27 January 2016 Available online 28 January 2016 Keywords: Potato Domestic cooking Phytochemical Antioxidant activity Glycemic index

a b s t r a c t Potato (Solanum tuberosum L) is an important food crop worldwide and a good source of vitamins, minerals and dietary fiber as well as phytochemicals, which benefits human body as nutrients supplementary and antioxidants. However, cooked potato is also considered as a high-glycemic-index food because of its high content of rapidly digestible starch, long-term consumption of which will increase the risk of cardiovascular disease and type-2 diabetes. Domestic cooking (boiling, frying, steaming, etc.) are usually adopted before potato consumption. The chemical, physical and enzyme modifications that occur during cooking will alter the potato’s antioxidant capacity and digestibility, which subsequently affected on the bioavailability of phytochemicals and the postprandial glycemic response of the human body. We reviewed the recent publications on the effects of domestic cooking on the nutrition, phytochemicals and the glycemic index changes of the cooked potato. Furthermore, the possible mechanisms underlying these changes were discussed, and additional implications and future research goals were suggested. Ó 2016 Published by Elsevier Ltd.

Contents 1.

2.

3.

4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Beneficial and adverse effects of the potato . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. Modifications during domestic cooking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3. Limitation of the existing domestic cooking studies on potatoes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of domestic cooking on minerals, vitamins, protein and dietary fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Vitamins. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Protein . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Dietary fiber. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of domestic cooking on phytochemicals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. A general conception of phytochemicals and the influence of domestic cooking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Total phenolic content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Anthocyanins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Carotenoids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Glycoalkaloids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of domestic cooking on the antioxidant capacity of potatoes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. The correlation of phytochemicals and antioxidant capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. New antioxidants generated during domestic cooking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of domestic cooking on the glycemic index (GI) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. A general conception of the glycemic index of the potato . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Glycemic index changes by different cooking methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Cold storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. Food additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding authors. E-mail addresses: [email protected] (X. Ye), [email protected] (S. Chen). http://dx.doi.org/10.1016/j.foodchem.2016.01.120 0308-8146/Ó 2016 Published by Elsevier Ltd.

166 166 166 167 167 167 167 168 168 168 168 168 169 170 170 170 170 170 171 171 171 172 172

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J. Tian et al. / Food Chemistry 202 (2016) 165–175

Conclusions and suggestions for future research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Future research suggestions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conflict of interest. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction 1.1. Beneficial and adverse effects of the potato As a globally consumed food crop in qualities that follow only rice, wheat, and maize, potato (Solanum tuberosum) is not only an important supplier of carbohydrates in the human diet (Fig. 1a) (King & Slavin, 2013) but also a key supplier of nutrients in the diet, including minerals, protein, vitamins and others (Burlingame, Mouillé, & Charrondière, 2009; Love & Pavek, 2008). Additionally, phytochemicals are bioactive components in potatoes that should not be overlooked because high contents of phenolic acids, anthocyanins, and carotenoids has been reported in its peel and flesh (Ezekiel, Singh, Sharma, & Kaur, 2013). Extensive research has indicated that these phytochemicals are closely associated with antioxidants and play an essential role in the prevention of many chronic diseases, such as atherosclerosis and cancers (McGill, Kurilich, & Davignon, 2013; Williams et al., 2013). A recent study showed that due to the high daily consumption, potatoes contribute the third highest total phenolic content to the diet among fruits and vegetables, listed after only the orange and the apple (Fig. 1b) (Camire, Kubow, & Donnelly, 2009; Song et al., 2010). Apart from the benefits, the adverse effects of the potato should be noted. Both in vitro and in vivo investigations of different potato cultivars have indicated that the cooked potato has the mostly rapidly digested starch, which will be absorbed quickly in the human body, resulting in high postprandial blood glucose levels (Soh & Brand-Miller, 1999). Over a long term period, this has been associated with increased risks of obesity and diet-related diseases, including type-2 diabetes and cardiovascular disease (Zaheer & Akhtar, 2014). In addition to the high glycemic index (GI), allergy-causing proteins and glycoalkaloids present in the potato

173 173 173 173 173 173

are also deemed unfavorable to human health and have been reported to induce anaphylaxis, intestinal inflammation or nausea (Knuthsen, Jensen, Schmidt, & Larsen, 2009; Lynch et al., 2012; Seo, L’Hocine, & Karboune, 2014).

1.2. Modifications during domestic cooking Prior to consumption, most dietary components are cooked using different methods according to the recipes and the culinary traditions of various countries. The application of heating during household cooking encompasses a variety of processes, i.e., boiling, frying, steaming, baking and roasting, in traditional and microwave ovens (Palermo, Pellegrini, & Fogliano, 2014). The biological, physical and chemical modifications that occur during cooking lead to sensory, nutritional and textural changes of food that may be beneficial or detrimental to human health. For example, cooking inactivates the microorganisms and anti-nutritional factors and enhances the digestibility of food and the bioavailability of nutrients. Additionally, cooking is involved in the formation of structural and desirable compounds, such as those that confer crispiness, flavor, antioxidants and coloring agents to the food. Domestic cooking also induces adverse effects, such as the loss of certain nutrients and the formation of undesired compounds (e.g., acrylamide or other toxic molecules) (VanBoekel et al., 2010). As the only major tuber food crop worldwide, most potatoes are consumed after cooking. The cooking methods are important factors affecting not only the chemical composition and physical structure of the potato but also the intake of bioactive compounds under normal dietary conditions. Thus, it may be interesting to analyze the changes that potatoes undergo during domestic cooking.

