Journal of Dairy Research (1979), 46, 369-376

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Utilization of milk proteins as starting materials for other foodstuffs BY CHARLES V. MORR* Department of Food Technology, Texas Tech University, Lubbock, Texas 79409, USA The modern food-processing industry is placing more and more emphasis upon the utilization of protein ingredients to provide specific functional properties to a wide range of formulated foods. Isolated milk protein products represent an important and valuable source of protein ingredients due to their recognized superior nutritional, organoleptic and functional properties. This paper provides up-to-date information on the quantities, production processes, composition, general properties, and specific functional properties of the major milk protein products, e.g. caseinates, co-precipitates, lactalbumin, whey protein concentrates and milk blends. The subject of chemical and enzymic modification to improve certain functional properties of milk proteins is considered briefly. SUMMARY.

Traditional and newly developed protein products are being extensively utilized as ingredients in an increasing number of formulated foods. They provide the food industry with additional opportunities in research and development of new and improved food products. These proteins are derived from a variety of sources, e.g. milk, meat, eggs, oilseeds, cereals, micro-organisms, by a number of diverse processes. Development and utilization of such protein ingredients represents one of the most important and interesting developments in the food manufacturing industry in recent years. Their importance stems from their nutritional contributions, and also from their ability to provide unique and essential functional properties to the food system. Due to increasing emphasis by the news media upon governmental regulations and nutritional and health topics, consumers are becoming more knowledgeable and concerned about the ingredients in their foods. Since milk, the source material for milk protein products, is associated with wholesomeness and health, and since milk proteins are universally recognized for their excellence in nutritional value, milk protein products are receiving increasing attention as formulation ingredients by the food manufacturing industry. Although non-fat dry milk and dried whey have been the major sources of milk protein ingredients for the food manufacturing industry in the past, a number of relatively new and versatile milk protein products are available, which offer special advantages for certain food applications. These milk protein products are generally produced from low-cost milk and whey, largely in New Zealand, Australia, USA, The Netherlands and Canada. Milk-pricing policies of the respective governments have a significant effect upon the relative importance of the various milk protein products that are being produced. A most important over-riding factor is the rapidly * Present address: Department of Food Science, Clemson University, Clemson S.C. 29631, USA 0022-0299/79/1728-2034 $01-00 © 1979 PrJDR

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Table 1. General requirements of food protein ingredients (1) Free of toxic and anti-nutritional factors: i.e. phytate; flatulence-causing factors; anti-proteolytic inhibitors; anti-haemagglutinins; mycotoxins and microbial toxins; pathogenio micro-organisms; amino acid derivatives (2) Free of off-flavours and pigments (3) High-protein concentration (4) Compatible with ingredients/processes (5) Provide functionality (6) Provide nutritional quality (7) Readily available (8) Low cost

expanding production of cheese whey, which provides a large supply of high quality protein for human consumption. This paper presents current information on the amount, production processes, composition, functional properties and major applications of the important milk protein products available today. Recent review articles by Borst (1971) and Muller (1971), as well as a personal communication from Dr N. J. Walker of New Zealand Milk Products Inc., and several booklets by Hugunin & Lee (1977) and Hugunin & Ewing (1977) provided much of the information used in this article. Milk protein products are complex protein systems that contain a number of major and minor components, all of which possess complex conformational states and undergo important reactions and interactions during their isolation and utilization (Morr, 1975, 1978). Although such information would readily explain many of the major chemical and functional properties of the milk protein products, it is largely outside the scope of this paper. General requirements of food protein products Food protein products must meet a number of general requirements to be selected as ingredients in formulated foods (Table 1). For example, they must be readily available, competitive in cost for the particular application, and be compatible with the processing parameters required as well as with the other formulation ingredients. In addition, they should contain low levels of residual components from their source material, e.g. toxic and anti-nutritional factors, pigments, and off-flavours that adversely affect their acceptability in the food formulation. One additional factor of major importance in certain applications is the relative nutritional value of the protein ingredient. It is universally recognized that milk protein products possess excellent nutritional properties due to their favourable balance of essential amino acids and to their high degree of digestibility. Functional properties of food protein products In addition to the above general requirements, food protein products must also provide at least one of a number of specific functional properties (Table 2) in order to be selected for a particular food application. Most of these functional properties are dependent upon the protein's conformation (Morr, 1978), solubility and waterbinding properties (Chou & Morr, 1978). Milk protein products satisfy most of these functional requirements and are therefore receiving considerable attention from the food industry.

