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Off‐flavors in foods Gary Reineccius

a

a

Department of Food Science and Nutrition, University of Minnesota, Room 147, 1334 Eckles Ave., St. Paul, MN, 55108 Version of record first published: 29 Sep 2009

To cite this article: Gary Reineccius (1991): Off‐flavors in foods, Critical Reviews in Food Science and Nutrition, 29:6, 381-402 To link to this article: http://dx.doi.org/10.1080/10408399109527534

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Food Science and Nutrition an introduction. Chlorophenols and chloroanisoles are frequently found in foods as off-flavors and generally contribute medicinal or disinfectant-like odors and moldy taints, respectively.5 They exhibit exceedingly low sensory thresholds with the 2,4- (I) and 2,6-dichlorophenol (II) and 2,4,6-trichlorophenol (III) exhibiting taste thresholds in water of 0.3, 0.2, and 2 ppb, respectively.7

Off-Flavors in Foods Gary Reineccius

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ABSTRACT The most common reason for consumer rejection of a food product is an unacceptable flavor. Food flavor may become unacceptable due to many reasons. For example, the food may become contaminated by airborne-, waterborne-, or packagingrelated chemicals. Alternatively, food components may undergo degradation due to oxidation, nonenzymatic browning, enzymatic reactions, or light-induced reactions. This review will provide a general description of the sensory properties, mechanism of occurrence, and recent research developments for the commonly occurring off-flavors in foods.

II The chloroanisoles have even lower odor threshold values (in water) with the 2,4,6-trichloroanisole (IV), 2,3,4,6-tetrachloroanisole (V), and pentachloroanisole (VI) giving values of 4 ppb, 4 X 10~ 3 , and 3 X 10~5 ppb, respectively.8

I. INTRODUCTION If one considers the complaints received in the food industry, unsatisfactory flavor undoubtedly accounts for the largest percentage of these complaints. While we can appreciate the value of fiber, vitamins, protein quality, etc., it would be rather unusual to receive a consumer complaint in this area. The sensory aspects of a food are extremely visible to the consumer and essential for consumer satisfaction. Unsatisfactory flavor means that the food does not meet the consumer's perception of product flavors. This could be due to a problem in initially choosing a flavor for the product or the product has picked up an off-flavor. Off-flavors may arise in a food product via many routes. For example, the food may become tainted due to contamination from airborne-, waterborne-, or packaging-related sources. Alternatively, microbial activity or chemical reactions in the food itself (e.g., lipid oxidation, nonenzymatic browning, or enzymatic reactions) may occur thereby resulting in off-flavors. Several good reviews have been written in the area of offflavors in foods, including the work of Whitfield and Tindale,1 Charlambous,2 Saxby,3 Whitfield and Shaw,4 Whitfield,5 and Heath and Reineccius.6 Since entire books have been devoted to this subject area, a comprehensive treatment of the subject is not possible within the format of this review. Instead, I will limit this review to updating work in the area of off-flavors due to contamination of our foods by an outside source.

OCH,

OCH

OCH.

Cl

IV

V

VI

Chlorophenols may arise from a reaction between phenol and chlorine in the water supply or from direct contamination of the food with chlorophenols. The reaction of phenol and chlorine yields monochlorophenols, 2,4- and 2,6-dichlorophenols and 2,4,6-trichlorophenol. Contamination of the food with industrially prepared chlorophenols involves the terra- and pentachlorophenols. Thus, the type of chlorophenol present (di- or trichloro vs. terra- or pentachloro) suggests the source of chlorophenol. Chloroanisoles are formed as a result of microbial methylation of the corresponding chlorophenol. Whitfield5 has provided an excellent overview on how chlorophenols may enter

II. CHLOROPHENOLS AND CHLOROANISOLES While contamination of our foods with chlorophenols and/ or chloroanisoles are discussed briefly in other sections of this review, their importance in contributing to off-flavors warrants

G. Reineccius, B.S., M.S., Ph.D., Department of Food Science and Nutrition, Room 147, University of Minnesota, 1334 Eckles Ave., St. Paul, MN 55108.

1991

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Critical Reviews In our food supply and result in off-flavors. I am providing a brief summary of his article.

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A. Chlorophenols via Reaction Chlorine in the water supply may react with cresols or phenols to yield the corresponding chlorophenols or chlorocresols. The phenol or cresol can come from several sources. Whitfield5 noted that phenol-based resins and paints used as protective coatings on liquid storage tanks and water filtration vessels may serve as sources of phenolics. He also noted that phenolbased piping or fittings can contribute sufficient phenolics to permit this reaction to occur. Phenolics from boiler treatment chemicals (e.g., lignin sulfonate) have been documented as off-flavor sources when the boiler water was used for direct steam injection heating of brines and syrups.5 B. Contamination with Chlorophenols Chlorophenols are used frequently in algaecides and fungicides. These compounds may enter the food through carelessness, airborne transport, or direct absorption into packaging materials and then into the food product. Whitfield5 recounted how refrigeration condenser water (treated with algaecides) was inadvertently sucked back into the processing water supply and thereby contaminated a food material with chlorophenols. He also noted how timber treated with pentachlorophenol and then used in pallets, shelving, or room construction may also find ways to enter the food supply. C. Chloroanisoles Cloroanisoles are not products of commerce, but result from the methylation of the corresponding chlorophenols via microbial activity. Curtis et al.9 have shown that many fungi, including Aspergillus and Penicillium sp., can methylate chlorophenols. Methylation is one means of their detoxification.10 There are numerous reports in the literature of off-flavors produced by chloroanisoles.5-8 A brief summary of this literature follows. Whitfield5 described an off-flavor in dried fruit shipped in cardboard boxes from Australia to Europe. Starch- and animalbased glues used in the construction and sealing of the cardboard boxes used to package the fruit contained polychlorophenols as antimicrobial agents. It was demonstrated that the high water activity in the shipping containers permitted microbial growth and the subsequent microbial methylation of the chlorophenols in the adhesives. Whitfield and Last" reported on their study of a related problem. Cocoa powder imported into Australia had been described as having a strong "musty" and "disinfectant" aftertaste. The cocoa had been packaged in multiwalled paper sacks and transported to Australia in metal shipping containers. The