Fig. 1. (a) General composition of raw potatoes. Adopted from Burlingame et al. (2009); (b) Contribution of total phenolics from vegetables consumed by Americans. Adopted from Song et al. (2010).

J. Tian et al. / Food Chemistry 202 (2016) 165–175

167

Murniece et al. (2011)

Lachman et al. (2013)

Navarre et al. (2010)

Finglas and Faulks (1984)

— — — 43 73 115 — — — — — — — — — — — — 71 44 79 20 40 81 — — — — — — — — — — — — 30 22 26 11.7 17.9 33.3 — — — — — — — — — — — — 550 379 451 250 358 715 — — — — — — — — — — — — 0.067 0.106 0.128 0.2 0.2 0.2 — — — — — — — — — — — — Zile/Brasla/Madara

Blaue St Galler/Valfi/Violette

Bintje/Piccolo/Purple Majesty

Desiree

Baked Boiled (flesh only) French fries, frozen and oven heated Boiled (20 min) Baked (200 °C; 90 min) Chipped (190 °C; 6 min) Raw Microwaving (1100 W; 2.5 min) Steamed (15 min) Boiling (18 min) Baking (375 °C; 30 min) Raw Microwaving (750 W, 10 min) Baking (180 °C, 45 min) Raw (FW) Roasted (210 °C, 25 min) Shallow-fried (150 °C, 7 min) Deep-fat-fried (180 °C, 4 min) Solanum tuberosum L (Russet)

8.3 13 13.3 9.00 10.00 13 55.17/58.46/44.61 65.57/67.40/52.56 72.39/70.45/57.54 71.40/73.84/52.12 77.3571.62/68.58 32.38/323.5/296.7 25.21/17.93/18.75 11.81/10.43/7.17 11.00/10.30/10.85 3.50/4.30/7.40 4.60/5.80/6.40 9.50/10.20/10.50

Cooking methods

Thiarmin

1.348 1.439 2.218 0.4 0.4 0.6 — — — — — — — — — — — —

The potato contains a variety of minerals that can be influenced by various domestic cooking conditions. The minerals were well retained despite the destruction of the potato microstructure during no-water-added domestic cooking (e.g., microwaving, baking, and roasting); however, when water was involved, a significant decrease could be observed due to leaching. Finglas and Faulks (1984) determined the mineral contents in potatoes after boiling, baking and frying, the result showed it was minimally affected by deep-fat frying and mostly affected by boiling. Bethke and Jansky (2008) reported boiling potatoes can even decreased potassium levels by 50%. A more recent study from the USDA National Nutrient Database indicated that boiling resulted in a more severe loss of minerals. The contents of potassium, phosphorus, and magnesium in baked russet potatoes with skin were 550 mg, 71 mg and 30 mg per 100 g, respectively, while boiling decreased those minerals to 379 mg, 44 mg and 22 mg, respectively (McGill et al., 2013). Interestingly, the contents of Fe and Zn in potatoes did not decrease after boiling, which may be due to a stronger binding of these metals to the macromolecules in potatoes (Burgos, Amoros, Morote, Stangoulis, & Bonierbale, 2007).

Niacin

P

2. Effect of domestic cooking on minerals, vitamins, protein and dietary fiber 2.1. Minerals

Variety

Table 1 Effect of domestic cooking on vitamin, minerals and protein in potatoes.

Many studies have been focused on the nutrition, phytochemicals and the glycemic index values of the potato using various domestic cooking methods (Burgos et al., 2012; Lemos, Aliyu, & Hungerford, 2015; Murniece et al., 2011; Nayak, Berrios, & Tang, 2014); however, a systemic evolution of the effects of domestic cooking on the potato nutrients has been given limited attention (Singh, Kaur, & Moughan, 2012). Moreover, some conclusions have been contradictory, and few articles have focused on the underlying reasons. Because the potato has become a staple in the diets of an increasing number of humans, the slight difference in nutritional composition will have significant effects on human health following long-term consumption. Thus, a clear classification of the beneficial and adverse changes of the potato during domestic cooking and a better method of cooking to retain the nutrients of the potato are urgently required. The current study aimed to review the effects of various domestic cooking methods on the phytochemical, antioxidant activity and postprandial glycemia changes in potatoes, and the possible mechanisms underlying these changes were discussed. In addition, the changes in hazardous substances, such as glycoalkaloids and allergenic proteins, were also reviewed.

Mg

Minerals (mg/100 g)c

K

Vitamin (mg/100 g)

Vitamin C

Cl

2.63 1.87 2.66 1.3 2.2 4.8 — — — — — — — — 1.54/1.66/1.30 2.07/2.33/2.46 2.86/2.63/2.61 3.48/3.41/4.27

Protein (g/100 g)

2.2 1.8 2.6 — — — — — — — — — – — 0.56/0.75/0.56 0.91/0.26/0.85 0.34/0.57/0.30 0.96/2.27/0.52

Dietary fiber (g/100 g)

References

USDA 2012

1.3. Limitation of the existing domestic cooking studies on potatoes

2.2. Vitamins Potatoes contain an excellent variety of vitamins, such as vitamin C, niacin, and thiamine (Kondo et al., 2012). Due to their heat sensitivity, cooking causes a significant loss of vitamin regardless of the cooking method used. Cooking in water or oil will induce a more severe loss of vitamins due to the leaching of hydrophilic or lipophilic vitamins. Han, Kozukue, Young, Lee, and Friedman (2004) also boiling caused a loss of 77–88%, pressure-cooking caused a loss of 56–60%, braising caused a loss of 50–63%, baking caused a loss of 33–51%, and microwaving caused a loss of 21– 33% (Table 1). The loss of vitamin C during domestic cooking can be attributed to the fact that vitamin C is easily dissolved in water and is not stable at high temperatures. In addition, some cooking methods (e.g., stir-frying) expose the food to atmospheric oxidation and induce a decrease in this vitamin level (Tian et al., 2016).