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Table 2. Specific functional properties of protein products Functional property Emulsification Stabilization Fat absorption Water absorption/retention Viscosity Gelation Fibre/texture Dough formation Adhesion/cohesion Aeration

Food application Coffee whitener, whipped topping, soup, meat products Soups, meats, desserts Meat products Meat products, bread, cakes, confections Soups, gravies, chili Simulated meats, cheese Simulated meats Baked foods Meat products, baked foods Whipped toppings, chiffons, confections, desserts

Table 3. Estimated annual USA production/utilization of milk protein products, 1977 Product Non-fat dry milk Casein/oaseinates Co-precipitates Partly delaotosed whey Partly demineralized whey Whey protein concentrates Whey solids in protein blends Whey solids in dry whey

Amount, 1000 metric tons 421*

66t 5t

14-5* 11-8* 3-6* 12-7* 210*

Protein content, % dry basis 36 95 90 20 13 50 13 13

Protein, lOOOmetrio tons 151-6 62-7 4-5 2-9 1-5 1-8 1-6

27-3

• Statistical Reporting Service, USDA (June 1977). •f N. J. Walker, personal communication.

General properties of milk protein products A general consideration of milk protein products indicates that they are available in a wide variety of forms, e.g. acid casein curd, rennet casein curd, Na/K/Ca caseinate, casein/whey protein co-precipitate, lactalbumin, partly delactosed whey, partly demineralized whey, whey protein concentrate and as blends of milk proteins with soy and cereal proteins. Each of these milk protein products has its own characteristic properties with respect to composition, nutritional value, flavour, solubility and functionality. For example, caseinates, co-precipitates and lactalbumin contain from 90 to 95 % protein, whereas whey protein concentrates generally contain only from 30 to 50 % protein with correspondingly higher levels of lactose and milk salts. Casein, Ca caseinate, lactalbumin and the co-precipitates are less soluble than Na and K caseinates and whey protein concentrates, and thus would not be selected for those applications for which functionality is dependent upon a high solubility. Although all milk protein products have excellent nutritional value, those that contain the highest proportions of whey proteins, with their rich supply of S-containing amino acids, exhibit the highest protein efficiency ratios (PER). Milk protein products are subject to development of off-flavours during storage, which are described as stale, musty or gluey, (Walker, 1977; Muller, 1971). These flavours are characteristic of dried milk and whey products in general and their origins are believed to be oxidized lipids, degradation of tryptophan, and the Maillard reaction between the proteins and residual lactose. As mentioned by Muller (1971), major emphasis is being given to production of

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casein curd, caseinate and other milk protein products by modern sanitary processes that will permit their use in human foods. These and other process modifications, such as the use of charcoal and other adsorbants for removing off-flavour components, are being considered for improving their acceptability for use in formulated foods. Estimated values for amounts of the major milk protein products produced and utilized in the USA are given in Table 3. Additional details on the applications of milk protein products in processed meats, synthetic meat products, beverages, cereals and pasta products, snack foods and imitation food products are given by Hugunin & Lee (1978), Hugunin & Nishikawa (1978), and Anonymous (1977). Preparation and properties of casein curd and caseinates Muller (1971) and Borst (1971) both reviewed the history and current status of the casein and caseinate manufacturing industry, which is heavily concentrated in New Zealand and Australia. The annual world production of these products has stabilized at about 100-120 thousand metric tons (N. J. Walker, personal communication). Although annual USA imports of casein and caseinates have fluctuated around 40-55 thousand metric tons over the last 20 years, there has been a steady shift from industrial to food uses (Borst, 1971; Muller, 1971). Casein curd is produced from pasteurized skim-milk by either treating with rennet, or direct acidification with acid, or by culturing with micro-organisms to produce acid. All of these treatments precipitate the casein or cause it to form an insoluble clot which can easily be separated from the whey fraction, washed and dried. Acid casein curd contains only about 2 % ash compared to 7-5% ash for rennet casein curd, which retains most of the colloidal phosphate of the casein micelles. Both of these caseins, unless subjected to enzymic or chemical modification, remain insoluble and are therefore useful for those functional applications which do not require solubility, e.g. breakfast cereals, protein supplements and baked foods. Rennet casein curd has traditionally been used in the commercial plastics industry, but there is relatively less demand for this product now, due to competition from other cheaper polymer products. Na, K and Ca caseinates are produced by neutralizing casein curd with the corresponding base to pH 6-8-7-5 and drying the solubilized form. Na caseinate, produced directly from wet casein curd, generally has a better flavoiir than if produced indirectly from previously dried casein or Ca caseinate (Towler, 1976). It is important to avoid elevating the pH above the 6-8-7-5 range to minimize development of offflavours by reaction of alkali with residual lipids, which are presumably bound to the hydrophobic amino acid residues of the casein molecules. It is also important to avoid exposure of the caseinate to elevated pH because of the likelihood of catalysing various cross-linking reactions that might produce potentially toxic amino acid derivatives. Na and K caseinates both have excellent solubility, heat stability, water absorption and, due to their unique amphiphilic conformation (Morr, 1978) perform without equal in stabilizing oil-in-water emulsions and aqueous foam systems. Na and K caseinates are experiencing excellent acceptance within the meatprocessing industry as water- and fat-binding agents in formulated meat products. They are being used in substantial amounts as binders in texturized vegetable protein products and in the food industry as emulsifiers in a wide range of products including margarine, toppings, cream substitutes, coffee whiteners, and as foam stabilizers in whipped/foamed foods. Na caseinate has a high viscosity and forms a