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disagreeable taints were found to be more intense near the walls of the sacks, and there was considerable sack to sack variation in intensity. Analytical studies demonstrated the presence of high levels of chlorophenols (ranging from 3 to 520 ppb depending on chlorophenol present) in the cocoa powder. The paper sacks were found to contain substantially more chlorophenols (ranging from 41 to 170 ppb), while the glued side seams contained extremely large amounts (ranging from 78 to 40,000 ppb). 2,4-Dichlorophenol was present in the cocoa at about 2000 times its sensory detection limit in water. Analytical studies also determined the presence of chloroanisoles in the cocoa (ca. 0.1 to 20 ppb) and the glued seams (2 to 34 ppb). 2,4,6-Trichloroanisole was present in the tainted cocoa about 70,000 times its odor threshold in water. Thus, the authors suggested that 2,4-dichlorophenol was primarily responsible for the disinfectant aftertaste and 2,4,6-trichloroanisole responsible for the musty off-odor. Whitfield and Last11 elaborated on the probable source of sack contamination. They noted that 2,4-dichlorophenol, 2,4,6trichlorophenol, and 2,3,4,6-tetrachlorophenol are the major chlorophenols formed during the bleaching of wood pulp for paper manufacture. Pentachlorophenol is used frequently as a biocide in adhesives. Thus, all of the chlorophenols found in the cocoa could be accounted for. Chloroanisoles were most likely formed by molds growing on the glued seams. Maarse et al. 8 and Whitfield5 have both studied the means by which foods have picked up musty, moldy taints from jute sacks. Whitfield5 proposed that the sacks could have become contaminated with chlorophenols either because they may have been applied to the jute to provide rot proofing or they may have been formed during the chlorine bleaching of the jute. Chloroanisoles are also considered to be responsible for the "cork taint" in cognacs12 and wines.13 Riguad et al.14 have suggested that the chloroanisoles came from the microbial conversion of chlorophenols used to treat cork. A final example of how chloroanisoles may taint a food relates to aged cheese.8 This problem was rather straightforward in that chlorophenols were found in the wooden shelves used for cheese storage. Mold grew readily on the shelves, thereby converting the chlorophenols used as a wood treatment to chloroanisoles that migrated into the cheese. Off-flavors from these sources (chlorophenols and chloroanisoles) continue to plague the food industry. Phenol- or chlorophenol-based compounds are rather ubiquitous in our environment and thus are common problems. The exceedingly low sensory thresholds of these compounds makes analytical detection very difficult.415 Whenever one detects a medicinal, disinfectant, or musty or moldy flavor taint, one should suspect the presence of chlorophenols or chloroanisoles and conduct a rigorous analytical study to determine their source. The remainder of this review is organized around the means

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Food Science and Nutrition of an off-flavor entering the food supply rather than the chemicals responsible. The importance of the chlorophenols and chloroanisoles as food taints justified a separate discussion.

Table 1 Off-Flavors Due to Airborne Contamination Contaminant

Method of transmission

Biscuits

Herbicides (Chlorophenols) Tar

Chocolates

"Aniseed taste"

Cakes

"Catty offflavor"

Wrapped bakery and confectionery

Diesel oil fumes

Wrapped bakery and confectionery products

Petrol vapors

Carried by wind from nearby herbicide factories (3—5 mi) Tar boiler gave off smell through gaps left in wall "Aniseed-type" odors airborne from adjacent factory Airborne contaminant produced by interaction of ketones and sulfur compounds in chemical factory in another town 20 mi away May occur in depots, warehouses, and loading bays, diesel oil fumes can penetrate some types of wrapping materials and be absorbed by foods, especially "fatty" foods; may also be caused by forklift trucks driven by diesel oil May occur in depots, warehouses, and loading bays, where petrol has been spilled Off-flavors due to paints used in stock rooms and production areas Oil used for spraying savory biscuits and other products overheated through failure of heater thermostat; oil oxidized in metal tank with "fishy" offflavor; products uneatable

Food

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III. AIRBORNE SOURCES OF CONTAMINATION

Biscuits

When one considers off-flavors entering a food product from airborne sources, one is typically dealing with foods that are very light in texture or have rather minimal packaging barriers. Off-flavors resulting from airborne contamination are often exceedingly difficult to trace back to the source. The compounds responsible for the off-flavor typically have exceedingly low thresholds and may be short-lived in duration or dependent upon many unrelated factors for producing the offflavor. The source of the off-flavor may be miles away from the food production or storage area and yet cause the off-flavor. One of the off-flavor problems reported by Goldenberg and Matheson16 illustrates how complex this type of off-flavor can be to identify (Table 1, example 1). They found that one of their bakeries received up to 45 consumer complaints each year over an 11-year period concerning a disinfectant, antiseptic, or soapy off-flavor. They could not trace this off-flavor to any ingredient or packaging material. They did find, however, that when baked goods production was shifted to another bakery (same ingredients), there were no complaints on the production that had been moved. Fortunately, 1 day one of their technologists noted a disinfectant smell in the air outside the bakery. An investigation of local manufacturers disclosed that there were two herbicide factories located within 5 mi of the bakery. Eventually, they were able to identify the odorant in the air and detect it in the baked goods. The odorant was a mixture of chloro-o-cresols (VII) that have sensory thresholds as low as 5 X 10~2 ppb in biscuits. Whitfield5 reported a similar occurence of airborne contamination in soft drinks. The soft drink manufacturer was able to determine that the off-flavor was not in the product prior to canning, and that it occurred in only a small proportion of the cans on the processing line. The manufacturer eventually determined that empty cans incompletely covered by cardboard spacers when stacked on pallets often gave the off-flavor. It appears that airborne chlorophenol condensed on the inside of the exposed cans, thereby providing an off-flavor to the beverage. The source of contamination was traced to an agrichemical factory that was using 6-chloro-2-methyl phenol (VIII) in the manufacture of 2,4-D. 6-Chloro-2-methyl phenol has a detection threshold of 8 x 10" 5 ppb in water. Goldenberg and Matheson15 have commented on several problems (e.g., with cakes biscuits, chocolate, confectionery, and meat pies) ultimately traced to airborne transmission of normal paint constituents. They also commented on problems with polymer-based systems. In one case, a plastic coating was applied to walls that ended up tainting chickens and trifles. A

Cakes, biscuits, meat pies Oxidized oil

Biscuits

From Goldenberg, N. and Matheson, H. R., Chem. Ind., 5, 551, 1975. With permission.

second case was an epoxy coating put on the floor of an extension off from the bakery. Two wall fans had been left on, permitting the transfer of styrene odors into the bakery and thereby tainting cooling cakes.

VII

VIII

Of more interest and challenge was the situation where a "catty" odor developed in meats. The "catty" odor was described as resembling the smell of "the urine of a mature tom1991

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Critical Reviews In cat'' . 3 The compound responsible for this odor was 4-mercapto4-methyl pentan-2-one (XI), which has an odor threshold in bland food of about 0.01 ppb. This compound was formed in the food via reaction of mesityl oxide (IX) with a sulfur source (most likely hydrogen sulfide (X)). The reaction is shown here. CH,

CH CH, CH,

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IX

SH

IV. WATERBORNE SOURCES OF CONTAMINATION

XI The "catty" taint was found in meat carcasses that had been stored in a cooler that had been painted recently with a polyurethane paint." The paint thinner used was a mixture of xylene and pentoxone (XII). CH3

CH3

CH

s

XII The similarity of pentoxone to mesityl oxide prompted Patterson17 to search for mesityl oxide as an impurity. He found about 0.5% mesityl oxide in this solvent. An equally complicated sequence of events led to the development of a "catty" defect in one of Marks and Spencer's bakeries.16 Mesityl oxide was again the culprit. It was produced in a factory 20 mi from the bakery, carried by the air to the bakery, absorbed into cooling cakes, and then reacted with sulfur compounds in the cakes to yield a chemical that produced the "catty" defect. The task of elucidating the sequence of events leading to this off-flavor had to be most challenging. This past year we were involved in two problems related to paint off-flavors. In one case the paint was applied to the floor of a refrigerated food storage area. The floor was in poor condition, and, thus, the contractor chose to apply enough paint to fill all the cracks and holes. He used 250 gal of paint to cover an area that should have required only 50 gal. The paint seeped down cracks in the floor to totally saturate and in some cases actually dissolve 7 in of styrofoam insulation. This warehouse has been empty for 3 years. Another situation was a refrigerated meat packing plant. In this case, the styrene-based resin was properly applied, but the

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floor was not given adequate time to cure before the meat was moved back into storage. The fatty tissues of the carcasses and processed meats readily absorbed the free styrene in the air and became heavily tainted with styrene. It was of interest that the company had an exceptionally difficult time getting rid of the meat even as a waste. Many of the off-flavor problems discussed could have been avoided if some thought had been given to the situation. Individuals handling foods should be aware of how susceptible foods are to absorbing off-flavors from their environment. If any objectionable odor is detected in the environment, foods without adequate protection (packaging other than glass; cans, or foil) should not be stored there.

