Module 2. Skim milk and its by-products

Lesson 8

8.1 Introduction

Lactic acid casein can be prepared either of direct acidification or by lactic fermentation process. In terms of cost of acid, a lactic fermentation process is attractive especially when, with large-scale processing by modern methods, the tendency for higher capital and operational costs are minimized. In New Zealand, lactic acid casein has been the most common casein product. Lactic casein is isoelectrically precipitated from skim milk at about pH 4.6 mostly by the lactic acid produced by starter bacteria growing on lactose. Commercial lactic casein is free flowing granular powder of light cream colour.

8.2 Traditional Batch Method of Lactic Casein Manufacture

In lactic casein production, lactic acid is produced in skim milk by lactose fermentation using cultures of Streptococcus lactis and/or Streptococcus cremoris. The steps involved in production of lactic casein, are given in Fig_8.1.swf .

8.2.1 Precipitation and curd formation

Coagulation of skim milk is performed in cheese vats. The pasteurized milk is cooled to 22-26°C and inoculated with about 0.5% starter of mixed strain (S. cremoris being the major culture) and incubated for 14-16 h. Mixed starters, composed largely of S. cremoris have been used successfully in New Zealand for many years, probably because of the build of the bacteriophages resistant strains. During incubation period, the starter ferments some of the lactose to lactic acid. As the pH of the skim milk falls slowly under quiescent conditions, the colloidal calcium phosphate in the casein micelles progressively dissolves and a casein gel network or coagulum with good water holding capacity is formed as the isoelectric pH of the casein is approached. The slow coagulating cultures are desirable as they exhibit less proteolysis and increased protein yield. The precise rate of acid production by the starter is not important as coagulation usually takes place several hours before processing begins and at about pH 4.5, the culture is in stationary phase of growth.

Traditionally, undefined mixed starters containing a gas producing component, usually “Leuconostoc spp or Streptococcus diacetylactis” have been used. A certain level of gas production is required to produce a type of curd particle that can be cooked and washed easily. The dried casein obtained from such gassy curd can be ground more easily. Too much gas production however will shatter the curd resulting in an increase in casein fine losses. Important factors in multiple strain starter selection are:

● The component strains must not produce inhibitory substances.

● It is necessary to select the strain, which demonstrates the resistance to a wide variety of phages and maintain that resistance during long term commercial use. Strains should be phage unrelated, that is, each strain in the multiple starters should be insensitive to phages, which attack any of the strains present.

● The multiple strain culture should be maintained in a deep frozen state.

8.2.2 Cutting of curd, cooking and dewheying

The set coagulum is cut with cheese knives and the agitation of the curd is started after about 2 minutes of cutting. Steam is then injected into the vat jacket at controlled rate (sufficient to raise the temperature by 0.5°C/min) to cook the casein curd to 50-60°C for whey synersis, to facilitate the separation of curd and whey and to create a curd firm enough for subsequent processing. Whey synersis is further assisted by gentle agitation of the curd for about one hour during cooking. The curd is then allowed to settle and the whey drained off.

8.2.3 Washing

The washing of lactic casein curd is done in the same way as for mineral acid casein except that a higher temperature (70°C or more) is necessary at some stage of washing to pasteurize the curd. Usually, a temperature of 75-77°C is used during second washing. After washing, the wash water is drained and the casein curd is pressed, milled and dried the same way as during the manufacture of hydrochloric acid casein.

8.3 Continuous Manufacturing Process

A more sophisticated method that reduces amount of labour and time and wash water requirement has been developed in New Zealand. This method is characterized by incubating the skim milk with starter culture in large silos (each with a capacity of up to 250 000 l) for a period of 14-16 h, which causes coagulation of milk. The curd formed is pumped and simultaneously heated to 50-60°C by direct steam injection. Further processing steps are similar to that are used in the continuous manufacture of hydrochloric acid casein. The flow diagram of the continuous lactic casein manufacture is illustrated in Continuous_manufacturing_plant_for_lactic_casein.swf .

Selected reference

Gupta, V.K. 1989. Technology of edible casein. Indian Dairyman, 41: 643-650.

Last modified: Tuesday, 16 October 2012, 6:35 AM