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gel at concentrations above 17% (N.J.Walker, personal communication). Additional details of food applications for these protein products are given by Hugunin & Lee (1978). Ca caseinate, on the other hand, forms a colloidal suspension rather than a solution as do Na and K caseinates. It exhibits a wide range of particle sizes, which destabilize upon heating, but tend to redissolve upon recooling (Roeper, 1977). It does not bind as much water (Knightsbridge & Goldman, 1975) and has a much lower viscosity than Na and K caseinates under similar conditions. Production and properties of co-precipitates Casein and whey protein co-precipitates are produced by heating skim-milk sufficiently to denature and complex the whey proteins with the caseins by disulphide interchange (Southward & Goldman, 1975). Following heating, the coprecipitate is formed either by adding acid to lower the pH to 4-6, or by adding CaCl2. These variations, as well as those in the curd-washing process, permit the production of high Ca or low Ca co-precipitates, whose ash contents range from about 2-5 to 8%, with an average value of 4-5%. These co-precipitates are insoluble and resemble acid casein curd, since they contain about 83% casein (Southward & Goldman, 1975; Borst, 1971). An additional variation in the process is to solubilize the co-precipitates by treating with NaOH or Na tripolyphosphate (TSPP). However, the denatured whey proteins remain largely insoluble and thus the final product resembles Na caseinate with the inclusion of insoluble lactalbumin (Borst, 1971). An additional variation is also available, whereby additional proteins and stabilizers may be incorporated into the co-precipitate by suspending them in the milk prior to heating (Southward & Goldman, 1975). This approach affords additional flexibility in designing food protein products for specialized requirements. Little work has been reported on the functional properties of co-precipitates (Southward & Goldman, 1975), but they would probably be most useful in those applications that require high nutritional quality and water absorption, but without significant protein solubility, such as with breakfast cereals, snacks and pastas. Production and properties of lactalbumin Conventional lactalbumin is produced by adjusting the pH of whey to 4-5-5-2 and heating to denature and precipitate 70-80 % of the whey proteins (Robinson, Short & Marshall, 1976; Borst, 1971). The resulting insoluble precipitates resemble casein and co-precipitates and exhibits similar properties in food applications. The composition varies from about 89 to 92 % protein and exhibits a variable amount of ash, depending on the washing steps of the preparation process. The ash content is higher for lactalbumin which has been solubilized by treating with TSPP, as with the co-precipitates above. Food applications for this protein product are similar to those for the other insoluble milk protein products, e.g. casein, Ca caseinate, and co-precipitate. It would be expected to have a slightly higher nutritional value than caseinates and caseins, due to its high concentration of lysine and S-containing amino acids. However, the likelihood of interaction of protein and lactose during the heating of whey to form the co-precipitate would probably result in a product with minor colour and flavour defects (Robinson et al. 1976). Due to its low solubility, and its high water absorption

