Functionality of Heated Milk Proteins in Dairy and Related Foods

Functionality of Heated Milk Proteins in Dairy and Related Foods C. V, M O R R Department of Food Science Clemson University Clemson, SC 29631 INTRODUCTION

The major milk proteins, e.g., caseins and whey proteins, contribute a number of critically important functions that make it possible to process and handle fluid milk and manufactured milk products. The functional attributes of milk proteins that contribute to milk processing include: milk fat globule emulsification, stabilization of ionic and colloidal minerals, pH buffering, cheese curd formation by the action of rennet, regulation of heat stability, development of viscosity and gelation in cultured and sterile milk products, foam expansion and stabilization in frozen dairy products, and control of ice and lactose crystallization in frozen milk and ice cream products. Commercial milk protein products represent an ever-increasing source of functional ingredients for use by the food industry (7, 10, 11, 17, 18; C. V. Morr and R. L. Richter, 1984, unpublished). High temperature heat processing, as in forewarming and sterilization, evaporation, and drying, cause changes in the physicochemical properties of milk proteins and minerals (1, 2, 3, 4, 5, 9, 20). The principle physicochemical changes in milk proteins and minerals caused by heating include: whey protein denaturation, interaction of denatured whey proteins with casein micelles, conversion of ionic and soluble calcium (Ca) and magnesium (Mg) phosphates and citrates to colloidal phosphate, deposition of the heat-induced colloidal phosphate onto casein micelles, and Maillard browning reaction of proteins with lactose or other added sugars. Heat and drying treatments of whey protein concentrates cause protein denaturation ar'd aggregation, resulting in loss of solubility and functionality (2, 7, 9, 10, 13). This paper considers the important physicochemical and functional properties of proteins

Received August 24, 1984. 1985 J Dairy Sci 68:2773-2781

in milk and manufactured milk products, the important heat-induced reactions of milk proteins during manufacture of milk products, and the effects of heat-induced reactions upon the functionality of milk proteins in milk products. It also considers the effects of heat and drying upon the physicochemical and functional properties of commercial milk protein products with emphasis upon those changes that interfere with their resolubilization and rehydration. Because this paper was presented as a broad, interpretive treatment of the subject, general review references are commonly used rather than the large number of specific research references that are available in the literature. PHYSICOCHEMICAL PROPERTIES OF M I L K PROTEINS

The pertinent physicochemical properties of the major caseins have been summarized in Table 1 (7, 9, 12, 14, 15, 16, 20). The caseins, which account for about 80% of the total milk proteins, occur in approximate weight ratios of 55% %1-, 30% 3-, and 15% K-, exhibit an amphiphilic state due to the segregation of acidic, ester phosphate, hydrophobic, and hydrophilic amino acid residues along their polypeptide chains. The abundance and uniform distribution of proline residues along their polypeptide chains results in a random coil conformational state that is immune to heatinduced denaturation but imparts a strong tendency for the caseins to undergo polymerization by hydrophobic, ionic, and Ca bonding and renders them insoluble at pH 4.6 and temperatures above 20°C. The casein subunits polymerize during milk synthesis due to the influence of calcium ions, to form submicelles of 10 to 20 nm diameter, which further associate with colloidal phosphate to form highly porous and solvated micelles of about 100 to 300 nm diameter that are subject to association-dissociation when exposed to heating and other processing treatments. The K-casein component of the micelles contains a

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TABLE 1. Physicochemical properties of caseins. 80% of Total milk protein Molecular weights of subunits range from about 19,000 to 25,000 daltons Unfolded conformational state of subunits Amphiphilie properties of subunits Subunits subject to interactions via hydrophobic and calcium ion bonding Insoluble at isoelectric point (4.5 to 5) at/> 20°C Casein micelles associated with colloidal phosphate Casein micelles are 100 to 300 nm diameter Casein micelles are coagulated by rennin Casein mieelles are stabilized by K-casein Casein micelles interact with t3-1actoglobulinby disulfide interchange mechanism