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J. Tian et al. / Food Chemistry 202 (2016) 165–175

Although a majority publication have reported the vitamin C was loss during different cooking methods, the degree of losses were quite different even using the same cooking methods, some of which were between 10% and 20% and others were between 70% and 80% (Love & Pavek, 2008). This was because the loss vitamins during domestic cooking were also depends on the heating level and time. Burg and Fraile (1995) reported a considerable reduction in the quantities during domestic cooking, and a more severe destruction was observed when extending the residence time beyond the minimum cooking time. In a word, the literatures about the effects of cooking on vitamin C in the potato shows that losses can be severe but that optimized cooking methods can minimize or eliminate these losses (Lešková et al., 2006). 2.3. Protein Despite the low concentration of protein in potatoes, its biological value is quite high (approximately 90–100 compared with whole egg (100), soybean (84), and beans (73)) (Waglay, Karboune, & Alli, 2014). In addition, potato protein is characterized by relatively well-balanced amino acid concentrations and a higher content of essential amino acids compared with other plant-origin proteins. Potato is regarded as the best plant source of lysine, which is often lacking in vegetable and cereal sources (Amanda Waglay, 2014). Generally, the protein content in cooked potatoes is retained or even increased due to the increase in dry matter content after water evaporation (McGill et al., 2013). Murniece et al. (2011) determined the protein content and amino acid profile of Latvian potatoes prepared using various cooking methods and found that the protein content increased from 1.55 ± 0.04 g/100 g in raw potatoes to 2.46 ± 0.10, 2.61 ± 0.13 and 4.27 ± 0.08 in roasted, shallow-fried and deep-fat fried potatoes, respectively (Table 1). Significant increases in Asp, Val, Met, Ile, Tyr, Phe, His, Lys and Arg were also observed. In another study, Finglas reported that the protein content (1.6 g/100 g in raw) increased to 2.2 g/100 g and 2.8 g/100 g after baking and roasting, respectively (Finglas & Faulks, 1984). Another important effect of cooking on potato protein is denaturation, which facilitates digestion of potato protein and avoids some negative aspects, such as allergic reactions and enzyme inhibition. Cooking treatments have been reported to decrease the binding of patatin and IgE, a protein that has been reported to induce allergic symptoms in sensitive individuals (Lynch et al., 2012). Additionally, it can inactivate protease inhibitors, which have been attributed to Kunitz-type proteases and other enzymes identified in potatoes (i.e., inhibitors I and II, with variable chymotrypsin and trypsin inhibitory activity, and carboxyl-peptidase inhibitors). However, the effect depends on the cooking conditions, with some cooking methods (e.g., frying, baking) showing greater ability to induce the denaturation of proteins than others (e.g., boiling, steaming) (Seo et al., 2014). 2.4. Dietary fiber Dietary fiber is the part of the plant material that is resistant to enzymatic digestion, it includes cellulose, hemicellulose, pectin, gums, mucilages and the non-carbohydrate component lignin (Slavin, 2013). Potatoes contain approximately 2.00 g/100 g of fiber, which is less than whole-grain cornmeal (7.3 g/100 g) but is more than white rice (0.3 g/100 g) and whole-wheat cereal (1.6 g/100 g) (Dhingra, Michael, Rajput, & Patil, 2012). Cooking will increase the content of dietary fiber. According to the USDA National Nutrient Database for Standard Reference (2008), the dietary fiber content of a boiled potato is 1.87 g/100 g (with skin; fresh weight), while that of a baked potato is 2.5 g/100 g, and that of French fries, frozen and then oven-heated potatoes is

2.66 g/100 g (Table 1). Thed and Phillips (1995) also reported that boiling and microwave-heating treatments increased the dietary fiber content of instant mashed potatoes, while baking caused only minor changes. The increase in dietary fiber after domestic cooking may because of the formation of complexes between polysaccharides and other compounds in the food (such as protein) and the formation of resistant starch (RS) in cooked potatoes (Dhingra et al., 2012). Murniece et al. (2011) concluded that after shallow frying (150 ± 5 °C), deep-fat frying (180 ± 5 °C) and roasting (210 ± 5 °C), the dietary fiber contents in all five varieties of potatoes were increased significantly, which may have been due to the formation of RS (amylose–lipid) during frying (Yadav, 2011). 3. Effect of domestic cooking on phytochemicals 3.1. A general conception of phytochemicals and the influence of domestic cooking Phytochemicals are bioactive, non-nutrient plant compounds in fruits, vegetables, grains and other plant foods, it is broadly classified as carotenoids, phenolics, alkaloids, nitrogen-containing, and organosulfur compounds (Nayak, 2011). Numerous studies have revealed a negative correlation between the intake of phytochemicals and chronic diseases (e.g., chronic inflammation, cardiovascular diseases, cancer and diabetes) (Acosta-Estrada, GutiérrezUribe, & Serna-Saldívar, 2014; González-Castejón & RodriguezCasado, 2011). In addition to supplying carbohydrates, minerals and vitamins, potatoes also contain a number of healthpromoting phytonutrients. Due to its high daily consumption, the potato contributes approximately 25% of the total phenolic content from vegetables to the diet (Fig. 1b) (Chun et al., 2005; Song et al., 2010). Red and purple-fleshed potato varieties contain high levels of anthocyanins, and these colored potatoes may serve as a practical source for natural anthocyanin pigments because they are inexpensive crops (Eichhorn & Winterhalter, 2005; Jansen & Flamme, 2006). Numerous studies have focused the changes in phytochemical during domestic cooking (Ezekiel et al., 2013; Palermo et al., 2014); however, the conclusions are quite different among different studies. As the slight changes in the phytochemical will significantly affected on our health, it is very important to retain the maximum phytochemical content during domestic cooking. Thus, these contradictory results should be summarized and analyzed to determine the source of their discrepancies. 3.2. Total phenolic content Numerous studies have revealed that the total phenolic content in cooked potatoes can be retained or even increased based on the cooking method used. Faller and Fialho (2009) compared the total phenolic content after boiling, steaming and microwaving and found that the phenolic content increased by 81.4%, 22.8% and 80.81%, respectively (Table 2). In another study, increases of 36.36%, 46.12%, and 47.48% were also observed after baking, frying and microwaving, respectively (Blessington et al., 2010). Similar increases were also reported in commercial processing such as cutting and thermal treatments (Reyes & Cisneros-Zevallos, 2003). Since there should not be any appreciable biosynthesis of phenolic compounds during cooking, an increase in phenolic content is indicative of an increase in recoverable compounds (Burgos et al., 2013). Earlier studies have demonstrated that thermal action during cooking induced a breakdown of potato structure and improved the extract ability of the phenolic compounds from the cellular matrix and release of dietary fiber-bound polyphenols, forming the corresponding free phenolic compounds (Ruiz-Rodriguez, Marín, Ocaña, & Soler-Rivas, 2008). In addition,