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and nutritional properties, lactalbumin is being used in a variety of baked products, meat products, yoghurt, processed cheese, confectionery foods and snack foods (Southward & Goldman, 1975). Jelen (1975) reported also that lactalbumin type whey protein products function well as ground meat extenders. A new, continuous lactalbumin process has been reported by Panzer et al. (1976), whereby whey is adjusted to pH 6 and heated to 120 °C by steam injection to denature and precipitate the whey proteins. The resulting lactalbumin contains 20-25 % ash, which is considerably higher than for conventional lactalbumin. An optional modification was also developed, in which the heated whey is adjusted to pH 4-6 to simultaneously aid in protein precipitation and lower the ash content to 2-5%. These protein products have also been found to function well in fortifying macaroni and similar pasta products, which require that the protein remains insoluble (Seibles, 1975; Schoppert et al. 1976). An additional modification of the procedure to precipitate and recover lactalbumin on a continuous basis is the Centri-Whey process which recovers heat-precipitated whey proteins as a concentrate for re-incorporation into cheese milk (Anonymous; Haustein, 1972). It is reported that this operation has been functioning well for over 10 years and has produced excellent results with Camembert and Port Salut type cheeses. It would be expected that cheese made with increased levels of whey proteins would exhibit a higher PER value. Heat precipitated whey proteins have been used for many years for production of whey cheese in European and Scandinavian countries (Robinson et al. 1976). Modler & Emmons (1977) developed a novel approach for preparing a lactalbumintype whey protein product which possesses a solubility that approaches that of whey protein concentrate. They adjusted whey to pH 2-5-3-5, heated at 90 °C, cooled, and adjusted to pH 4-5 to precipitate the denatured whey proteins. The intriguing aspect of this process is that the resulting product, which contains about 40 % protein on a dry basis, exhibits such a high protein solubility. Heating at pH below the isoelectric point apparently permits protein denaturation without adversely altering solubility of the protein at pH above the isoelectric point. Enzymic modification of lactalbumin As indicated above, lactalbumin exhibits only limited functionality in most food systems, due mainly to its extremely low solubility. Researchers have been attempting to modify it by enzymic treatment to improve its functionality (Robinson et al. 1976; Jost & Monti, 1977). Although it remains to be proven that the resulting product possesses acceptable flavour and functionality for the food industry, the heat precipitation/enzymic solubilization approach might offer significant economic advantages over pressure/membrane and related molecular sieve approaches for preparing whey protein concentrates. Production and properties of whey protein concentrates Considerable interest has been generated for developing processes suitable for large-scale production of whey protein concentrates with a minimum of protein denaturation, and concomitant losses of solubility and functionality (Morr, 1976). The broad subject of whey protein concentrates has been reviewed with respect to whey utilization, methods and cost of reclaiming whey protein, and utilization and functional characteristics of whey protein concentrates (Mathur, 1977).

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Such whey protein concentrates are being produced on a fairly extensive scale by several major food companies in the USA and others are seriously considering the advisability of entering the field. Major processes have been developed on a pilot or commercial scale for recovering whey proteins from whey. These include: ultrafiltration, gel-filtration, metaphosphate complex, carboxymethyl cellulose complex, and electrodialysis/lactose crystallization processes to preferentially concentrate and fractionate the proteins from the lactose, minerals and other whey components. Although whey protein concentrates generally have good solubility and function reasonably well in certain food applications, they are not as functional in certain important food systems as are caseinates and soy proteins (Morr, Swenson & Richter, 1973). For example, whey proteins do not function as well as egg proteins in baked food applications because they do not form a sufficiently strong foam structure to retain loaf volume. However, they do contribute beneficial attributes to such products as confections and related foods. Similarly, they are not capable of stabilizing emulsions or foams as effectively as caseinates. The explanation for the relatively poor functional properties of whey protein concentrates in baked food applications probably resides in their relatively low sulphydryl and disulphide contents, compared to those of egg proteins. Thus, they may not be capable of forming a firm, 3-dimensional structure to stabilize loaf or cake volume. Their inferior ability to stabilize emulsions and foams is likely to be due to their compact, globular conformation, which does not exhibit sufficient amphiphilic properties to orientate the water/oil or water/air interface (Morr, 1978). However, it appears that their surfactant properties may be significantly improved by heat denaturation of the proteins, providing that the denaturation is conducted just prior to forming the foam (Richert, Morr & Cooney, 1974). One potentially important area for whey protein concentrates in food formulations is in acid type foods, e.g. carbonated, fortified beverages (Holsinger et al. 1974). Whey protein concentrates, provided they are in an undenatured state, retain sufficient solubility to provide reasonable clarity in beverages in the acid pH range. Further developments are possible in this area, but must await development and marketing input from the industry. Whey protein concentrates prepared as metaphosphate and carboxymethyl cellulose complexes have serious limitations in functionality, especially at pH levels approaching the isoelectric point, e.g. pH 4-6-5-0. Reduced solubility and excessive viscosity tend to limit their utility in a number of food applications. Whey protein concentrates prepared by electrodialysis/lactose crystallization do not contain a sufficiently high protein content, e.g. usually 20-35 %, for certain food applications. However, such products do offer special utility in formulating milk-based foods that compare closely with human milk in casein to whey protein ratio as well as various mineral ratios (Stribley, 1963). Thus far whey protein concentrates have not competed satisfactorily with non-fat dry milk, caseinate and soy proteins in many important formulated food applications. This situation is due to an unfavourable cost comparison, as well as their failure to to provide substantial functional advantages. Production and properties of milk protein blends One of the newest and most interesting trends in food protein products is the development of milk protein blends. Such blends are formulated from whey, whey 13