Water that is contaminated with off-flavors may taint a food product. This tainting can occur even during the growth of the food stuff. Here I am thinking of the water being fed to an animal or the water used to support the growth of aquatic animals. Animals, particularily nonruminants, transfer odors quite readily to their fatty body tissues. Also, large quantities of water are used in food-processing operations. Water may be used to wash, heat (direct steam injection), or reconstitute (e.g., cheese for spray drying or orange juice concentrate) our foods. The sensory quality of the water can be very critical in terms of the flavor quality of a food. A. Off-flavors in Incoming Water Supply Off-flavors in the water supply, as received by a food-processing plant, are most commonly due to microbial growth. The two most troublesome groups of microorganisms are algae and actinomycetes,18 with the algae most commonly being the greater problem. Algal odors are divided into two groups: those that are caused by the green and blue-green algae (which give an "aromatic" defect) and those caused by diatoms and pigmented flagelates ("grassy" or "fishy" defects).19 Under the proper environmental conditions, algae can produce surface covering blooms in several days. As the algae population reaches apeak, odors described as "grassy", "geranium", "earthy", or "vile" occur. As the algal bloom declines, other off-flavors occur. These off-flavors may be the result of cellular degradation yielding intracellular metabolites that then also degrade. Alternatively, the off-flavors may be due to the growth of other microorganisms (such as Streptomyces) on the dead algae. If the algal bloom dies off suddenly, decomposition products of the algae are often worse than the initial bloom. Bacterial growth on the blue-green algae will yield methyl (XIII), isobutyl (XIV), and n-butyl mercaptans (XV), as well as dimethyl

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Food Science and Nutrition sulfide (XVI).18 Off-flavors may also enter the water supply from pipeline-encrusting organisms. Tracy and Leonard18 noted "rotten eggs", CH,

H

H

H

CH

XIII

XIV

creosote have been associated with off-flavors even though the supports did not actually even contact the chlorinated water. The creosote components must have been carried through the air to the water, reacted with the chlorine, and given off-notes. Off-odors in the water supply have been also traced to plastic piping. The plastic piping may contain substances that diffuse into the water supply, thereby tainting the water. Alternatively, plastic piping has been shown to permit soil contamination to enter the water supply. Tracy and Leonard18 noted a case in which gasoline leakage into the soil reached plastic water lines, penetrated the lines, and eventually tainted the water. B. Off-Flavors Due to In-Plant Contamination The problems mentioned thus far have been related to offflavors in the normal water supply. Water may also become contaminated in the food-processing facility. Goldenberg and Mathesen15 have described three such incidents. The first problem was traced to chlorinated water being used to cool lithographed cans. Apparently the chlorine in the water supply reacted with the phenol-based lithograph print, and the tainted water was then used for the direct heating of brines and syrups. Any time one uses direct steam injection for heating of a food product, extreme care must be used to assure water quality. The second problem was briefly mentioned in an earlier section of this article (chlorophenols by reaction). In this case, lignin sulfonate was used as a boiler treatment. While there were no significant off-odors from the lignin sulfonate, reaction of phenolics in the lignin sulfonate with chlorine in the water formed chlorophenols. These chlorophenols gave an off-flavor to brines and syrups when the boiler water was used in direct heating. The third problem Goldenberg and Matheson16 described was more poorly defined. In this case, they were finding a disinfectant taint in spray-dried cheese powder. Apparently, the water used to dilute the cheese for spray drying contained compounds that gave the defect since placing an activated carbon filter on the water supply solved the problem. They did not determine the source of this off-flavor. One also has to note that in-plant water purification devices may result in off-flavors if not properly maintained. Neglect may permit bacteria to accumulate in the filters, producing offodors.

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CH, XVI

XV

"metallic", and "putrid" odors in water coming from a particular section of the Coast Range Tunnel (San Francisco water supply). The found that iron bacteria, Crenothrix and Gallionella, were using dissolved iron and manganese to grow on the tunnel walls. As the biomass grew large enough, it sloughed off into the water system where it rotted, producing an offflavor. Tracy and Leonard18 have made some interesting observations about off-flavors due to chlorination. They noted that water chlorinated beyond its "breakpoint" has less potential for taste and odor problems. The breakpoint is that amount of chlorine at which one unit of applied chlorine yields one additional unit of free chlorine. If water high in ammonia or other nitrogen-containing compounds is not chlorinated to the breakpoint, a taste and odor problem can occur due to the production of chloramine compounds (mono (XVII) and dichloramine (XVIII) and nitrogen trichloride (XIX)). Interestingly, most consumer complaints of chlorine taste results from insufficient chlorine rather than too much. Chlorination to 0.6 to 0.8 mg/1 destroys the chloramines and

Cl

H

Cl

1

I

N

Cf

c XVII

XVIII

C. Tainting of Aquatic Animals by Water Off-flavors in water may contribute taints to our food due to their assimulation during growth. As was mentioned earlier, off-flavors of this type are typically limited to aquatic animals. It is rather impressive how quickly fish will pick up odors from the water. Magligalig et al.21 demonstrated that only a 10-min exposure of fish to 2-pentanone or dimethyl sulfide (15 ppm in water) will result in off-flavors in the fish muscle. This

XIX

nitrogen trichlorides that have significant flavor and yet is below the sensory threshold of 0.8 to 1.0 mg/1 for chlorine. The tremendous potential for off-flavors due to chlorophenols and chloroanisoles is also evident in the water supply. Connors20 noted that wooden reservoir supports treated with

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Critical Reviews In

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demonstrates, very clearly, that water quality is absolutely essential to the production of fish with good sensory quality. Off-flavors in fish and seafood have been the subject of numerous research articles and very aptly summarized in the reviews of Berg,22 Whitfield and Freeman,23 Persson,24 and Whitfield.25 While off-flavors in fish may arise due to autoxidative processes during storage, this review does not cover off-flavors due to chemical changes in foods. Therefore, the off-flavors that arise in fish and seafood (marine animals) only from polluted water and the marine organism diet are discussed. 1. Water Pollution Contamination of the water by petrochemicals (e.g., crude oil) is a major source of off-flavors in marine animals.25 Aromatic hydrocarbons, particularly the alkylbenzenes containing nine to ten carbons, are commonly responsible for off-flavors.26 Vale et al.27 and Shipton et al.28 reported on a "kerosene" taint in mullet taken from the Moreton Bay area and the Brisbane River in Queensland, Australia. This taint appeared in the mullet at the start of winter when they began their migration from the southern estuaries to the wanner waters to the north. Areas where tainted fish were caught generally had docks, sewage outlets, or heavy industry. Volatiles isolated from the tainted fish closely resembled kerosene hydrocarbons, suggesting water pollution was responsible for the off-flavor. It is of interest that oysters and mussels have a greater tendency to accumulate petroleum hydrocarbons than free-swimming fish.29 Pollution from paper mill effluent has been shown to be a problem in certain Norwegian rivers.22 The fish caught in rivers with paper mills were generally badly tainted and the taint was described as being "typically paper mill" and "sulfite-like". Numerous compounds were associated with this off-flavor. Terpenes, terpene derivatives, alkyI benzenes, alkenyl benzenes, and organochlorine compounds were always higher in concentration in fish characterized as having "sulfite-like" or "paper mill" taints. Paasivirta et al. 30 have found 2,3,4,6-tetrachlorophenol and hexachlorobenzene as sources of off-flavors in fish from rivers and lakes contaminated by processing effluents. Contamination of water with herbicides and fungicides has also been implicated in fish off-flavors. Miyazaki et al.31 found that excessive use of pentachlorophenol-based herbicides in rice paddies has resulted in polyhaloanisole contamination of oysters in Tokyo Bay. It is quite evident from the literature that there are many sources of water contamination that can result in taints in marine animals. 2. Taints Related to Diet Unfortunately, assuring that the water is not contaminated via oil spills or industrial pollution does not guarantee the absence of off-flavors in marine animals. Other aquatic or-

