Int. Dairy Journal 1 (1991) 1-11

Off-flavors of Whey Protein Concentrates: A Literature Review

C. V. M o r r & E. Y. W. H a

Department of Food Science and Technology, The Ohio State University, Columbus, Ohio 43210-1097, USA

(Received 17 September 1990; revised version received and accepted 18 December 1990)

ABSTRACT

Considerable progress has been made in technology development for manufacturing whey protein concentrates (WPC) that contain up to 75% protein. However, these products also contain variable amounts of residual lactose and 3-7% lipid materials that are susceptible to chemical reactions resulting in the development of typical stale and aged off-flavors in the product. New technologies including chemical pretreatment, microfiltration and ultrafiltration/diafiltration are now available for removing these materials more effectively from whey," this should enable the industry to manufacture WPCs with improved flavor stability and functionality. This paper considers the chemistry of off-flavor formation in WPC products in comparison to milk and whey by Maillard reaction, lipid oxidation, and riboflavin decomposition during storage and suggests whey pretreatment processing technology to improve the flavor stability of WPC.

I N T R O D U C T I O N

The US dairy industry manufactures in excess of an est imated 140 mil l ion lbs of spray dried whey protein concentrate (WPC) annual ly and an addit ional , but substantial ly smaller, a m o u n t o f whey protein isolate (WPI). In addit ion, the US impor ted an est imated 1-4 mil l ion lbs of WPC

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Int. Dairy Journal 0958-6946/91/$03.50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Ireland

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and 3.3-8-5 million lbs milk protein concentrate in each of the last two years (American Dairy Products Institute, 1990). Although WPC products generally exhibit a bland flavor immediately after drying, they develop a typically stale off-flavor during storage due to a set of complex, inter-related chemical reactions which include lipid oxidation and Maillard browning (Hill et al., 1969; Ferretti et al., 1970; Ferretti & Flanagan, 1971a, b, 1972; Hammond, 1989). There is no information in the literature on the volatile organic compounds responsible for off- flavors in WPC.

The off-flavors developed in WPCs during storage greatly limit their use in many bland food products (Hugunin, 1987). Factors that may impact on the development of off-flavors in WPC include processing treatments which affect the composition of the whey and WPC; drying conditions which control moisture content of the WPC and initiate lipid oxidation and non-enzymatic browning reactions; and moisture and temperature conditions which affect the kinetics of the lipid oxidation and browning reactions during WPC storage (Morr, 1976, 1979a, b; Morr & Foegeding, 1990).

Ultrafiltration and ion exchange adsorption technology have been used for the manufacture of WPCs and WPIs, respectively (Morr & Foegeding, 1990). The high quality of WPIs, in terms of compositional, sensory, and functional properties, sets the standard for the industry to produce WPCs with improved properties.

FACTORS FOR OFF-FLAVOR DEVELOPMENT IN DRIED WHEY PRODUCTS

The subject of flavors and off-flavors in milk and dairy products has received much attention over the past 10-15 years (Shipe et al., 1978; Forss, 1979; Baddings & Neeter, 1980; Thomas, 1981; Adda et al., 1982; Heath, 1983; Manning & Nursten, 1985; Hammond, 1989). However, the amount of published research on off-flavors of dried milk and whey products has been relatively limited (Ferretti & Flanagan, 1971 a, b, 1972; McGugan et al., 1979).

Aged, stale off-flavor is the single most important flavor criticism of dried whey and WPC products. Although numerous factors may be responsible for the formation of off-flavors in stored whey and WPC, two major factors are usually involved, i.e. lipid oxidation and Maillard browning reaction (Ramshaw & Dunstone, 1969; Ferretti & Flanagan, 1972; Min et al., 1990).

McGugan et al. (1979) reported that a major portion of fresh cheese

Off-flavors of wh~' protein concentrates: a literature review 3

whey flavor is dialyzable, thus it should be readily removed by ultrafiltration and diafiltration steps of WPC manufacture. This, plus further losses due to volatilization during drying accounts for the low concentration of whey flavor in freshly manufactured WPC.