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J. Tian et al. / Food Chemistry 202 (2016) 165–175 Table 2 Effect of domestic cooking on phytochemicals in potatoes.* Cooking methods

Total phenolic

Anthocyanin

Carotenoid

Glycoalkaloids

Antioxidant capacity

Reference

Boiling (100 °C, 15 min) Baking (178 °C; 40 min) Microwaving (1100 W; 5 min) Crips (Moisture content < 2%, reheated 170 °C) Boiling (100 °C, 60 min) Microwaving (20 min) Baking (204 °C, 60 min) Boiling (100 °C, 25 min) Steaming (100 °C, 35 min) Baking (200 °C, 60 min) Microwaving (900 W, 5 min) Steaming (peeled, 20–30 min) Pre-drying (160 °C/120 min) Boiling (100 °C, 20–25 min) Microwaving (1100 W, 2.5 min) Baking (375 °C, 30 min)

4.68–43.09%; 1.09–30.26%; 5.43–33.22%; ND-59.05%;

— — — 34.64–83.31%;

— — —

— — — —

2.53–20.14%; 4.43–25.09%; 1.64–15.90%; 33.75%"–37.50%;

Xu et al. (2009)

— — — 100.00%" 96.80%" 3.35%" 109.59%" — — ND-23.15%; — —

— — — — — — — — — — — —

— — — — — — — 59.45%; 68.00%; — — —

2.67–66.71%; 11.37–75.81%; 7.94–82.16%; 17.19% 0 23.88% 8.96% — — 16.96–52.23%" 4.98–19.51%" 35.59–70.03%"

Perla et al. (2012)

Steaming (15 min) Boiling (100 °C, 18 min) Microwaving (750 W, 10 min)

19.25–62.02%; 37.93–70.53%; 37.62–73.51%; 33.97%; 33.80%; 81.82%; 64.59% ; — — 18.47–69.66%" 2.35–52.00%" 49.04– 119.76%" 15.63–93.41%" 27.16–71.26%" —

— — —

—

Baking (204 °C, 15 min)

37.27%"

—

ND

— — 39.59– 95.03%; 31.63– 72.56%; —

ND-14.98%" 8.19–26.48%" —

Baking (180 °C, 45 min)

— — 129.79– 847.83%" 85.62–557.14%"

Boiling (100 °C, 25 min) Frying (191 °C, 1 min) Microwaving (800 W, 2.5 min) Boiling (100 °C, 23–34 min)

3.32%; 46.86" 49.08" —

— — — —

— — — —

11.97%" 64.70%" 69.48%" —

Boiling (100 °C, 20 min) Microwaving (700 W, 8 min) Boiled (100 °C, 30 min) Boiling (100 °C/25 min)

ND ND 6.37–52.60%; —

16.25–20.04%; 18.01–29.73%;

16.61%; ND 8.17%; 17.46%"– 18.69%; — — ND-68.68%; 7.32–50.44%;

ND ND — —

— — — —

—

—

Kita et al. (2013)

Lemos et al. (2015)

Rytel (2012) Burgos et al. (2013) Navarre et al. (2010)

Lachman et al. (2013)

— 45.62%"

Blessington et al. (2010)

Burgos et al. (2012) Mulinacci et al. (2008) Tierno et al. (2015) Burmeister et al. (2011)