386

ganisms, either growing in the water or serving as food for the marine animals are frequently the source of off-flavors. "Earthy muddy" taints occur very commonly in marine animals. These taints have been traced to the growth of a species of Actinomyces in the water by Thaysen32 and Thaysen and Pentelow.33 More recent studies have demonstrated that blue-green algae are probably more commonly the source of off-flavor than the actinomycetes.25 The off-flavor components responsible for the muddy defect in fish are generally geosmin (XX) or 2-methylisoborneol (XXI).23 Recently, Martin et al.34 have implicated two dehydration products of 2-methylisoborneol, 2-methylenebornane (XXIT), and 2-methyl-2-bornene (XXIII), as additional compounds that will contribute to the muddy, musty off-flavor in fish. All of these compounds are known to be metabolites of certain species of cyanobacter35"37 and actinomycetes.3S-39 The occurrence of 2-methylisoborneol and geosmin in water has been shown to be correlated with increasing

*

XX

CH S

XXI

XXII eutrophication.40-41 Warm water temperatures, high fish standing crops, and excess amounts of fish food all contribute to the high incidence of off-flavor in catfish from culture ponds.42-43 Martin et al.44 studied the uptake of 2-methylisoborneol in catfish. When the fish were placed in water containing 2methylisoborneol at a concentration of 5 n.g/1, in only 2 h the level found in the fish muscle was 32 ± 0.2 |xg/kg. Maximum muscle concentration (54 ± 4 \x-%Dng) was reached in 48 h. When 2-methylisoborneol concentrations in the water were increased to 50 |xg/l, muscle levels also went up by a factor of ten. In general, the methylisoborneol was concentrated in the fish muscle at a level approximately ten times that added

Volume 29, Issue 6

Food Science and Nutrition One can see that water quality is important in determining the flavor quality of foods. The quality of water may influence the flavor of marine animals, thereby providing off-flavors to our food before the food is even collected. Several examples have been discussed whereby incoming tainted water may contaminate the food. Alternatively, water may become contaminated in-plant. The water may pick up various chemicals from boiler treatments, polymeric piping, or cooking processes. Water can be a very effective medium of transferring objectionable odorants to our food supply.

to the water. This reflects the partition coefficient of methylisoborneol for fat vs. water. Martin et al.44 also determined where in the catfish the methylisoborneol was concentrated. Methylisoborneol was found in the greatest concentration in the subepidermal layers and abdominal fat. While' 'muddy'',' 'musty'' off-flavors are the most common taints found in marine animals used as foods, a number of other flavor defects also may occur.45 Motohiro46 described an offflavor in canned salmon that was "petroleum-like" in character. He found

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V. PESTICIDES, DISINFECTANTS, AND DETERGENTS CH3

+

0

Pesticides, disinfectants, and detergents comprise a group of chemicals that will occasionally contribute to off-flavors in foods. Pesticide contamination is outside the control of the food processor, but disinfectant or detergent contamination is within his/her control.

CH,

CH.

XXV

XXIV

that this off-flavor was due to the thermal degradation of dimethyl-P-propiothetin (XXIV) to dimethyl sulfide (XXV). Dimethyl-P-propiothetin was present in the salmon as a metabolite of Limacina helicina, a major constituent of the salmon diet. Dimethyl sulfide was also found to be responsible for an offflavor described as "blackberry" in cod from the Labrador coast.47'48 Whitfield25 has summarized work on two distinctive offflavors in Australian prawns. The first defect was a garlic-like note found in the red prawn, Aristeomorpha foliacea, which was traced to trimethylarsine (XXVI). The odor threshold of trimethylarsine in water is about 0.002 fig/kg and about 0.1 jig/kg in prawns. Trimethylarsine was postulated to arise from the microbial alkylation of arsenic. Alkyl arsines have been isolated from soil, sewage sludge, ocean mud, and brown algae.25 A second off-flavor discussed by Whitfield25 was an "iodoform-like" defect found in both prawns and shrimps. This off-flavor was traced to the presence of 2.6-dibromophenol (XXVII)49 in the diet. Certain local algae and bryozoa were found to contain this compound. The sensory threshold of this compound was determined to be 32 (xg/kg in prawn muscle.

A. Chlorophenols Goldenberg and Matheson16 have presented a listing of offflavors that have occurred at Marks and Spencer due to disinfectants, pesticides, and detergents (Table 2). The central role of chlorophenols in contributing off-flavors is again evident. The chlorophenols are not used as an active ingredient in any case, but are present as a contaminant or formed via the reaction of chlorinated water with a phenolic constituent. The first example involves airborne transmission of chlorophenol from contaminated timber in a cargo ship. The timber was originally contaminated with the chlorophenol by airborne means, and the timber then served as a resevoir for future contamination. The second example with spray-dried milk powder was apparently less defined. Goldenberg and Matheson16 did not elaborate on the source of the chlorophenols in the water supply. The chlorophenol contamination noted for canned peas fits the scenario presented earlier for chlorophenol formation via reaction with chlorinated water. While it would seem obvious that fresh tar would be picked up by the tires and possibly enter the pea troughs, one can imagine the driver blithely going along the road with little concern. The defect in smoked bacon is due to using hypochlorite to disinfect the bacon. This off-flavor seems rather obvious and should have been considered prior to its occurrence. Smoke is well known to contain substantial quantities of phenolics readily reacting with chlorine to produce chlorophenols.

CH.

CH

CH. XXVI

B. Pesticides The example of pesticide contamination of canned raspber-

XXVII

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Critical Reviews In Table 2 Off-Flavors Due to Disinfectants, Pesticides, and Detergents

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Food

Contaminant

Cake

Chlorophenol

Spray-dried milk powder

Chlorophenol

Canned peas

Chlorophenol

Canned raspberries

Pesticide

Pork

Disinfectant

Tomatoes

Pesticide

Smoked bacon

Disinfectant

Method of transmission Due to sultanas, contaminated on board ship by chlorophenol; on a previous voyage holds had been used for cargo of herbicide containing 0.5% chlorophenol as impurity; hold washed out but timber still impregnated; finally timber parts had to be destroyed "Off-flavor" of chlorophenol type: waterbome; use of activated carbon eliminated offflavor Peas transported to cannery in open steel troughs on newly tarred road; peas contaminated by tar-covered stones flying up from road; peas then blanched in chlorinated water and chlorophenol formed "Catty" off-flavor may have been due to mesityl oxide in ketonic solvent Preventative spraying with disinfectant in slaughterhouse against spread of swine vesicular disease "Off-flavor" of chlorophenol type Hypochlorite used as sterilizing agent; excess used and inadequately washed off; chlorine combined with phenolic compounds present in smoked bacon to give chlorophenol; bacon uneatable; sterilizing agents containing chlorine should not be used where smoked meats or fish are handled, unless special precautions are taken

From Goldenberg, N. and Matheson, H. R., Chem. Ind., 5, 551, 1975. With permission.