Ferretti and Flanagan (1971b) identified 24 volatile compounds in a steam distillate from commercial spray dried whey powder that included: seven alkylpyrazines, three furans, two pyrroles, alkylpyrazines, furans, pyrroles, a-methyl-y-butyrolactone, isobutyramide, N-methyl-2- pyrrolidinone, 3-hydroxy-2-butanone, benzaldehyde, phenol, benzyl alcohol, maltol, dimethylsulfone, and propionic, butyric and benzoic acids.

Based on results obtained for non-fat dry milk (Driscoll et al., 1985), storage temperature would be expected to be one of the most important factors affecting the quality of spray dried WPC. Non-fat dry milk (NFDM) developed a detectable concentration of off-flavor compounds after 6 months storage at 32°C, but became unacceptable for use after 24 months storage at this temperature due to the development of an oxidized, stale off-flavor. Kieseker and Clarke (1984) reported that the flavor of NFDM with < 4% moisture was stable when stored at 20°C, but underwent rapid flavor deterioration and became unacceptable for food use within 6 months at 40°C.

Sensory data for three commercial whey protein isolates (WPI) that contained < 1% total lipid and < 0.5% lactose were recently reported in comparison to eight commercial whey protein concentrates (WPC) that contained 3.3-7.4% total lipid and 2-5% lactose (Morr & Foegeding, 1990). The three WPIs exhibited a totally bland flavor with a slight indication of old whey powder off-flavor with a 0-2 intensity on a 0-5 scale. The eight WPCs exhibited a stale, old whey powder flavor with a 4-5 intensity rating on a 0-5 scale. These results indicate that lipid and lactose concentration may be very important in controlling the flavor stability of WPCs and WPIs. Whey processing modifications that further reduce the concentration of these two components should be beneficial for improving the flavor stability of WPC.

MAILLARD BROWNING REACTION IN DRIED MILK AND WHEY PRODUCTS

The Maillard browning reaction, which involves interaction of protein and lactose as its initial step, is believed to be important in the formation of stale off-flavors in dried milk and whey products (Morr & Richter, 1988). Fors (1983) reviewed the literature pertaining to nearly 450 products

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arising from this reaction. Work by Ferretti and Flanagan (1971a, b, 1972), who investigated the volatile compounds formed by the Maillard browning reaction using model lactose--casein and whey systems, demonstrated that stale flavor formation in dried milk and whey pro- ducts is due to a combination of at least five different chemical compounds.

Aldehydes produced by the Strecker degradation of the amino acids phase of the Maillard browning reaction undergo condensation and a series of further reactions to form furfurals, dehydration products and polymers that result in the brown pigments plus off-flavor (Hodge et al., 1972; Hammond, 1989). The amount of carbon dioxide formed, which is also a major product of the Strecker degradation of amino acids phase of Maillard browning, is correlated to the intensity of brown pigment formed as well as to the oxidative stability of the lipids in the milk powder (Min et al., 1990).

The factors affecting browning reaction rate in dried food products include temperature, water activity, pH and availability of reactant (O'Brien & Morrissey, 1989; Labuza & Saltmarch, 1981a, b). The rate of the reaction at low temperature is quite low, however, it is the predominant reaction at temperatures of > 35°C, especially in food systems with an intermediate moisture content (Labuza & Saltmarch, 198 l a). Maillard reaction has a relatively high temperature coefficient, e.g. the Qi0 for pigment formation is in the range 3-6 (Labuza & Saltmarch, 1981b). Labuza and Saltmarch (1981a) reported the maximum browning reaction rate in whey powder is at water activity 0-44, which is considerably lower than the usual values of 0-6-0.8 for many dried foods (Labuza & Saltmarch, 1981a, b).

The rate and extent of Maillard reaction in dried whey and WPC can be controlled by altering their composition. For example, Ramshaw and Dunstone (1969) reported that non-enzymatic browning of non-fat dry milk could be inhibited by removal of lactose. The rate of brown pigment formation increases significantly at elevated pH values (O'Brien & Morrissey, 1989). The extent of carbon dioxide formation was decreased when antioxidants were added to NFDM (Minet al., 1989, 1990). Added antioxidants prevent the formation of lipid oxidation products as well as non-enzymatic browning by inhibiting oxidation of protein sulfhydryl groups (Pokorny, 1981).