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protein concentrates, caseinates and other proteins, e.g. mainly soy proteins (Anonymous, 1978). They are produced and marketed by milk protein product producing companies, using proprietary methods, which usually involve dry or wet blending or co-precipitation of the protein components, heating to modify the proteins, and spray-drying. It is claimed that these blends match or even exceed specific functional requirements of the individual protein components, are more compatible with processing conditions, reduce the cost, and result in a superior final food formulation. The list of applications for these protein blends is impressive and it is likely that the trend towards their increased production and utilization will continue in the future, especially if governmental food assistance programmes are expanded (Holsinger et al. 1974). REFERENCES ANONYMOUS (1977). Dairy Council Digest 48, Utilization of Milk Components by the Food Industry Rosemont, HI, USA: National Dairy Council. ANONYMOUS. Gentri Whey Bulletin, Publication No. 60067, Alfa-Laval Company. ANONYMOUS (1978). Food Product Development 12, 52, and 54. BORST, J. R. (1971). Food Technology in Australia 23, 544. CHOU, D. & MOBB, C. V. (1978). Journal of American Oil Chemists' Society (in the Press). HAUSTEIN, G. G. (1972). Deutsche Molkerei-Zeitung 93, 1492. HOLSINGER, V. H., POSATI, L. D. & DEVILBISS, E. E. (1974). Journal of Dairy Science 57, 849. HOLSINGER, V. H., SUTTON, C. S., VETTEL, H. E., EDMONDSON, L. F., CROWLEY, P. R., BERNSTON, B. L.

& PALLANSCH, M. J. (1974). Proceedings IV International Congress Food Science and Technology, vol. 6, pp. 25-33. HUGUNIN, A. G. & EWING, N. L. (1977). Dairy Based Ingredients for Food Products. Rosemont, 111., USA: Dairy Research Inc. HUGUNIN, A. G. & LEE, S. M. (1977). A Fresh Look at Dairy Based Ingredients. Rosemont, 111., USA: Dairy Research Inc. HUGUNIN, A. G. & NISHIXAWA, R. K. (1978). Food Product Development 12, 46. JELEN, P. (1975). Journal of Food Science 40, 1072. JOST, R. & MONTI, J. C. (1977). Journal of Dairy Science 60, 1387. KNIOHTSBRIDGE, J. P. & GOLDMAN, A. (1975). New Zealand Journal of Dairy Science and Technology 10, 152. MATHUB, B. N. (1977). Thesis, Panjab University , Chandigarh, India. MODLER, H. W. & EMMONS, D. B. (1977). Journal of Dairy Science 60, 177. MORR, C. V. (1975). Journal of Dairy Science 58, 977. MORR, C. V. (1976). Food Technology 30, 18. MORR, C. V. (1978). Journal of Agricultural and Food Chemistry (in the Press). MORR, C. V., SWENSON, P. E. & RICHTER, R. L. (1973). Journal of Food Science 38, 324.

MULLER, L. L. (1971). Dairy Science Abstracts 33, 659. PANZER, C. C , SOHOPPET, E. F., SINNAMON, H. I. & ACETO, N. C. (1976). Journal of Food Science 41,

1293. RIOHERT, S. H., MORR, C. V. & COONEY, C. M. (1974). Journal of Food Science 39, 42.

ROBINSON, B. P., SHORT, J. L. & MARSHALL, K. R. (1976). New Zealand Journal of Dairy Science and Technology 11, 114. ROEPER, J. (1977). New Zealand Journal of Dairy Science and Technology 12, 182. SOHOPPET, E. F., SINNAMON, H. I., TALLEY, F. B., PANZER, C. C. & AOETO, N. C. (1976). Journal of Food

Science 41, 1297. SEIBLES, T. S. (1975). Cereal Foods World 20, 487. SOUTHWARD, C. R. & GOLDMAN, A. (1975). New Zealand Journal of Dairy Science and Technology 10,101. STRIBLEY, R. C. (1963). Food Processing 24, 49. TOWLER, C. (1976). New Zealand Journal of Dairy Science and Technology 11, 140. WALKER, N. J. (1977). Proceedings Jubilee Conference on Dairy Science and Technology. Palmerston North, New Zealand: New Zealand Dairy Research Institute.

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Utilization of milk proteins as starting materials for other foodstuffs.

Journal of Dairy Research (1979), 46, 369-376 369 Utilization of milk proteins as starting materials for other foodstuffs BY CHARLES V. MORR* Depart...
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