BY-PRODUCTS TECHNOLOGY

Module 4. Functional properties of milk protein products

Lesson 40
FUNCTIONAL PROPERTIES OF CASEIN PRODUCTS

40.1 Introduction

The term functionality as applied to food ingredients, is defined as any property, other than nutritional attributes, that influences ingredients usefulness in food. The functional properties of proteins are those physico-chemical properties that enable proteins to contribute to the desirable characteristic of food. Besides providing nutritional value, casein preparations are used in food system due to their various functional properties. (Table 40.1) The hydrophobicity (due to aliphatic and aromatic side chains) and open, amphipathic structure (due to high content of proline) of the caseins are very significant in deciding the functional properties like viscosity, gelation, swelling, foaming and emulsification and are of great technological significance. Caseins are good film-formers and find use in whipping and foaming applications, and in emulsions of fats or oils. Sodium caseinate is more effective as an emulsifier, thickener and foaming agent than calcium caseinate and absorbs water more effectively in wheat flour system. Important functional properties of casein and casein products are discussed in detail in this lesson.

40.2 Water Absorption /Moisture Binding Properties

Casein micelles bind large amount of (2-4 g/g protein) water as compared to globular protein (50 g/100 g of protein). This is due to the mechanical entrapment of water in the micellar matrix (via colloidal calcium phosphate). The k-casein 'hairs' (having carbohydrate moiety) that protrude from the surface of the micelles also contribute to the large amount of water associated with the micelles. So casein can modify the texture of dough or baked products, serve as the matrix former in cheese-type products, produce specialised plastic materials, or increase the consistency of solutions such as soups. Ability of caseinates to bind moisture through hydrogen bonding and entrapment has also been advantageous in meats and sausages. This property can be modified by heat treatment and by ionic environment. For example, syneresis of yoghurt is prevented by a high heat treatment. Replacing calcium ions by sodium, increases moisture binding properties of caseins.

Table 40.1 Functional role of food proteins in food system

40.3 Solubility

Solubility is an important functional property and is a pre-requisite for most other functionalities. A typical solubility-pH profile of casein shows that close to its isoelectric pH, i.e. pH 4.0-5.0, the acid casein is completely insoluble, while at pH values >5.5, it is converted to cationic salt (Na, K, and NH₃) and is solubilized. Caseinate solutions (10-15%) can be readily prepared at pH 6.6-7.0. Sodium and ammonium caseinates show better solubility and viscosity than calcium caseinates which is perhaps due to higher ionisation of the former caseinates. Calcium caseinate in water exists as large aggregates that are stable at pH more than 5.5. Casein is also soluble at <~pH 3.5 but the viscosity coefficient is higher at acid than at neutral pH values and gel like system is formed. Virtually all applications of casein products require them to be dissolved first.

Acid soluble caseins may be produced by enzymatic or chemical modification. High calcium co-precipitates and rennet caseins are very insoluble in water because of their high calcium content, and it is necessary to use calcium sequestering agents (citrate or polyphosphate) to solubilize them. Higher pH above 9.0 also increases the solubility of rennet casein.

40.4 Viscosity

High solution viscosity of casein is a result of the very open, nearly random structures of casein molecules. Sodium caseinate finds application in products where high viscosity is required. Caseinates from highly viscous solution at concentration greater than 15% and even at high temperatures containing greater than 20% protein is so high as to make them difficult to process. The viscosity of sodium caseinate is logarithmically related to concentration, while there is a linear relationship between log viscosity and the reciprocal of absolute temperature. The viscosities of casein solutions differ, not only with different caseins, but with the concentration, cation present, pH value, temperature and the age of the solutions. The viscosity of sodium caseinate is strongly dependent on pH, with a minimum at about pH 7.0. The viscosity of casein is much higher at low pH (2.5-3.5) than at neutral pH, gel-like structures are formed at >5% protein at temperatures <40°C. For many food applications, high viscosity is advantageous. Sodium caseinate finds application in products where high viscosity is required. High solution viscosity of casein is a result of the very open, nearly random structures of casein molecules. The lack of solubility in the presence of calcium changes the behaviour of casein. As calcium is added to a solution of sodium caseinate, a number of changes are evident. The calcium causes aggregation of casein into structures that resemble micelles. As these aggregates increase in size and number, the viscosity of the solution decreases. The solution also becomes turbid as the particles become large enough to scatter light. Calcium caseinate is selected when a solution of relatively low viscosity and high turbidity (milky appearance) is desired.

Limited proteolysis by indigenous milk proteinase reduces the viscosity of caseinate solutions and may explain the low viscosity of caseinates produced from late lactation milk, which has a high level of indigenous proteinase. The viscosity of caseinates can also be reduced by treatment with disulphide-reducing and/ or sulphydryl blocking agents.