Preheating milk to temperatures of 85-88°C for 15-20 min produces free sulfhydryl groups which reportedly inhibit browning and mask undesirable flavors that develop during storage of the dried product (Mook & Williams, 1966). In practice, preheating is commonly used in the manufacture of evaporated milk and powdered milk to increase heat stability (Morr & Richter, 1988; Singh et al., 1989).

Off-flavors of whey protein concentrates." a literature review 5

LIPID OXIDATION IN DRIED MILK AND WHEY PRODUCTS

Oxidized milkfat is commonly described as having an 'oxidized' or 'cardboard' flavor (Hammond, 1989), which is different from the stale flavor formed in dried whey and WPC. Oxidation of milkfat results in the formation of undesirable volatile compounds in milk powder and whey products (Hill et al., 1969) and is one of the major factors hindering full commercial exploitation of whole milk powder. The fatty acids of major importance to this reaction in dried milk and whey products include linoleic acid, linolenic acid and the other polyunsaturated fatty acids that are especially susceptible to oxidation (Hammond, 1989). The reaction of oxygen with unsaturated fatty acids results in the formation of hydroperoxides which decompose to form a multitude of aldehyde and related chemical compounds (Frankel, 1985; Hammond, 1989) that are reactive and are believed to promote the Maillard browning reaction. Oct-l-en-3-one, 2,4-dienals, trienals and cis-4-heptenal have been implicated as being responsible for off-flavors in products that have undergone milkfat oxidation.

Lipid oxidation is a source of a variety of non-volatile components as well as volatile components responsible for undesirable flavors in milk and whey products. Non-volatile components include hydroperoxides, trihydroxy fatty acids and other oxygenated forms of fatty acids, oxidized lecithins and fatty acid dimers. Autoxidation of unsaturated fatty acids can occur during the process. The degradation of milk lipids has been studied extensively and the progress made has been quite substantial (Day & Lillard, 1960; Lillard & Day, 1961; Sattar & deMan, 1975; Schwarz & Parks, 1978; Walstra & Jenness, 1984). The reaction of unsaturated fatty acids with oxygen to form hydroperoxides is generally a free radical process. The formation of free radicals in the initiation step of autoxidation can take place by thermal or photodecomposition of peroxides or hydroperoxides, by metal catalysis and by ultraviolet irradiation. The exact nature of the initiation step is not fully understood, although it is known that initiation can be encouraged by suitable radicals, including those produced by a metal-catalyzed decomposition of preformed hydroperoxides (Gunstone, 1984). The role of singlet oxygen in the primary initiation reaction has been postulated (Rawls & van Santen, 1970). The reactivity of singlet oxygen with unsaturated fatty acids is about 1450 times greater than triplet oxygen (Rawls & van Santen, 1970; Min & Lee, 1988). The possible formation of free radicals in dry milk proteins by mechanical energy has been reported (Hansen et aL. 1970).

Through autoxidation, phospholipids have the potential to participate in the generation of off-flavors in milk and whey products. It has been

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reported that phospholipids have an antioxidant activity due to their mineral ion chelating ability or to their synergistic relation with other antioxidants. At the same time, phospholipids are rather easily oxidized or discolored during heating or storage, and may become precursors of off-flavors in the finished products (Porter, 1985; Min & Jung, 1989; Szuhaj & Sipos, 1989; Hamzawi, 1990). The phospholipid fraction of milk is oxidized more readily than the glyceride fraction and is therefore considered as one of the off-flavor precursors in milk (Sattar & deMan, 1975).

RIBOFLAVIN

Decomposition of riboflavin by photolysis has been reported to contribute to the undesirable flavors in milk (Sattar & deMan, 1975; Allen & Parks, 1979; Allen & Joseph, 1985; Gaylord et al., 1986; Deger & Ashoor, 1987; Olson & Ashoor, 1987) and would presumably be expected to function in dried whey products as well. Aurand et al. (1966) stated that riboflavin is the primary factor responsible for light-induced 'oxidized' flavor in fluid milk, whereas ascorbic acid is a secondary factor. The mechanism for the photolysis of riboflavin is dependent on the pH of the solution. In acid or neutral solutions the product is lumichrome, which is intensely fluorescent, and in alkaline solution, the product is lumiflavin (Wishner, 1964). Light-induced off-flavor was observed in skim milk, whey, and serum protein fractions, but not in casein and free amino acid fractions, indicating that serum proteins are the primary source material of this off-flavor (Sattar & deMan, 1975). Toyosaki & Mineshita (1990) studied the mechanism of the photolysis of riboflavin in a model system; they reported the formation of two riboflavin decomposition products and acceleration of riboflavin photolysis in the presence ofa-lactalbumin and fl-lactoglobulin. They also reported that hydroxyl radicals as well as active oxygen were involved in the decomposition of riboflavin. Whether riboflavin photolysis is also involved in off-flavor development of WPC has not been determined.