ries was poorly defined, but appeared to be due to mesityl oxide in the pesticide solvent. This conclusion was probably based on sensory detection of a "catty off-flavor" that is associated with mesityl oxide reactions as is noted earlier. C. Disinfectants Pork contamination by spraying the hogs with a disinfectant demonstrates the importance of education all the way back to the producer. A residue of pesticides or herbicides on plant

388

tissue or in animal tissues can readily survive the food-processing operation and result in off-flavors. D. Detergents Woo and Lindsay50 have reported on a source of soapiness in butters. Soapiness is typically associated with high concentrations of C, o or C I2 free fatty acids. These acids most commonly arise in a food due to lipase activity on coconut fat. However, Woo and Lindsay50 did not detect sufficient levels of these two fatty acids to explain the soapy defect. They were aware that alkylbenzene sulfonates are widely used as sanitizing agents for cow's udders and for dairy equipment. Therefore, they chose to develop methodology to determine the most commonly used alkylbenzene sulfonates in butter processing plants and to see if sensory/analytical data would agree. Sensory evaluation of skim milk containing either added sodium dodecyl benzene sulfonate (XXVIII) or sodium dodecyl sulfate (XXIX) demonstrated soapy flavors very similar to the defect character noted in the off-flavored butters. Quantitative studies also demonstrated the presence of these two detergents (7 to 22 ppm) in tainted butter samples. Woo and Lindsay50 could not determine how the detergents came to contaminate the butters. The relatively high levels of

Na+~0

XXVIII

NaT

XXIX detergent suggested that residues left on sanitized equipment would not be adequate. It appeared that significant contamination of the cream by prepared cleaning solution was necessary to result in such high levels. They hypothesized that perhaps a worker had substituted the alkylbenzene sulfonated detergents for chlorine containing sanitizers when the batchchurned butter was washed. The fact that the chemicals causing off-flavors were generally not the active ingredients should bring a message home to the manufacturers of pesticides, disinfectants, and detergents. They may have tested their product for ability to taint foods, but not focused on the proper constitutent of their prod-

Volume 29, Issue 6

Food Science and Nutrition A later publication by Heydanek et al.53 presented work on evaluating the effectiveness of various coupon coating films in reducing volatile transmission from coupons to breakfast cereals. They started the study by determining the volatile compounds found in various types of premiums and coupons. As can be seen in Table 4, the premiums and coupons may contribute quite a number of volatiles to a food product. Some of these volatiles (e.g., styrene (XXX) and methylisobutyl ketone (XXXI)) have very characteristic odors and low sensory thresholds. Many of these constituents could readily provide an offflavor to a food if placed in direct contact or in a closed package with food.

uct. The culprit may well be the occasional and variable contaminant in their product. One also cannot overlook the fragrances used in various cleaning products. The fragrance may be used to cover up objectionable chemical odors or just make product use a little more pleasant. These fragrances can contribute an off-flavor, albeit more pleasant, just like the chlorophenols or any other off-flavor chemical. Sound knowledge of potential to provide food taints as well as strict qualitycontrol procedures are necessary to avoid such problems.

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VI. CONTAMINATION BY PACKAGING MATERIALS

CH,

Off-flavors due to packaging materials are frequently encountered in the food industry. The only packaging material that will not potentially give an off-flavor to a food is glass. Yet we have to be aware that glass containers must have some type of closure, typically metal with a protective polymeric coating and a gasket of a second polymer. Therefore, even glass containers could provide components that may taint a food. Some off-flavors in foods traced back to packaging sources are listed in Table 3. One will note again that the off-flavor components are not the major food-packaging materials themselves, but rather minor constituents that may be functional or present as contaminants.

CH, XXX

A. Paper Packaging Paperboard/pulp sources of food-packaging materials enjoy the largest market share — about 34%.51 These materials can be sources of off-flavor in foods. Goldenberg and Matheson16 have noted several cases of off-flavor associated with paperpackaging materials (Table 3). The source of the first off-flavor was not absolutely determined. They noted that the use of recycled paper contaminated cakes, biscuits, and chocolate and that these products picked the off-flavor up from either additives in the recycled paper or from the print in this paper. The second set of off-flavors, chocolate cakes and confectionery, were better defined and found to be due to inadequate curing of inks used in printing. Traditionally, organic-solvent based inks have been used in the trade. The inks themselves are seldom the problem, but the solvents used to dilute the pigments or provide a coating for printing are commonly a problem. Heydanek52 found a "piney" note in breakfast cereal that he traced to the glassine inner liner. The glassine liner was made of two sheets of Kraft® paper laminated with a resin in microcrystalline wax and overwaxed on both sides with paraffin. Further study determined that the resin used in bonding the paper liner was the source of the off-flavor. Apparently this resin was derived from a pine oil that would be more or less odorous depending upon processing of the oil.

1991

XXXI

We have found a situation where the white paperboard dividers used to separate doughnuts in clear cello wrap gave a hydrocarbon off-flavor to the product. The manufacturer needed to use a package organizer (paperboard divider) to maintain doughnut integrity, but the doughnuts would leave a grease circle on the paperboard unless it was wax coated. The wax was applied in a solvent that was hydrocarbon based and would provide the off-flavor when production was "pushed" too much to permit complete evaporation of the solvent. A second problem we have encountered with paper foodpackaging materials was with yogurt. Yogurt is often packaged using roll stock. The roll stock is made by preparing one side of a sheet of packaging for food contact and printing on the other side. The sheet is then rolled, printed side against product side, for shipping, storage, etc. We found that solvents remaining from the printing system readily migrated from the print side of the package to the product side and then, ultimately, to the yogurt. The yogurt next to the package always had a "chemical" taste, while that toward the center was free of off-flavor. B. Plastic Packaging Plastic packaging has undergone substantial growth in recent years. Rauch51 estimated that plastic would have 22% of the food-packaging market share by 1990. Improved consumer acceptance, FDA approval of low-temperature sterilization using hydrogen peroxide, and technological developments in plastic materials and their manufacture have all combined to boost market share. While plastic packaging offers numerous advantages to the manufacturer and consumer, it also brings along unique off-flavor problems. Plastics are made from lowmolecular-weight monomers that, if incompletely polymer-

389

Critical Reviews In Table 3 Off-Flavors in Foods Due to Packaging Materials Food

Source and method of transmission

Packaging material Paper

Cakes, biscuits,' chocolate

Chipboard

Doughnutsb

Coated paper divider plus cellophane overwrap Glassine liner coupons

Cold cereals'

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Chocolate cakes* Yogurt containers'" Confectionery*

Coated paper — print on one side

Off-flavor due to either preservatives added to repulped waste paper or compounds in print in waste paper Inadequate removal of hydrocarbon solvents used to apply divider coating Residual solvents; residual solvents in inks Inadequate "curing" of inks used in printing

Plastic Orange and lemon drinks* Chocolate and lemon cookies'1 TV dinners'"

PVC bottle Polystyrene trays wrapped in printed cellophane Polystyrene trays

Mercaptide stabilizer used in PVC — split off in processing of beverage Residual styrene monomer Residual styrene monomer

Laminates Fruity soft drinks"

Polyester/aluminum foil/ polethylene laminate

Residual toluene in laminate adhesive — faulty drying process

Metal-Paper Refrigerated doughs'"

Paper tube with metal caps

Residual hydrocarbon solvents in triglycerides used to lubricate metal caps to facilitate packaging operation Metal

Canned pork products'

Lacquered can

Refrigerated milk beverage1"

Lacquered can

Beet*

Can

"Catty" OF due to use of lacquer to cover sideseam to prevent blackening; solvent used contained mesityl oxide which reacted with sulfur compounds in food to cause OF Trace contaminant (isophorone) in solvents used in lacquer which did not "flash off' Lubricants used in can manufacture

From Goldenberg, N. and Matheson, H. R., Chem. Ind., 5, 551, 1975. With permission. From Reineccius, G. A., 1983, unpublished. From Heydanek, M. G., Anal. Chem., 49, 901a, 1977; Haydanek, M. G., Woolford, G., and Baugh, L. C , J.FoodSci., 44, 850, 1979. From Passey, N., Instrumental Analysis of Foods, Recent Progress, Charalambous, G. and Inglett, G., Eds., Academic Press, Orlando, FL, 1983, 413. From Hardwick, W. A., Brew. Dig. June 22, 1985.