single disulfide group that reacts with denatured /3-1actoglobulin and possibly other whey proteins during heat processing of milk. The K-casein component of the micelles also contains a highly acidic and polar glycomacropeptide (GMP) group attached at one of several threonine residues that functions to stabilize the micelle in the presence of Ca ions in milk (9). The general properties of whey proteins are summarized in Table 2. The major whey proteins, /3-1actoglobulin, ~-lactalbumin, and serum albumin, account for roughly 50, 12, and 5%, respectively, of the total whey proteins in milk (7). Undenatured whey proteins are not precipitated at their isoelectric points that range from about pH 4.2 to 5.1, but heatdenatured whey proteins precipitate in this pH range. Whey proteins contain significant amounts of sulfhydryl and disulfide groups that render them sensitive to denaturation and intermolecular interaction during heating of milk and whey products (2, 7, 9, 20). Upon heating of milk or whey, whey proteins are denatured by unfolding their polypeptide chains (Figure 1) and activating their sulfhydryl groups. They subsequently interact with the casein micelles during the heating of milk (9) or undergo interaction and aggregation during the heating of whey products (16 ;Morr and Richter, 1984, unpublished). EFFECT OF H E A T ON P H Y S I C O C H E M I C A L PROPERTIES OF M I L K PROTEINS

Heat processing treatments such as ultra-high temperature pasteurization; forewarming, evapJournal of Dairy Science Vol. 68, No. 10, 1985

oration, and sterilization of evaporated milk; forwarming, evaporation, and drying of highheat milk powder; and heat pretreatment for manufacturing lactalbumin and milk protein coprecipitate; cause a number of important chemical and physicochemical reactions in milk proteins (1, 2, 3, 4, 5, 9, 20;Morr and Richter, 1984, unpublished). Examples of these reactions in Table 3 include whey protein denaturation, interaction of denatured whey proteins with casein micelles (8, 9), interaction of casein micelle-denatured whey protein corfiplex particles with colloidal phosphate and divalent cations, polymerization of casein micelledenatured whey protein complex particles, and interaction of proteins with lactose by the Maillard browning reaction. In lieu of interaction with casein micelles in heated milk systems, heat-denatured whey proteins aggregate by disulfide interchange and Ca-bonding in heated whey products. Heating also causes conversion of soluble Ca and Mg salts to colloidal phosphate in milk or to insoluble complexes in whey (Figure 2). These reactions promote the aggregation of whey proteins in heated whey products. Processing factors that control interaction, polymerization, and aggregation of milk proteins include protein content and composition, time and temperature of heating, pH, total solids content, ionic composition, viscosity, and agitation. It is not uncommon for several of these processing and compositional factors to simultaneously vary during the manufacture of milk and whey products. For example, protein content, total solids content, ionic composition, viscosity, and product temperature all vary simultaneously over a rather wide range during evaporative concentration and drying of milk and whey products. Thus, it is often difficult to predict or extrapolate the degree of protein

TABLE 2. General properties of whey proteins. 20% of Total milk proteins Molecular weights range from 14,000 to 1,000,000 Compact, globular conformation Subject to denaturation and sulfhydryl group activation Subject to protein-protein interaction by disulfide interchange and calcium bonding Denatured form is insoluble at isoelectric point (pH 4.5 to 5)

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microscopy. Unheated milk contains about 1540 whey protein molecules per casein micelle (7). However, each casein micelle contains roughly 18,000 K-casein molecules and thus is theoretically capable of interacting with all of the whey protein molecules during heat proessing. The rate of interaction between denatured whey proteins and casein micelles should decrease as the reaction proceeds, due to the reduction of protein particle number concentration from roughly 1540 × 107 liter - 1 to 1 × 107 liter - 1 . This theoretical reduction in number concentration of milk protein molecules in forewarmed milk may be one of the most important factors responsible for imparting physical stability to the concentrate during sterilization of evaporated milk and sterile milk concentrate. Heating whey or sterilizing nonforewarmed milk, which lacks casein micelle-denatured whey protein complexes, leads to gross whey protein aggregation and physical instability in these products. EFFECT OF H E A T ON F U N C T I O N A L PROPERTIES OF M I L K P R O T E I N S IN M I L K PRODUCTS

