By-Products Technology

Module 3. Processing and utilization of whey

Lesson 20
PHYSICO-CHEMICAL CHARACTERISTICS OF WHEY

20.1 Introduction

There is wide variation in composition depending on milk supply and the process involved in the production of the whey. In general, whey produced from rennet-coagulated cheeses and casein is sweet whey, whereas the production of acid casein and fresh acid cheeses, such as Ricotta or Cottage cheese, yields acid whey. When we use rennet, most part of calcium and phosphorus of the casein complex remain with the curd. The ash content of the whey is, therefore, less than when the coagulating agent is acid, which transfers part of the phosphorus and most of the calcium to the whey. Production of channa and paneer yields medium acid whey. Based on acidity, whey can be conveniently classed into groups:

Sweet whey : Titrable acidity, < 0.20%, pH 5.8- 6.6.
Medium acid whey : Titrable acidity, 0.20-0.40%, pH 5.0-5.8.
Acid whey : Titrable acidity greater than 0.40%, pH < 5.0.

20.2 Chemical Composition of Different Types of Whey

Different types of whey produced during the manufacture of cheese, casein, chhana, paneer, chakka and co-precipitates vary in chemical composition and acidity (Table 20.1). It, in general, contains about half of the total solids of milk, and is a source of precious nutrients like lactose, whey proteins, minerals and vitamins.

Table 20.1 Composition of different whey systems

Whey is a multicomponent solution of various water-soluble milk constituents in water; the dry matter of whey consists primary of carbohydrate (lactose), protein (several chemically different whey proteins) and various minerals. Fat content of the freshly separated liquid whey may be up to 0.5-1% depending on the type of milk used and the efficiency of the cheese-making operation. Fat content of most acid whey from fresh cheeses such as cottage cheese or ordinary quarg is negligible as skim milk is used in the manufacture of these cheeses. The proximate composition of various whey may show significant variations due to many factors including the pretreatment of the cheese milk (heating, centrifugation, cultures used, mechanical handling, use of processing aids such as the yellow color, use of membrane processes); and the whey handling and pretreatment processes (pasteurization, pre concentration, recovery of casein fines).

Lactose, the principal component of the whey constitutes about 4.4-4.9% of the whey (almost 75% of the dry matter) depending on the whey type. Lower lactose content is usually found in the acid whey due to the fermentation process in which some of the lactose is converted to lactic acid. Although lactose is the most abundant material of whey, the most valuable whey component is the whey protein, constituting approximately 0.7% of the whey (about 9-11% of the dry matter). In addition, whey may contain about 0.2-0.3% of nitrogenous matter denoted as non protein nitrogen (principally inorganic compounds, urea etc): this is sometimes included with the true whey protein and reported as total whey protein (N x 6.38). In many cases, like in the manufacture of co-precipitates, channa and paneer, high heat treatment is given to milk resulting in varying degree of whey protein precipitation along with the product. As a result, whey obtained from the production of such products will contain mainly the heat non-coagulable whey proteins and its total protein content will be substantially lower.

In addition to lactose and whey proteins, minerals constitute the third major component of dry matter of whey, which contribute to the electrical conductivity of whey. The mineral composition shows the greatest variations between different types of whey, together with pH and lactic acid content. In addition to lower pH and higher lactic acid (and correspondingly lower lactose) content, the acid cheese whey shows substantially higher calcium and phosphorus contents caused by the solubilization of the calcium-phosphate complex of the casein micelle at the acid pH range. In contrast, the calcium removal from the casein micelle does not occur during rennet clotting at pH 6.0 or higher; thus, much of the milk calcium is retained in the cheese rather than being lost in the sweet whey. The differences in acidity as well as the higher calcium content of the acid whey appear to be the main reason for variations in physico-chemical properties of different whey, including the substantially lower heat stabilities of acid whey in comparison to sweet whey.

20.3 Physical Properties

There is comparatively little data available on the detailed physical properties of whey.

20.3.1 Colour

Whey is the greenish translucent liquid. The greenish color of most traditional whey systems, regardless of the processing conditions used, is caused by the water-soluble and heat-stable riboflavin. However, riboflavin is sensitive to light as well as to ionizing radiation treatments and whey systems exposed to these conditions will show fading of the green color.

20.3.2 Flavour

Studies on the flavour of acid whey, based on eight flavour characteristics, have been reported by McGugan et al. (1979). When increasing concentration of whey were added to skim milk, ‘brothiness’ was first noted when 20% whey was added, and diacetyl, bitterness and sweetness at 40% addition. Volatile acidity, non-volatile acidity, saltiness and astringency were only noted in 100% whey. Neutralization of the whey resulted in a change in all flavour characteristics.

20.3.3 Surface tension

The surface tension of cheese whey has been shown to vary between 40,000 and 84,000 N/m, increasing with increasing total solids, and decreasing with temperature. It is possible that some of the observed variation is due to difference in lipid content of whey. According to one report, the surface tension of whey is low (42 dynes/cm) compared to 48 dynes/cm of skim milk.

20.3.4 Viscosity

The viscosity of whey at different temperatures has been presented in Table 20.2. The viscosity characteristics of whey and concentrated whey are important, not only in terms of evaporation efficiency, but also for operation such as lactose hydrolysis, which might be expected to function more efficiently if applied to concentrates. Lactose hydrolysis reduces viscosity and the degree of non-newtonian behaviour.

Table 20.2 Viscosity of whey at different temperatures (Viscosity in cP)

20.3.5 Heat stability

The individual proteins in whey have a wide range of denaturation temperature, about 65-75°C. In general it has been found that, with higher TS, the denaturation of β-Lg slows down, but the denaturation of α-La increases. Increased lactose concentration reduced the denaturation of both proteins, perhaps as result of the formation of heat induced complexes. Increased calcium contents, up to 0.4 mg ml^-1, tended to slow denaturation, but above this level, little further effect was observed. The rate of denaturation of both proteins was slower at pH 4 than at pH 9. Other comparisons have shown that the method used for concentration may significantly affect the nature and detailed confirmation of whey proteins, with membrane processing having much less severe effects than conventional evaporation. The changes occurring in the proteins at these temperatures may partly explain the improvement in flux commonly obtained holding whey at 55°C before ultrafiltration.

20.3.6 Solubility

The solubility of lactose is only about 20 g/100 g water at room temperature and 60 g/100 g water at 60°C. Therefore, whey concentration to more than about 36-38% total solids results in formation of crystalline lactose. This is the principle of the lactose manufacturing process based on crystallization from highly concentrated whey.

Selected reference

http://www.angelfire.com/zine2/mr_x/WheyProteinByMrX.pdf