": Increased; ;: decreased; ND: no difference; —: no data. Curtivars. Xu et al. (2009): Dakota Pearl, Goldrosh, Nordonna, Norkotah, Red Norland, Sangre, Viking, Dark Red Norland. Burgos et al. (2013): Bolona, Challina, Leona, Guincho. Kita et al. (2013): Salad Blue, Vitelotte, Valfi, Blue Congo, Highland Burgundy Red, Rosemarie, Herbie 26. Perla et al. (2012): Mesa Russet, CO99256-2R, Silverton Russet, CO98012-5R, CO95172-3RU, Colorado Rose, Russet Nugget, VC0967-2R/Y, CO99045-1W/Y, CO01399-10P/Y, AC99329-7PW/Y, Purple Majesty, CO97222-1R/R, CO97226-2R/R. Navarre et al. (2010): Bintje, Piccolo, Purple Majesty. Lachman et al. (2013): Agria, HB Red, Rote Emma, Blaue St Galler, Valfi, Violette. Burgos et al. (2012): 705821 (Solanum phureja), 705172 (S. phureja), 704393 (Solanum goniocalix), 701862 (S. goniocalix) and three 702472 (S. goniocalix), 705799 (S. phureja), 704218 (S. phureja). Mulinacci et al. (2008): Vitelotte Noire, Highland Burgundy Red. Blessington et al. (2010): Atlantic, ATX85404-8W, Innovator, Krantz, NDTX4930-5W, Russet, Burbank, Santana, Shepody. Lemos et al. (2015): Purple Majesty. Tierno et al. (2015): Vitelotte, Kasta, Monalisa, Jesus, Morada, Zamora, NK-08/284, NK-08/286, NK-08/336, NK-08/348, NK-08/348, NK-08/355, NK-08/356, NK-08/362, NK-08/ 370, NK-06/130. Burmeister et al. (2011): Mayan Twilight, Mayan Gold, Red Laura, Shetland Black. * Detail information of potato varieties.

the heat treatments could inactivate polyphenol oxidases, preventing the oxidation and polymerization loss of polyphenols (Navarre, Shakya, Holden, & Kumar, 2010). In contrast, several studies reported that cooking only caused limited changes or decreases in total phenolic content (Ezekiel et al., 2013). Lemos et al. (2015) reported that cooking had a detrimental effect on the levels of total phenolics, as the total phenolics in uncooked potatoes were 209 ± 35.7 mg GAE/100 g (fresh weight), while baking decreased it to38.1 ± 7.5 mg GAE/100 g, boiling decreased it to 137.6 ± 0 mg GAE/100 g, microwaving decreased it to 74.0 ± 1.3 mg GAE/100 g, and steaming decreased it to 130.4 ± 3.7 mg GAE/100 g (Table 2). The decrease may be attributed to water-soluble phenols leaching into the cooking water and breakdown of phenolics during heat processing (Kita, Ba˛kowska-Barczak, Hamouz, Kułakowska, & Lisin´ska, 2013; Tudela, Cantos, Espín, Tomás-Barberán, & Gil, 2002). Moreover,

during processing, the phenolic compounds participate in the Maillard reaction, which results in an increase in Maillard reaction products and decrease in the phenolic level (Perla, Holm, & Jayanty, 2012). In commercial processing, the commonly used pretreatments, such as blanching and dices processing, could also induced a significant loss of total phenolic (Jaiswal, Gupta, & Abu-Ghannam, 2012; Rytel et al., 2014). In conclusion, the final phenolic content was the sum of the residual and the recovery. The cooking methods with lower temperatures, shorter times and reasonable potato sizes, such as steaming and microwaving will be better choices for the retention of total phenolic content. 3.3. Anthocyanins Anthocyanins are highly unstable compounds, and their stability can be affected by temperature, pH, oxygen and light. Earlier

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studies have demonstrated that the magnitude and duration of heating during cooking have a strong influence on anthocyanin stability (Patras, Brunton, O’Donnell, & Tiwari, 2010). Brown, Durst, Wrolstad, and De Jong (2008) reported that after boiling, microwaving and baking, the total anthocyanin contents of five cultivars of potatoes significantly decreased. Compared to boiling (100 °C) or steaming (100 °C), frying (170 °C) induce a greater decrease due to thermal degradation. A commercial frying process (170 °C) for different cooking times also induced approximately 38–70% loss anthocyanin in purple potatoes, a prolonging of cooking time can even cause almost complete degradation (Kita et al., 2013). While in their later research, the process of potato chips production caused almost total degradation of anthocyanin compounds (Kita, Ba˛kowska-Barczak, Lisin´ska, Hamouz, & Kułakowska, 2015). Because anthocyanins are highly watersoluble pigments, pretreatment by soaking or blanching, which were both commonly used in domestic and commercial processing, will also cause the leaching of anthocyanins into water. Interestingly, similar to total phenolic content, a small change or increase in the anthocyanin content has been reported in cooked potatoes (Brown et al., 2008; Lachman et al., 2012; Lemos, Aliyu, & Hungerford, 2012; Tierno, Hornero-Méndez, Gallardo-Guerrero, López-Pardo, & de Galarreta, 2015). Lemos et al. (2015) determined the anthocyanin content after boiling, steaming and microwaving and reported increases of 100.46%, 97.21%, and 109.79%, respectively. This increase may be attributed to two factors. First, similar to other polyphenols, anthocyanins are enzymatically degraded in the presence of polyphenol oxidase; however, this enzyme can be inactivated by the thermal treatments during cooking, which aids the anthocyanin retention in potatoes. Second, some cooking methods, such as microwaving, will destroy the microstructure of potatoes and induce a better extraction of anthocyanins than can be detected (Lemos et al., 2012). 3.4. Carotenoids Carotenoids are highly unsaturated structures, which makes them sensitive to light, oxygen and heat. Therefore, depending on the cooking conditions, such as time and temperature, carotenoids can be more or less affected (Blessington et al., 2010). Generally, cooking will cause a decrease in the carotenoid content in potatoes; however, in some cases, with heating during cooking, carotenoids bonded with proteins in potatoes (e.g., protein-xanthophyll aggregates) are dissociated, allowing a detectible increase in carotenoids in cooked potatoes (Burmeister et al., 2011). Typically, boiling in water or frying in oil will diminish the carotenoids due to the thermal effect or the lipophilic properties of carotenoids (Tierno et al., 2015). 3.5. Glycoalkaloids Potatoes are not commonly associated with toxicity; however, as part of the maturing process, potato tubers accumulate small quantities of alkaloids, which may cause some discomfort because of cholinesterase inhibition, teratogenicity, intestinal inflammation and nausea (King & Slavin, 2013). Glycoalkaloids is the main alkaloids in potato, a concentration of glycoalkaloids above 200 mg/kg fresh weight has been considered to pose a risk to human health. The majority of glycoalkaloids are removed during peeling (70%) and blanching (29%). The domestic cooking can reduce the remaining glycoalkaloids (Mulinacci et al., 2008). Steaming, pre-drying and dehydrating result in decreases of 59.39%, 69.38% and 75.34% of glycoalkaloids, respectively (Table 2) (Rytel, Tajner-Czopek, Aniołowska, & Hamouz, 2013). Tajner-Czopek, Rytel, Kita, Pe˛ksa, and Hamouz (2012) reported that the peeling process of coloredfleshed potatoes decreased glycoalkaloids content, on average, by