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Milk proteins provide a number of key functions that facilitate successful manufacture of milk products (Table 4). Casein micelles provide excellent emulsification properties for stabilizing milk fat globules in homogenized milk products (9, 14; Morr and Richter, 1984, unpublished). Colloidal phosphate in milk

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Figure 1. Diagramatic representation of undenatured whey protein molecule (A); denatured whey protein molecule (B); and two interacted (aggregated or complexed) whey protein molecules (C). Hexagonal symbols denote hydrophobic residues.

damage caused during the processing of certain milk products. The molecular size distribution of milk proteins is significantly altered by heating milk and whey products (9; Morr and Richter, 1984, unpublished), as shown by ultracentrifugation, gel filtration, and electron

TABLE 3. Effect of beat on milk proteins in milk products. Skim milk Whey protein denaturation Interaction of denatured whey proteins and casein micelles Complexing of calcium, magnesium, and other ions by milk proteins Lower rate of rennin coagulation of casein micelles Lower solubility of milk powder Improve baking properties of milk powder Color and flavor development by Maillard browning reaction Whey Denaturation and aggregation of whey proteins Complexing of calcium, magnesium, and other ions Lower solubility and functionality of whey protein products

Journal of Dairy Science Vol. 68, No. 10, 1985

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MILK Ca & Mg/ HPO4 & Cit

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Figure 2. Effect on heat upon soluble and colloidal minerals in milk. Ca = Calcium, Mg = magnesium, HPO 4 = phosphate, Cit = citrate.

contains approximately 63% of the Ca, 47% of the phosphorous (P), and 33% of the Mg and is an integral part of the casein micelles (16). Thus, casein micelles and colloidal phosphate mutually stabilize each other in milk, and the micelles also stabilize additional colloidal phosphate that forms during heating and concentration of milk products (9). As indicated, casein micelles also provide the critically important function of intereacting with and stabilizing heat-denatured whey protein molecules as they form during forewarming of milk for manufacture of evaporated milk or high-heat milk powder. This latter function prevents gross aggregation of denatured whey protein molecules as in heated whey products. Casein micelles are coagulated by the action of rennin (chymosin) during cheese manufacture. Rennin hydrolyzes K-casein molecular polypeptide chains to release the GMP fragment, thus destroying micelle stability (9, 20). The enzyme-modified casein micelles undergo rapid polymerization by hydrophobic and ionic bonding to form a coagulum. Heat processing of milk prior to rennet treatment may cause sufficient whey protein denaturation and whey protein interaction with casein micelles to inhibit the action of rennin. The reduced susceptiblity of K-casein to the action of rennin in highly heated milk products is probably due to their being shielded from the enzyme by the presence of interacted /3-1actoglobulin at disulfide groups in their polypeptide chains. Thus, highly heated milk offers considerable difficulty in cheesemaking operations. It is necessary to forewarm milk prior to con centrating for the manufacture of evaporated milk in order to provide adequate heat stability to the concentrate during subsequent sterilization (9; Morr and Richter, 1984, unpublished). Journal of Dairy Science Vol. 68, No. 10, 1985

The forewarming treatment denatures and complexes denatured whey proteins with the casein micelles, as well as modifies the mineral components to provide improved heat stability (8). As indicated, such heat treatments reduce the theoretical protein number concentration in milk by a factor of about 1540 (Figure 3). Ultra-high temperature sterile milk concentrates are given minimal heat treatment prior to concentration and sterilization and thus contain a higher number concentration of undenatured whey protein molecules, which would be free to interact with other whey proteins or casein micelles during sterilization and storage (Figure 4). These latter interactions in the product result in progressive viscosity increases and eventual gelation (5, 9, 20; Morr and Richter, 1984, unpublished). Casein micelle-denatured whey protein complexes formed in highly heated milk products imbibe considerable water to increase the viscosity and gel strength of yogurt and other cultured milk products (7, 20). It is necessary to heat milk to at least 85 to 90°C for /> 15 min to produce sufficient protein polymerization to result in the desired product viscosity. Casein micelles are unstable and undergo gradual polymerization in frozen milk products stored under conditions that allow lactose crystallization (9; Morr and Richter, 1984, unpublished). Casein micelles remain stable and provide an important stabilizing function