390

Volume 29, Issue 6

Food Science and Nutrition Table 4 Volatile Components Identified in Premiums and Coupons Premium Polyethylene

FlexoPrint

RotoPrint

+

+

GC

MS

Decane

+ •

• b

+

+

Dodecane Hexane Hexene Heptane Heptene Nonane Nonene Octane Octene Undecane Undecene Toluene Xylene Ethyl benzene Propyl benzene Styrene Acetone Ethanol Ethyl acetate Ethyl cellusolve Hexanal Methanol Methyl cellusolve Methyl ethyl ketone Methyl-iso-butyl ketone 1-Nitropropane 2-Nitropropane Pentanal n-Propyl acetate i-Propyl acetate 1-Propanol 2-Propanol

+

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

+ + +

+ +

+

+ + + + +

Compound

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Coupon

Polyethylene/ Polystyrene

• b

+ + + +

+ + + + + + + + + + + + + + + + + + +

+ matches authentic compound RT. + matches authentic compound mass spectra or published spectra.

stituents, absorbing flavor from the food, or permitting either oxygen or water vapor to enter the food, accelerating degradation reactions.

ized, may taint the food. A problem that is discussed later in this review is the "scalping" of flavor by plastic packaging. Flavors will have some solubility in the plastic packaging and migrate from the food into and perhaps through the package. Flavor constituents are lost from the food and flavor may become unacceptable. Plastics may also contribute to unacceptable flavor of a food by allowing either oxygen or water vapor to enter the food. There is little question that metal cans and glass jars have zero permeability for either oxygen or water vapor. Thus, the switch from glass or metal to plastics may change the sensory properties of a food by contributing off-flavor con-

1. Contamination In terms of contributing off-flavors to foods, styrene-based polymers are often a problem. Styrene monomer has a very characteristic odor and low threshold (about 1 ppm depending on medium). Passey54 has described styrene odor absorption by chocolate and lemon cookies that had been placed on polystyrene trays wrapped in printed cellophane. We have spent considerable time studying off-odors arising

1991

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Critical Reviews In from thermoset trays used primarily for up-scale TV dinners. These trays performed well in microwave applications, but occasionally tainted foods when heated in a conventional oven. At that time, the microwave market would not support a product devoted solely to it and therefore the food and tray had to also be able to withstand conventional oven heating times and temperatures. The major constituent responsible for objection odor was, of course, styrene. Polyvinyl chloride (PVC) is a versatile polymer that can be used in food packages from bottles to films. Goldenberg and Matheson16 recounted a problem they had with orange and lemon drinks that were packaged in PVC. Apparently, HC1 used in processing attacked a mercaptide stabilizer used in the PVC, liberating a free sulfur compound. Sulfur compounds are well known for their low sensory thresholds and objectionable odors. More recently, Kim et al.55 have studied the volatiles found in PVC films. They identified a total of 33 volatiles (Table 5). While some of the volatiles have high sensory thresholds, compounds such as naphthalene (XXXII), 1-methylnaphthalene, benzaldehyde, benzothiazole (XXXin), 2,6-dimethyl-4-methyl phenol (XXXIV), and nonyl phenol (XXXV) isomers are of very great concern. The presence of the 2,6-dimethyl-4-methyl phenol and nonyl phenol isomers should be of particular concern, since contact with chlorinated water at elevated temperatures will readily transform these phenols to chlorophenols; volatiles with exceedingly low sensory thresholds and objectionable "musty"

XXXII

XXXIII XXXIV

Table 5 The Volatile Compounds Identified in the PVC Film Aliphatic hydrocarbons 3,6-Dimethyl undecane Tetradecane 3-Methyl eicosane 2,6,10,14,18,22-tetracosahexaene, 2,6,10,15,19,23-hexamethyl 5-Methyl-2-undecane 7-Hexadecene Aromatic hydrocarbons Xylene isomers Toluene Indene Naphthalene 1-Methyl naphthalene Alcohols 2-EthyI hexyl alcohol 2-Ethyl decanol Butylatedhydroxytoluene (BHT) 2,6-dimethyl-4-methyl phenol Nonyl phenol isomers Ketones 1 -(3,4-Dimethylpheny l)-ethanone Aldehydes Benzaldehyde Esters Dipropyl adipate Dimethyl phthalate Dibutyl adipate Diethyl phthalate bis-(2-ethylhexyl) adipate bis-(2-ethylhexyl) phthalate bis-(l-mthylheptyl) phthalate 2-Ethoxyethyl acetate Pentadecanoic acid, 14-methyl, methyl ester Propanoic acid, 2-methyl, l-(l,l-dimethylethyl)-2-methyl-l,3-propanediyl ester Acids 2-Ethyl hexanoic acid Pentadecanoic acid 4-Hexenoic acid, 3-methyl-2, 6-dioxoCyclohexane carboxylic acid, l-(l.l-dimethylethyl)Heterocyclic compounds Benzothiazole

2. Flavor Scalping

XXXV "chemical" properties. The volatiles listed in Table 5 were postulated to arise from the PVC polymer itself, the plasticizers [2>«-(2-ethylhexyl) adipate and Ws-(2-ethylhexyl) phthalate], antioxidants (tris-nonylphenyl phosphite), and epoxidized soybean oil (used to improve the long-term heat and light stability of the polymer and contribute some plasticization).

392

Flavor migration (scalping) into the plastic packaging is also a concern. There are some plastic packaging materials that are very "clean" in terms of providing malodorous compounds to foods (e.g., polyethylene). However, these plastics may well be guilty of scalping flavor from a food product, thereby reducing overall flavor strength or selectively absorbing only certain flavor constituents, resulting in a change in flavor character.56 As was mentioned earlier, polymer-based materials may absorb flavor compounds. The polymer may be a coating on the inside of a metal can, a jar closure, barrier for paperboard or be the container itself (i.e., plastic bottles or aseptic packages).