approximately 50%, cutting decreased it by approximately 53%, and blanching, on average, decreased it by approximately 58% compared with the raw material. In commercial potato processing, higher decreases in glycoalkaloids contents were found in fried potatoes, and different frying methods also led to different contents. The glycoalkaloids decreased approximately 83% in potato crisps, while in French fries decreased 92% and 94% in frying stage I and II, respectively; It was because in the commercial processing, the common washing and blanching process can leach the glycoalkaloids out of potato chip, and a further frying in hot oil will also decomposition the compounds (Tajner-Czopek et al., 2012). Similar decreases of glycoalkaloids were also observed in coloured-fleshed potatoes during commercial French fries processing (TajnerCzopek, Rytel, Aniołowska, & Hamouz, 2014). 4. Effect of domestic cooking on the antioxidant capacity of potatoes 4.1. The correlation of phytochemicals and antioxidant capacity Antioxidants are substances that significantly delay or inhibit the oxidation (process of losing electrons) of oxidizable substrates (Wolfe et al., 2008). Extensive research has indicated that foods rich in antioxidants play an essential role in lowering the risk of many chronic diseases (Poljsak & Milisav, 2014; Vinson, Demkosky, Navarre, & Smyda, 2012). Vitamin C has been considered an indicator of antioxidant activities in fruits and vegetables (Burg & Fraile, 1995); however, recent reports indicated that vitamin C contributed less than 0.4% of the total antioxidant activity in apples while phytochemicals played a more important role (Eberhardt, Lee, & Liu, 2000). The phenolic, carotenoid, and anthocyanin contents are the most important phytochemical contributions to the total antioxidant capacity in fruits and vegetables. Although there is a relatively low content of these phytochemicals in potatoes relative to other fruits and vegetables, the high daily consumption globally makes it one of the most important antioxidants in human daily life (Liu, 2013; Song et al., 2010). The contents of phytochemicals in potatoes are altered during different cooking methods, resulting in different antioxidant properties. Thus, it is important to identify the best cooking methods that can retain highest level of antioxidant potential (Nayak, 2011). Similar to photochemical changes among different cooking methods, the radical scavenging activity of cooked potatoes was in consistent among different studies (Ezekiel et al., 2013; Lemos et al., 2015). Perla et al. (2012) reported a decrease of 42.22%, 50.47% and 63.77% in the radical scavenging activity due to boiling, microwaving and baking treatments, respectively, while Burgos et al. (2013) found that the antioxidant activity of boiled purplefleshed potatoes increased by 10.67% compared with the raw tuber (Table 2). Nevertheless, in most of the studies, the trend of antioxidant activity changes during domestic cooking was positively correlated with the phytochemical changes (e.g., total phenolic content, anthocyanins, and carotenoids). 4.2. New antioxidants generated during domestic cooking Although phytochemicals play a very important role in the antioxidant properties of the cooked potato, they are not the only contributor to antioxidant activity (Nicoli, Anese, & Parpinel, 1999). Kita et al. (2013) determined the antioxidant activities of purple potatoes during frying and found that despite the decrease in anthocyanin and polyphenol levels after frying, potato crisps still displayed increased antioxidant activity. Similarly, Xu, Li, Lu, Beta, and Hydamaka (2009) found that the phytochemicals and antioxidant activities of potatoes did not decline in the same order

J. Tian et al. / Food Chemistry 202 (2016) 165–175

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The long-term intake of high-GI foods is associated with an increase in potential health risks (e.g., type-2 diabetes and cardiovascular disease). Beyond the nutritional properties of a quality staple food, potatoes have been classified as high-GI value food that will causes high postprandial glycaemia (Ek, Brand-Miller, & Copeland, 2012). However, numerous studies have reported that the GI of the potato changes with different cooking methods. Frying, baking, cooling, and cooking with additives are good methods of lowering the GI. Thus, the simple ranking of the potato as a highGI food could be misleading because the GI is greatly affected by the cooking methods (King & Slavin, 2013). Here, we focus on the effect of domestic cooking methods on the GI, and the effects of adding phytochemicals and food additives during cooking on the GI are included. Fig. 2. Changes in total antioxidant activity in vegetable matrix subjected to different processing conditions. Adapted and modified from Nicoli et al. (1999).

during different domestic cooking. This was because although domestic cooking from 100 °C to 180 °C can induce significant decreases in naturally occurring antioxidants, it can also induce many chemical reactions due to the high cooking temperature, such as caramelization, the Maillard reaction, Strecker degradation (a reaction of dicarboxylic compounds with amino acids), and the hydrolysis of esters and glycosides of antioxidants and oxidation of phenolic antioxidants to quinones and their polymers (Nicoli et al., 1999). As shown in Fig. 2, the total antioxidant capacity is the sum of residual intrinsic antioxidants and new antioxidants that formed during cooking. Apart from the intrinsic phytochemicals, the new compounds generated during cooking will also contribute to the antioxidant capacity of potatoes.