TABLE 4. Functionality of milk proteins in milk products. Fat emulsification (milk fat globule membrane and homogenization) Casein micelle stabilization of colloidal phosphate Colloidal phosphate stabilization of casein micelles pH and calcium ion buffering action Heat stability (stabilization of whey proteins in sterile milk products) Rennet coagulation of casein micelles in cheese making Age thickening of Ultra-high temperature milk concentrates Interaction with water (casein micelle-whey protein complex imbibition of water to provide viscosity in yogurt and cultured milk products) Cryo-destabilization of casein micelles in frozen milk products Crystal growth control in ice cream and frozen desserts Insolubilization of high-heat milk powder products

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in frozen desserts and ice cream where added stabilizers and sweeteners prevent lactose crystallization. The casein micelles and whey proteins provide added viscosity and b o d y to ice cream, aid in foaming during the freezing process, and also aid in preventing ice and lactose crystal growth in the frozen product during storage. High-heat milk powder contains highly denatured and complexed whey protein-casein micelles that are only poorly resolubilized and rehydrated when the product is reconstituted (Figure 5). Low-heat milk powder contains the whey proteins in an undenatured state and its particles are more readily resolvated and solubilized. Similar considerations are in order for the manufacture and use of other whey protein-containing milk products, e.g.,

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whey protein concentrates, lactalbumins, and coprecipitates, which have been exposed to variable heat intensity processing during their manufacture. The object for manufacturing a soluble and functional whey protein concentrate is to minimize heat exposure and other process treatments that denature whey proteins. The opposite relationship is true where a high degree of whey protein denaturation and insolubilization is needed for manufacturing lactalbumin and coprecipitate. COMMERCIAL MILK PROTEIN PRODUCTS

In addition to the mentioned important functional contributions of milk proteins in manufactured milk products, large quantities of milk proteins are isolated from milk and used as functional ingredients in other food products (7, 10, 11, 17, 18). For example, annual world casein and caseinate production is estimated at about 500 million lb. The US imports about 130 million lb of casein and caseinate annually and uses roughly 60% of this amount in formulated food applications. The US dairy industry manufactures and uses about 400 million lb of whey and modified whey products and about 18 million lb of whey protein concentrate annually in food manufacture. Our industry also uses about 377 million lb of nonfat dry milk as functional ingredients annually (17, 18). Processing conditions used for manufacturing nonfat dry milk (NFDM), concentrated and dried whey, whey protein concentrate (WPC)

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Figure 4. Diagrammatic representation of polymers of denatured whey protein-casein micelle complexes in ultra-high temperature sterile milk concentrate. Symbols are the same as in Figure 3. of Journal

Dairy Science Vol. 68, No. 10, 1985

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and other milk protein products have been reviewed recently (6, 19). The NFDM is produced by varying the extent of heat treatment to provide a range of product solubilities and functionality (7; Morr and Richter, 1984, unpublished). High-heat NFDM, which contains the whey proteins in a highly denatured and complexed state, is relatively insoluble and therefore used in those food products where solubility is undesirable or unneccessary, e.g., bakery products, pasta, breakfast cereal, nutrition bars, etc. Low-heat NFDM contains highly soluble and functional milk proteins that are used in food products where flavor, color, physical stability, and solubility are beneficial. Journal of Dairy Science Vol. 68, No. 10, 1985

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Figure 9. Schematic diagram of process for manufacturing whey protein concentrate. Concentrated and dried whey and modified whey products for human food uses are generally manufactured under minimal heat processing conditions to retain the proteins in a soluble and functional form (Figure 6). Liquid whey is subjected to ion exchange or electrodialysis treatments to demineralize partially or modify mineral composition, thereby improving nutritional and functional properties of these products. Whey concentrates are also processed to remove substantial amounts of lactose prior to drying. Such products contain a higher protein content than conventional whey powder and therefore function well in certain specialized food product applications. Caseins and caseinates are produced from skim milk by processing under conditions that minimize heat exposure and whey protein denaturation and interaction with casein micelles (Figure 7). Pasteurized skim milk is