Volume 29, Issue 6

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Food Science and Nutrition Obviously, the larger the surface area and mass of the polymer, the faster the rate and the greater the quantity of flavor absorbed from the food. When the polymer is the only packaging material, the opportunity exists for the flavor to migrate into and through the polymer. While one finds quite different concerns for this situation vs. simple absorption, for the purposes of this review, I do not differentiate. If some or all of the flavor leaves the product, there may well be flavor problems. Scalping is seldom uniform, i.e., the polymer will typically offer greater solubility to some flavor components than others. If the greater solubility is for flavor impact compounds (i.e., those which carry the characteristic flavor), the named flavor decreases in intensity with storage. If greater solubility is for supportive compounds, the flavor may change in character. Thus the problem encountered due to scalping may be a general change in flavor intensity or an unbalancing due to selective absorption.

b. FOOD SYSTEMS

An alternate approach to working with model systems has been to place real foods in various packages and monitor flavor analytically and sensorially. Many studies have been conducted using citrus products since they are particularly prone to flavor scalping problems and are commonly packaged in aseptic containers (have polymer food contact surfaces). Mannheim et al.59>6° reported on changes in limonene content and sensory properties of citrus juices packaged in glass jars vs. a laminated carton of polyethylene/carton/aluminum/polyethylene. Their results are illustrated in Figures 2 to 4. The greater loss of limonene during storage in the laminated package is obvious in Figure 2. This loss translates directly to flavor changes obvious to the palate (Figure 3). Sensory differences between glass and laminate packaged orange juice were significant after about 9 weeks of storage. The grapefruit juice stored in glass and that stored in the laminated carton were not significantly different until about 12 weeks of storage.

a. MODEL SYSTEMS

3. Packaging Catalyzed Off-Flavors A second and perhaps important effect of polymer coatings is their cataytic effect on flavor deterioration. Mannheim et al.59 demonstrated this effect by placing strips of carton laminates in juices stored in hermetically sealed glass jars. Surface to volume ratios of 1:4 and 1:6, when compared with the actual ratio in 1-1 packs, were used. The contact of orange juice with a laminated carton resulted in accelerated ascorbic acid loss and browning (Figure 4) relative to controls stored in all glass. It was noted that sensory panels could detect flavor differences between juices in contact with laminates vs. glass in as little as 14 d of storage.

While there is substantial literature in the area of flavor scalping,56 only a couple of studies are presented to illustrate the problem. Most studies have involved model systems whereby the absorption of selected compounds by various polymers or permeability of polymers to flavorants was monitored. When absorption is considered, rate of flavor loss is determined by both solubility and diffusion factors. If the flavor compound has little solubility in the polymer, it will not enter the polymer and thus not be lost from the food. If the flavor compound is soluble in the polymer, then its rate of loss from the food depends upon diffusion rate through the polymer. We can readily see that properties of the flavor compound (e.g., polarity and molecular size) as well as polymer properties (e.g., chemical composition, density, and thickness) will influence flavor loss. Specific data to illustrate these factors are presented below. Landois-Garza and Hotchkiss57 studied the effects of molecular weight, relative humidity, and concentration on the permeability of alkyl esters through polyvinyl alcohol (PVOH) films. They found film permeability to increase with increased molecular weight of alkyl esters. Increased molecular weight results in greater solubility than the decreased diffusion rate. Larger-molecular-weight esters would be preferentially lost from the food, thereby changing flavor profile and sensory character. Concentration and humidity were also found to influence film permeability. As concentration of flavor compound increased, permeability increased. Permeability of PVOH was found to be directly related to humidity. A rather comprehensive treatment of the odor permeability of polypropylene has been provided by Zorbel.38 A summary figure (Figure 1) has been selected from their work. As can be seen from this figure, molecular weight and chemical properties (primarily polarity) are important determinants of permeability of flavorants. Within a given class of flavor compounds, permeability is reasonably predictable.

4. Oxygen/Water Vapor Transmission A final consideration of plastic packaging relates to its relative oxygen or water vapor transmission rates. It is reasonable that oxidation rate is influenced by availability of oxygen. Plastic packaging materials vary greatly in oxygen permeability and in all cases exceed that of the glass or cans the plastic often replaces. Thus flavor quality may deteriorate rapidly in plastic packaging due to oxidation. Water vapor transmission through a package can also influence flavor quality. The obvious effect is a loss in crispness of products such as potato chips. Less obvious is the influence water activity has upon chemical reactions in foods. It is well documented that many deterioration reactions in foods are influenced by water activity and therefore perhaps packaging materials. C. Metal Packaging Metal rounds out the top three food-packaging materials (26%) of market).51 Metal cans may also provide off-flavors to a food product. The food product may be contaminated by residual volatiles in the sealant used to assure an aseptic seal

1991

393

Critical Reviews In BENZYL ACETATE

-10.5T

.

DICH LORANISOLE

VANILLIN \ GUAIACOL \ * METHYL FORMATE *•

ACETB.ACUI. /

\

m

W

L

S °«.

/

"• ..PROPANAL /

MENTHOL

•t. ^ LIMONENE ACETIC ACID (DIMER)

' 0

\Oy

!

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-+-

H

ALKANES ALCOHOL

X

ESTERS

-

MISC.

2-BUTANONE

TETRAHYDROFURAN

0

0

AROMATIC H

20 40 60 80 100 120 140 160 180 200 MOLECULAR WEIGHT FIGURE 1. Permeability of odors through polypropylene packaging films.38

between the can body and top (or bottom). The sealant is a polymer that has the potential for providing off-flavors. Pressure- or heat-sensitive pull tabs are becoming common as can closures (or openings). These foil tabs actually contact the can contents in the area of opening and thus can directly transfer residual volatiles to the food product. Cans may become contaminated during manufacture. Lubricants used in manufacture may not be properly removed and thus taint the food. A final and most significant source of off-odors is the material used to coat the internal food contact — surfaces of the can. One can generally not permit direct can/food contact. This is both to protect the integrity of the can as well as isolate the food from contamination with metals. Low pH foods can readily dissolve metal cans. The can lining is again a polymer applied in a solvent. The polymer or solvent may contaminate the food providing off-flavor. 1. Can Coatings As with other off-flavor sources, it is generally not the major constituents that cause the problems but minor constituents. Goldenberg and Matheson16 reported finding a "catty" defect in canned pork products. Investigation into the source of the problem showed that the solvent used to thin the lacquer sideseam coating contained mesityl oxide. As is noted previously in this review, mesityl oxide readily reacts with sulfur compounds in foods to produce a characteristic "catty" defect. Cabezudo et al.61 presented their work related to off-flavors contributed to beer by the epoxy resin coatings used on the interior of beer cans. Standards have been set for the presence of methylethyl ketone, methylisobutyl ketone, toluene, butanol 394

plus xylene, and cyclohexane in epoxy-coated steel cans. The amount of these solvents remaining in the epoxy can coating is dependent upon several factors, including curing time and temperature. Too low temperatures may result in incomplete evaporation of residual solvents, while too high temperatures may result in too rapid solidification of the resin and therefore bubble development (containing solvent). In either case.the resin may contribute volatiles that may taint the beer. We have worked on a problem where a refrigerated milkbased beverage had a "chemical" defect. The defect was variable occurring only in some cans. In taking cans apart and heating individual parts in water, we determined by sensory examination of the water that the side-seam joint was the source of the off-flavor. Analytical examination of the side-seam determined that isophorone (XXXVI) was the source of the offflavor. Isophorone was found as a trace contaminant in one of the solvents used to thin a side-seam coating polymer. Isophorone was not present in all batches of solvent, only in some. The side-seam striping application was not well done, in some cases being too thick. Thus isophorone became a problem when the solvent was contaminated at the highest level and was applied too thickly. 2. Lubricants Rolling lubricants are typically based on mineral oil, but must contain some polyunsaturated fatty acids to make the lubricant slightly hydrophilic. Hard wick62 reported on a flavor problem where the fatty acid constituent, in this case oleic acid plus its minor impurities, was found to be very rancid and thereby contributed a "rancid", "old beer" off-flavor to beer.