40.5 Gelation and Coagulation

Gels are systems in which a small proportion of solid is dispersed in a relatively large proportion of liquid but which have the property of mechanical rigidity or the ability to support shearing stress at rest properties of solids. Gelation or coagulation occurs when milk is subjected to limited proteolysis by acid proteinases, e.g. rennets, which hydrolyse the micelle-stabilizing κ-casein, producing para-κ-casein-containing micelles, which coagulate at the concentration of Ca²⁺ in the milk serum. This phenomenon forms the basis for the manufacture of rennet casein and most cheese varieties. Gel-like structures are formed at > 5% protein at temperatures < 40°C, which may be exploited in the preparation of milk protein-containing fruit gels.

Concentrated Ca-caseinate dispersions (> 15% protein) gel on heating to 50-60°C. Gelation temperature increases with protein concentration from 15 to 20% and with pH in the range 5.2-6.0. The gel liquefies slowly on cooling but reforms on heating; calcium caseinate is the only milk protein system reported to exhibit reversible thermal gelation properties.

40.6 Melting Properties

Casein also exhibits melting properties that are unique among proteins. Following limited proteolysis, casein will become thermoplastic and will flow upon heating. A similar affect can be achieved by chelating of some of the calcium ions present. These phenomena are the basis for the melting of natural cheeses and the production of process or imitation process cheese. Structure must exist before a substance can be said to melt. With caseins, this structure may be obtained by precipitation with calcium, acid or the addition of rennin. Casein does not form thermal gels and has little functionality in applications that require temperature set. High heat stability and the ability to melt are the two properties of caseinates that make them difficult to replace in many food applications. The demand for casein for products like cheese analogues (Processed cheese, Mozzarella cheese) depends on the formation of a protein matrix from calcium caseinate which will undergo thermo melting similar to its processed cheese counterpart.

40.7 Surface Active Properties

The surface activity of proteins is important in number of functional applications (film-formation, foaming and emulsification). These applications require the formation of continuous cohesive films at surface interphase. This event of film formation (diffusion, adsorption, spreading and partial unfolding) is influenced by the inherent structural characteristics of protein e.g. composition, conformation, molecular flexibility and extrinsic factor e.g. pH, temperature, protein concentration, type of ion species etc. The caseins due to their small size migrate quickly to air/water or oil/water interfaces and their open conformations allow them to spread readily at interfaces. The amphipathic structure of caseins facilitates orientation of the hydrophobic residues into the air or oil phases with the hydrophilic residues in the aqueous phase, thus exhibiting their surfactant properties. Sodium caseinate is a more effective interfacial tension depressor than whey protein, blood plasma, gelatin or soy protein; it diffuses more quickly to an interface and on reaching the interface adsorbs more quickly than the other proteins, probably because of direct and rapid anchoring of freely-available hydrophobic segments.

The surface activity of whole and individual caseins may be modified enzymatically. Dephosphorylation or treatment of sodium caseinate with plasmin (to produce γ-caseins and proteose peptones) greatly increases its surface activity.

40.8 Whipping/Foaming Ability

Caseinates generally give higher foam overruns, but produce less stable foams than egg white or whey protein concentrates. The excellent surfactant property of the amphiphillic casein is also responsible for its use in whipped toppings, cake mixes, and ice-cream.

40.9 Emulsion Properties

The emulsifying power of a protein is its capacity to stabilise oil-water or water-oil emulsion with minimum concentration under specified conditions. This power is mainly due to numerous surface hydrophobic (due to polar amino acids) sites which promote a greater affinity of protein for the oil phase. Casein forms complexes with milk fats and other lipids and acts as emulsifier by forming a stable coating around fat globules. Due to emulsifying properties, casein is also used in paint industry. In general, milk protein products, especially caseinates, are very good fat emulsifiers and are widely used in emulsifying applications in foods. Na-caseinate containing emulsions are resistant to heat shock of pasteurization, have an increased freeze-thaw tolerance, and remain unharmed during spray drying treatment.

Selected references

Kinsella, J.E., Damodaran, S. and German, J.B. 1985. Physico-chemical and functional properties of oilseed proteins with emphasis on soy proteins. In: New protein foods: Seed storage proteins, ed. A.M. Altshul and H.L. Wilcke. Academic Press, London: 107-179.
Damodaran, S. 2005. Amino acids, peptides and proteins. In: Food chemistry, 3rd edition, ed. Owen R. Fennema. Marcel Dekker Inc., New York: 365-376.
Damodaran, S. 1997. Food proteins: An overview. In: Food proteins and their applications. ed. S. Damodaran and A. Paraf. Marcel Dekker, Inc., New York: 1-24.