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treated with acid to isoelectrically precipitate the casein. The casein precipitate is recovered, washed, and dried as acid casein. Casein curd is also neutralized with alkali to pH 8 to 9, and the resulting caseinate solution is spray dried. Rennet casein is produced by treating skim milk with rennet to coagulate the casein micelles, which are then recovered, washed, and dried. Because the resulting enzyme modified casein micelles are insoluble in the presence of Ca ions, rennet casein must be used for those food product applications where this property is useful. Lactalbumins are highly heat denatured whey protein concentrates (Figure 8). They are produced by heating whey under severe temperature and time conditions to promote whey protein denaturation and render them insoluble

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I CO-PRECIPITATE Figure 10. Schematic.diagram of process for manufacturing coprecipitate. UHT = Ultra-high temperature. Journal of Dairy Science Vol. 68, No. 10, 1985

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Figure 11. Schematic diagram of process for manufacturing total milk proteinate.

in the presence of added acid and Ca ions. These whey protein products lack solubility and related functionality for most food product applications but offer special functionality for certain food products such as baked foods, nutrition bars, pasta, etc. Whey protein concentrates are produced from whey by ultrafiltration and other protein recovery processes that provide mild temperature conditions to minimize whey protein denaturation. (7, 9, 10, 13, 17, 18). The whey is most commonly processed by ultrafiltration to provide protein concentrations in the retentate fraction of 50 to 75% on a solids basis (Figure 9) and spray dried. Most of the commercially manufactured WPC products exhibit a 30 to 50% protein denaturation range. Coprecipitates (Figure 10) are produced from skim milk by processing under drastic heating conditions that favor whey protein denaturation and interaction with casein micelles. Calcium ions and acid are then added to precipitate the denatured protein complex, which is recovered, washed, and dried. These milk protein products generally exhibit only limited solubility and related functionality. The solubility and functionality of coprecipitates can be improved by treating them with polyphosphates and other Ca-chelating chemicals. Total milk proteinates (TMP) are a new form of milk protein isolate (Figure 11) similar in composition to coprecipitate, but retaining their proteins in a highly soluble and functional form (17, 18). Skim milk is adjusted to about pH Journal of Dairy Science Vo]. 68, No. 10, 1985

10 and heated to about 70°C to solubilize casein micelles, adjusted to pH 3.5 to complex the whey proteins and caseins, adjusted to pH 4.6 to precipitate the complexed proteins, washed, and dried. It is claimed that this milk protein isolate contains caseins and whey proteins in an 86:14 weight ratio, as in skim milk, and that the whey proteins are in an undenatured and functional form.

F U N C T I O N A L I T Y OF I S O L A T E D M I L K P R O T E I N S IN F O R M U L A T E D FOOD PRODUCTS

The previously discussed milk protein products - caseins, caseinates, whey protein concentrates, lactalbumins, coprecipitates and total milk proteinates - are used as functional ingredients (Table 5) in a number of food products (7, 10, 11, 12, 17, 18). Caseinates and caseins are used in formulated food products to provide fat emulsification in coffee creamers, whipped toppings, and meat emulsions; to provide foam expansion and stabilization in whipped toppings; to provide gelation and curd formation in cheese analogs; and to contribute viscosity in various food products. Whey protein concentrates and modified whey products are used as functional ingredients in infant formula; for foam expansion and stabilization of meringues; for fat emulsification in processed meat products; for water binding and gelation in meat products; low pH solubility of the proteins in fruit juices and drinks; and as nutritional supplements in special dietary and pharmaceutical products. Lactalbumins and coprecipitates are most commonly used in pasta, bakery, and confectionery products. Nonfat dry milk and dried whey products are

TABLE 5. Functional requirements of isolated milk protein products. Fat emulsification (coffee whiteners) Foam stabilization (whipped toppings, meringues) Gelation (cheese analogs) Viscosity (whipped toppings) Solubility as a function of pH (fruit juices, carbonated beverages) Heat coagulation (meat products, meringues, bakery products)