WHEY PROCESSING TECHNOLOGY TO IMPROVE THE FLAVOR OF WPC

Commerical WPIs exhibit considerably higher flavor quality and functionality than WPCs (Mort & Foegeding, 1990). WPCs contain sufficient residual lactose, lipids, phospholipids, lipoproteins and Cu E+

Off-flavors of whey protein concentrates: a literature review 7

and other pro-oxidants to render them susceptible to lipid oxidation and MaiUard browning reactions that result in the development of aged, stale off-flavors during dry storage. The poor flavor and relatively high lactose and mineral content of the WPCs represent a serious problem which limits their acceptance and use by the food industry (Hugunin, 1987; Morr & Foegeding, 1990).

The functional properties of whey protein concentrate are adversely affected by the residual lipid content ofWPC (Morr, 1979a; Richert et al., 1974). These residual lipids are believed to be mainly phospholipo- proteins that are released from the milkfat globule membrane into the whey during cheesemaking. These phospholipoproteins along with small sized milkfat globules that are not removed from whey are concentrated along with the proteins in the retentate fraction during ultra filtration (UF). They lead to UF membrane fouling and associated loss of flux (Lee & Merson, 1976; Morr, 1987; Kim et al., 1989; Rinn et al., 1990). It is generally understood that the removal ofphospholipoproteins from cheese whey prior to ultrafiltration (UF) and diafiltration (DF) in the manufacture of WPC and WPI improves compositional and functional properties of the final products (Maubois et al., 1987; Kim et al., 1989; Morr & Foegeding, 1990; Rinn et al., 1990). However, nothing has been published to confirm that such treatments result in improved WPC flavor stability.

Newer processing technologies, such as chemical and physical pretreatment, have been developed to manufacture WPC with reduced concentrations of residual milkfat and phospholipoprotein complex material (Maubois et aL, 1987; Kim et al., 1989; Rinn et aL, 1990). Storage under N2 atmospheres and use of Cu2+-complexing substances and antioxidants results in some flavor improvement, but inclusion of additional clarification steps to reduce the lipid content of whey prior to and during concentration by ultra filtration may provide the most effective improvement in flavor stability. New and improved ultra- filtration and diafiltration (UF/DF) technology is also being used to manufacture WPC with lower lactose content (< 5% w/w), which should effectively improve the flavor stability of WPC by reducing the extent of Maillard browning reaction (Morr & Foegeding~ 1990).

Care must also be exercised to minimize the temperature and time of heat processing treatments of whey and UF/DF retentate prior to and during spray drying of the WPC. Such heat treatments may initiate the browning reaction, which could result in accelerated brown pigment and off-flavor production in spray dried WPC product during storage. The relatively short product residence time required to concentrate the UF/DF retentate fraction with a falling-film evaporator results in significantly

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less browning and off flavor production than by the conventional vacuum evaporators (Morr, 1976; Morr & Richter, 1988).

FUTURE RESEARCH NEEDS

Flavor is generally recognized as the single most important factor affecting consumer acceptance of food products. Commercial WPCs generally exhibit a significant level of aged, stale off-flavor, which greatly limits their utility as food ingredients. In order to control this off-flavor problem in WPC, it will likely be necessary to utilize chemical pretreatment alone or with ceramic or metallic membrane micro- filtration technology and UF/DF processing to remove residual phospholipoprotein complex. Research is needed to investigate the chemical reactions, i.e. lipid oxidation and Maillard browning reactions, responsible for off-flavor production in spray dried WPC. This information, along with reliable sensory data, should be beneficial for devising processing schemes for producing WPC with improved flavor stability. Additional research is also needed to characterize the resulting WPCs to determine whether more complete removal of the residual phospholipid complex results in improved flavor profile in addition to functionality as indicated by results from preliminary studies.

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