5. Effect of domestic cooking on the glycemic index (GI) 5.1. A general conception of the glycemic index of the potato The GI was first defined by Jenkins as the incremental blood glucose area following the test food, expressed as a percentage of the response to an equivalent carbohydrate portion of a reference food taken by the same subject (Jenkins et al., 1981). Foods with a GI value above 70 are classified as high-GI foods, those with values between 56 and 69 are classified as medium-GI foods, and those with values lower than of 55 are classified as low-GI foods.

5.2. Glycemic index changes by different cooking methods Starch accounts for approximately 90% of the potato mass (based on dry weight). According to its digestibility in the small intestine, starch is divided into DS (digestible starch) and RS (resistant starch) (Nayak et al., 2014). The existing research has demonstrated that the consumption of RS has a negative accordance with high postprandial blood glucose (Sajilata, Singhal, & Kulkarni, 2006). The changes in the potato starch and its effects on enzymes during domestic cooking are described in Fig. 3. In general, the heating during domestic cooking has two functions. On one hand, the modification of starch, such as gelatinization and the formation of RS, changes the properties of the starch, which makes it easier or harder to hydrolyze. On the other hand, the cooking process changes the microstructure of the potato, which makes the contraction of the enzymes much easier or harder with the starch. For example, mashed and boiled potatoes are considered to have higher GIs than fried, microwaved or baked potatoes, which were because of the effects of the gelatinization degree and the destruction of the microstructure by these methods (Tahvonen, Hietanen, Sihvonen, & Salminen, 2006). For example, during frying, the existing water within the cells of the inner part of the potato leads to complete starch gelatinization, while on the surface, the high temperature also leads to the formation of lipid– amylose (RS). Additionally, as the water inside the cells is quickly evaporated, and cell dehydration occurs, the surface of a fried potato becomes compact, which might be a barrier for the reaction between enzymes and starch. However, during boiling, the cell structure of the potato is completely collapsed, and the intracellular starch is

Fig. 3. Mechanism of cooking methods that effect glycemic index of potato.

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Table 3 Effect of domestic cooking on GI and resistant starch in potatoes. Variety

Cooking methods

GI

Resistant starch (%DW)

References

Solanum tuberosum L

Raw Boiled (25 min) Cooled Mashed Oven-baked (200 °C; 50 min) Fries (peeled; 180 °C; 8 min) Crips (merchandise) Boiled (25 min) Microwaved (1450 W; 2.5 min) Baked (232 °C; 16 min) Deep fat fried (180 °C; 2 min) Boiled/cooled (whole, unpeeled 30 min) Boiled/cooled (whole, unpeeled 30 min) Boiled/cooled (whole, unpeeled 30 min) Boiled Boiled and mashed Microwaved Baked Canned and microwaved Steam boiled/cooled Oven baked/cooled and reheated Mashed/cooled Baked Frence fries Boiled hot/cooed Boiled/cooled Boiled + cooled + vinegar Boiled French fries

— 99.6 — 107.5 67.8 56.6 74.3 — — — — — — — 88 ± 9 91 ± 9 79 ± 9 93 ± 11 65 ± 9 104 ± 39/76 ± 32 95 ± 30/73 ± 25 106 ± 42/81 ± 28 76.5 ± 8.7 63.6 ± 5.5 89.4 ± 7.2/56.2 ± 5.3 168 ± 31/125 ± 17 96 ± 15 111 ± 13.7 77 ± 10

69.05 1.18 4.63 2.08 3.70 6.64 3.27 2.90 7.30 6.20 9.10 6.5/10.90 3.4/7.90 3.3/9.30 — — — — — — — — — — — — — — —

Garcia-Alonso and Goni (2000)**

Solanum tuberosum L

Nadine Moonlight Draga Pontiac

Variety Nicola

Solanum tuberosum L

Sava

Thed and Phillips (1995)

Monro et al. (2009)

Soh and Brand-Miller (1999)**

Tahvonen et al. (2006)**

Fernandes et al. (2005)

Leeman et al. (2005)* Leeman, Östman, and Björck (2008)*

GI: Glycemic index, glucose = 100. * White bread = 100; DW: dry matter. ** Cooking methods were described detailly.