Volume 29, Issue 6

Food Science and Nutrition

x -e- CARTON -*• GLASS

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10

20 30 TIME (DAYS)

40

FIGURE 2. D-Limonene concentration in orange juice filled into glass jars and carton packs stored at 25°C.60

materials seemingly flux to the coating surface. The fatty materials are thus exposed to air and may oxidize during storage of the empty cans to provide off-flavors. It is of interest that the level of fatty acids contaminating the cans is probably less than 1 mg per can (half inside and half outside). As in most situations of off-flavor, the component causing the off-flavor typically has a low sensory threshold so very little precursor material is necessary to provide an off-flavor.

The primary off-flavor compound resulting from fatty acid oxidation was 2-nonenal (XXXVII). 2-Nonenal has a detection threshold of less than 1 ppb in beer.

0. Miscellaneous Packaging All of the other packaging materials combined make up only 18% of the market (including glass at about 7%).51 These packaging materials can also present off-flavor problems. We studied an off-flavor that involved a refrigerated dough product that was packaged in a foil-lined paperboard tube. This tube had metal caps on both ends. Occasionally, the dough packaged in this container would have a very objectionable hydrocarbon odor. The hydrocarbons were found to come from the process of applying metal caps to the tubes. The capping operation is high speed, which requires that the end caps be lubricated. While the lubricant was a good-quality edible triglyceride, the triglyceride had to be cut with a hydrocarbon solvent so that it could be applied thinly and uniformly. In some cases, the hydrocarbon solvent was not given adequate time to flash off; thus, the residual hydrocarbon was sealed inside the dough package. Passey54 examined an off-flavor in fruity soft drinks. The soft drink was packaged in a multibarrier laminate. The adhesive used to hold the layers together contained toluene that was not adequately removed in the drying process and contaminated the soft drink, thereby contaminating the product with toluene.

XXXVI

CHO

XXXVII The occurrence of this off-flavor was actually linked to an interaction of the oxidized fatty acid and the interior can coating. An organic coating is applied to the interior of beer cans. This organic coating is typically a solvent-based epoxy that has a weak affinity for aldehydes (e.g., 2-nonenal). This weak affinity apparently prevented the 2-nonenal from being washed from the inside of the cans or being evaporated during oven curing. The 2-nonenal was released from the resin coating to the beer during storage, thereby providing an off-flavor. As in many areas of manufacture, there is a trend to replace the solvent-based epoxy coatings with water-based coatings. Hardwick62 commented that this change has worsened the offflavor problems arising via the above-mentioned mechanism. Apparently the water-based resins have little affinity for any fatty acid material left on the interior of the cans and the fatty

E. Packaging Closures In closing this section, I would like to mention two additional

1991

395

Critical Reviews In Most food-packaging-related off-flavors should be caught at the quality control level before use rather than after use.

ORANGE JUICE 9 5 % SIGNIFICANCE

VII. MICROBIAL CONTAMINATION

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4 6 8 TIME (WEEKS)

10

12

FIGURE 3. Triangle taste comparisons between juices aseptically filled into glass jars and carton packs stored at IS'C.19

packaging-related problems concerning closures. Simpson et al. 63 reported on a taint in wine caused by guaicol. One would expect some guaicol in corks since guaicol is a degradation product of lignin. Guaicol may also be found in wines since microorganisms are capable of degrading vanillin (from oak) to guaicol. However, Simpson et al. 63 found guaicol levels in tainted wines as high as 2.63 mg/1, which were too high to come from either vanillin conversion or lignin degradation. Simpson et al.63 postulated that the guaicol most likely came from a chemical source, i.e., the corks came in contact with guaicol vapors at some point of manufacture, transport, or plant storage. The second off-flavor related to packaging closures involved a turpentine-like odor in cola. Wiley et al.64 noted that some colas stored in either glass or plastic developed this off-odor toward the end of their shelf life. The off-odor was traced to somewhat typical oxidation of citrus terpenes. This in itself would not be unexpected or of particular interest to this review except that terpene oxidation was related to the bottle closure. The bottle cap had a white ethyl vinyl acetate liner whitened through the use of titanium dioxide. Wiley et al.64 found that the titanium dioxide was acting as a catalyst for oxidation, thereby producing an off-odor. In conclusion of this section, it becomes quite clear that virtually any packaging material may contribute an off-flavor to a food product. It is noteworthy, however, to recall that packaging is generally in lesser quantity than the foodstuff. Therefore, in order for a packaging material to lend an offflavor to a food product, it must be present in the packaging material at a much greater concentration that in the food itself. It generally is possible to detect off-flavor contaminants in the packaging either via good sensory testing or analytical work.

396

I believe it is quite safe to say that microbial spoilage is the most common source of off-flavors in nonpreserved foods. Microbial activity can product off-flavors in several ways: the production of undesirable primary metabolites, incidental chemical conversion of food constituents of otherwise little flavor significance, or through residual enzyme activity after cell death. Goldenberg and Matheson16 have again presented a good summary of off-flavors due to microbial activity (see Table 6). Their examples include all three basic mechanisms of off-flavor production by microorganisms. The first example (Table 6) involved the use of "bad" eggs. Apparently the eggs were either abused prior to freezing or after thawing. Microbial activity occurred and resulted in an off-flavored cake. Mold growth is a common source of off-flavors. Goldenberg and Matheson16 have provided three examples of off-flavors due to mold. In two examples, some different types of cakes and biscuits obtained an off-flavor due to direct microbial activity. Certain microorganisms may produce 2-methoxy-3-isopropyl pyrazine (XXXVIII), 2-methylisobomeol, or geosmin that have exceptionally low sensory thresholds and all carry very characteristic earthy, moldy, musty notes.

XXXVIII The third example provided by Goldenberg and Matheson16 involved microbial (molds) generation of off-flavor in a less direct manner. In this case wood shavings used in the chicken house were contaminated with chlorophenols. Molds grew in the litter thereby converting the chlorophenols to chloroanisoles. The chloroanisoles have sufficient vapor pressure and extremely low sensory thresholds such that breathing the chicken house air resulted in a taint in the meat. The fourth example given by Goldenberg and Matheson16 is a good example of bacterial enzymes producing an off-flavor in foods. Soapy flavors are characteristic of medium chainlength free fatty acids (decanoic (XXXIX) and lauric acids

Volume 29, Issue 6

Food Science and Nutrition 0.14

0.135-

(1:6) CARTONS (1:4) CARTONS NO CARTONS 0.11

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1 1.5 2 TIME (WEEKS)

2.5

FIGURE 4. Browning of orange juice stored at 25°C as affected by contact with carton strips.5'

development from microbial growth. The cells of thefishmuscle readily hydrolyze once death of the fish has occurred. Cell hydrolysis yields a very rich nutrient broth for microbial growth. Fresh fish is generally considered to have little odor. As the fish is stored (>0°C), an odor develops that is initially characterized as being "fishy". The odor changes as product deterioration advances to what is often described as being "intense, putrid, and foul". Substantial changes occur during storage of sterile fish tissue that are due to enzymatic action. These changes are very obvious if one measures any of several chemical parameters (volatile acids, amines, volatile reducing substances, carbonyls, etc.). However, it has been proven that tissue enzyme activity contributes little to the flavor changes that occur during spoilage.66 Bacterial action is primarily responsible for spoilage odors. There has been a great deal of work published determining the organisms responsible for off-flavors in fish. Since the subject of this article is not microbiology but chemistry of offflavors, I only mention that the organisms responsible for spoilage of fish generally are Gram-negative bacteria of the genera Pseudomonas, Achromobacter, and Vibrio.61 It is of interest that within these genera of organisms, only some species can cause spoilage odors. Also, only a small proportion (