SYMPOSIUM: MODIFICATION OF MILK PROTEINS used as f u n c t i o n a l i n g r e d i e n t s in t h o s e f o o d p r o d u c t s w h e r e high lactose a n d m i n e r a l c o n t e n t s are n o t serious p r o b l e m s . F o o d products that commonly contain NFDM and dried w h e y p r o d u c t s i n c l u d e b a k e r y p r o d u c t s , b r e a k f a s t cereals, soup mixes, b r e a d i n g mixes, confectionery products, and meat products where functional requirements include emulsification, w a t e r a n d fat b i n d i n g , color a n d flavor d e v e l o p m e n t b y t h e Maillard b r o w n i n g r e a c t i o n , viscosity, gelation, a n d n u t r i t i o n a l properties.

REFERENCES

1 Dalgleish, D. G. 1982. Milk proteins in chemistry and physics. Food proteins, P. F. Fox and J. J. Condon, ed. Appl. Sci. Publ., Essex, England. 2 de Wit, J. N., and G. Klarenbeek. 1984. Effects of heat treatments on whey proteins. J. Dairy Sci. 67:2701. 3 Fox, F. P. 1981. Heat-induced changes in milk preceding coagulation. J. Dairy Sci. 64:2127. 4 Fox, F. P. 1982. Heat-induced coagulation of milk. Developments in dairy chemistry - 1. P. F. Fox, ed. Appl. Sci. Publ., Essex, England. 5 Harwalker, V. R. 1982. Age gelation of sterilized milks. Developments in dairy chemistry - 1. P. F. Fox, ed. Appl. Sci. Publ., Essex, England. 6 Marshall, K. R. 1982. Industrial isolation of milk proteins: Whey proteins. Developments in dairy chemistry - 1. P. F. Fox, ed. Appl. Sci. Publ., Essex. England. 7 Modler, H. W. 1984. Tailoring functionality of non-fat dairy ingredients for food product ap-

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plication. FIL-IDF Can. Sere. Dairy Ingred., Ottawa, Can. 8 Morr, C. V. 1969. Protein aggregation in conventional and ultra high-temperature heated skim milk. J. Dairy Sci. 52:1174. 9 Morr, C. V. 1975. Milk proteins in dairy and food processing. J. Dairy Sci. 58:977. 10 Morr, C. V. 1976. Whey protein concentrates: an update. Food Technol. 30 (3):18. 11 Morr, C. V. 1979. Utilization of milk proteins as starting materials for other foodstuffs. J. Dairy Res. 46:369. 12 Morr, C. V. 1979. Conformation and functionality of milk proteins. Functionality and protein structure. A. Pour-El, ed. Am. Chem. Soc., Washington, DC. 13 Morr, C. V. 1979. Functionality of whey protein products. N . Z . J . Dairy Sci. Technol. 14:185. 14 Morr, C. V. 1981. Emulsifiers: milk proteins. Protein functionality in foods. J. P. Cherry, ed. Am. Chem. Soc., Washington, DC. 15 Morr, C. V. 1982. Functional properties of milk proteins and their uses as food ingredients. Developments in dairy chemistry - 1. P. F. Fox, ed. Appl. Sci. Publ., Essex, England. 16 Morr, C. V. 1983. Physico-chemical basis for functionality of milk proteins. Kiel. Milchwirsch. Forschungsber. 35 : 333. 17 Morr, C. V. 1984. Production and use of milk proteins in foods. Food Technol. 38 (7):39. 18 Morr, C. V. 1984. Manufacture, functionality, and utilization of milk products. Proc. Milk Proteins '84 Conference, Luxemberg. 19 Muller, L. L. 1982. Manufacture of casein, caseinates and coprecipitates. Developments in dairy chemistry - 1. P. F. Fox, ed. Appl. Sci. Publ., Essex, England. 20 Schmidt, R. H., and H. A. Morris. 1984. Gelation properties of milk proteins, soy proteins, and blended protein systems. Food Technol. 38 (5):85.

Journal of Dairy Science Vol. 68, No. 10, 1985