fully swelled and gelatinized, which makes the starch easier to digest. Thed and Phillips (1995) reported that after boiling, microwaving, baking and deep-fat frying, the RS content in potatoes was 2.9%, 7.3%, 6.2% and 9.1%, respectively. In another study, GarciaAlonso and Goni (2000) determined the RS and GI values with different cooking methods and found that the RS contents ranged from 1.18% in boiled potatoes to 6.64% in French fries, and the GI value was 99.6 for boiled potatoes and 107.5 for mashed potatoes; however, it was only 67.8 for baked, 74.3 for chips and 56.6 for French fries (Table 3). Similar results were reported by Soh and BrandMiller (1999). 5.3. Cold storage In addition to being served hot, sometimes, the cooked potato may be stored at a lower temperature prior to consumption (e.g., prepared food), which has also been shown to increase the RS content and decrease the GI. Monro, Mishra, Blandford, Anderson, and Genet (2009) determined the RS content in three varieties of potatoes, which were 6.5%, 3.4% and 3.3% after boiling, while after cooling, the RS content increased to 10.9%, 7.9% and 9.3%, respectively (Table 3). Another study showed that after cold storage of the boiled and fried potatoes at 4 °C for 24 h, the RS increased from 1.18% and 1.17% to 4.63% and 5.16%, respectively (Garcia-Alonso & Goni, 2000). The formation of RS under cold storage may be attributed to the retrogradation of gelatinized starch (Fig. 3). During cooling, the gelatinized starch, which was considered to be disorganized, became ordered and made the starch once again more resistant to digestive enzymes, consequently lowering the postprandial glycemia (McGill et al., 2013). As a result of increasing the RS content, cooling significantly decreased the GI of the potato regardless of the cooking methods used (Fernandes, Velangi, & Wolever, 2005; Tahvonen et al., 2006). Thus, a cooling step for cooked potatoes may be a good choice to lower the GI.

5.4. Food additives Several studies suggested that different GI values are obtained from potatoes that are consumed alone or with food additives (Gullfford, Bicknell, & Scarpello, 1989; Hätönen et al., 2011; Mitrou et al., 2015). It is clear that the addition of protein and fat to a carbohydrate-containing meal could appreciably moderate the influence of the carbohydrate on the glucose response due to the insulinogenic effect or the reduction of insulin responses (Collier & O’Dea, 1983; Ma, Stevens, Cukier, et al., 2009; Welch, Bruce, Hill, et al., 1987). Henry, Lightowler, Strik, and Storey (2005) evaluated the adding of commercial toppings (oil, chicken breast, salad, etc.) on the GI of baked potatoes and found that the addition of protein can lower the GI of potatoes by 18%, while the co-ingestion of fat lowered the GI of potatoes by 58%, changing the GI classification from high GI to low GI (Table 3). Adding vinegar is another effective method to lower the postprandial glucose level, it is a very popular cooking method in China and other Asian countries. With less fat consumption, it reduces the GI value by the reduction of the gastric emptying rate (Liljeberg & BjoÈrck, 1998). Leeman, Ostman, and BjRck (2005) reported that the addition of vinegar and olive oil in the form of a vinaigrette dressing to cooked and cooled Sava potatoes in a salad reduced the GI by 43%, and the author suggested that vinegar and oil mixed with cooked potatoes might lower the GI of the potato. However, the adding of vinegar will reduce the content of oil and will be a healthy way of eating potato. Phytochemicals from plants can also lower the GI of the potato. In an in vitro study, using potato starch as a substrate, chlorogenic acid (5-caffeoylquinic acid, 5CQA) showed excellent inhibition of the porcine pancreatic alpha-amylase activity at levels that are expected in potato tissue, which may support the hypothesis that potato varieties with high 5-CQA levels would have lower glycemic effects than those with lower levels (Karim, Holmes, &

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Orfila, 2015). In an in vivo study to evaluate the GI of pigmented potatoes with different polyphenol contents, four varieties of pigmented potatoes (purple, red, yellow and white) were consumed with skin after cooking in a convection oven, and the results showed that the mean GI values for the potato types varied (purple = 77; red = 78; yellow = 81; and white = 93). The author concluded that the GI of colored potatoes was significantly related to their polyphenol content, and the lower GI was possibly mediated through an inhibitory effect of anthocyanins on intestinal aglucosidase (Fig. 3) (Ramdath, Padhi, Hawke, Sivaramalingam, & Tsao, 2014). 6. Conclusions and suggestions for future research 6.1. Conclusions The application of the domestic cooking of potatoes encompasses a variety of processes, such as boiling, frying, steaming, baking and roasting. The modifications (physics, chemical, and enzyme) during domestic cooking affect the nutritional value of the potato to different degrees. Generally, the nutrients, including minerals, vitamins, proteins and dietary fiber are well retained with the appropriate cooking methods. Despite the leaching or degradation of the intrinsic phytochemicals, higher recoveries are obtained due to the release of bound-phytochemicals during domestic cooking. Compared to boiling or mashing, some cooking methods (e.g., frying, microwaving and baking) could reduce the postprandial glycemic response significantly, and a cooling step or co-digestion with protein, lipid and vinegar were effective ways to decrease the GI of potato. 6.2. Future research suggestions Despite the numerous studies that have focused on the effect of domestic cooking on potatoes from different perspectives, additional work should be performed. First, most of the current studies have been focus on the phytochemicals and antioxidant activities of cooked potatoes using chemical assays are inadequate (Huang, Sun, Lou, Li, & Ye, 2014), the in vivo bioaccessibility of phytochemicals is an interesting topic that should be investigated (Palermo et al., 2014). Second, domestic cooking affects the structure of potatoes intuitively; therefore, studies focused on the structural changes of the potato during cooking, as well as its relationship with the release of phytochemicals and the digestion of starch, are urgently required. Most importantly, almost all of the studies have been based on Western cooking methods; however, because almost half of the global potato supply is now consumed in Asia, more work should focus on Oriental cooking methods, such as Chinese stir-frying with vinegar and Indian curry with plant spices, which can also reduce the GI value of potatoes by different ways and can consume less oil. Furthermore, the nutritional value and antioxidant activity will be different due to those distinct cooking methods. Thus, it will be interesting to analyze the changes in potatoes prepared using Oriental cooking methods. Conflict of interest The authors declare that they have no conflicts of interest. Acknowledgments This work was supported by National science-technology support plan projects (2014BAD04B01) and the center basic funds of university (2-2050205-15-001).

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