Module 2. Skim milk and its by-products

Lesson 14

14.1 Introduction

Protein hydrolysis is an ancient technology used for centuries all around the world. Today, protein hydrolysates have assumed a new dimension in food industry. The manufacture of protein hydrolysates is a well-established process for the utilization of protein rich food by-products, wastes and non-conventional food proteins. Historically, soy sauce was the first protein hydrolysate. The fundamental flavour characteristic of a protein hydrolysate is suggestive of meat flavour. Therefore, hydrolysates are used by packer, canners and other manufacturers to accentuate or to suggest the flavour of meat. Casein is an important substrate for the preparation of milk protein hydrolysates.

The extent of protein hydrolysis, which represents the extent of protein breakdown to peptides and amino acids, is expressed either as percent amino nitrogen or as degree of hydrolysis (DH). Degree of hydrolysis is the ratio of the number of peptide bonds cleaved and the total number of peptide bonds in the intact protein. In the manufacture of casein hydrolyates, per cent DH is one of the important controlling factors, which reflect on the product quality.

14.2 Processes of Casein Hydrolysates Manufacture

Basically, there are two methods for the manufacture of protein hydrolysates, viz. acid hydrolysis and enzymatic hydrolysis, however, there are also reports of use of alkali for the manufacture of hydrolysed protein.

14.2.1 Acid hydrolysis

In this process, casein or caseinates are allowed to react with arsenic-free hydrochloric acid (35-45%) or sulphuric acid for a period of 4-18 h. The reaction is carried out at higher temperature (80-100°C) either at atmospheric or higher pressure. Thereafter, the content is neutralized to pH 6.0-7.0 by an alkali. The product is then either concentrated or dried. The process details given in a patent comprises of mixing 70 kg casein with 62 kg water and 92 kg purified arsenic-free hydrochloric acid (sp.gr. 1.17 and 34% concentration), and boiling with stirring for approximately 18-20 h until casein is fully broken (Fig. 14.1). This is followed by cooling to 28-32°C and gradual addition of 105-115 kg pure sodium hydroxide (as a 30% aqueous solution) to the mixture to bring the pH to 6.6-6.7. The neutralized mixture is then cooled to 15-25°C and then purified by centrifugation and filteration. The clear liquid is spray dried at 95-100°C obtaining a readily water soluble powder. It contains 53.5-53.7% amino acids, 41-41.5% NaCl and 2.0-2.5% other minerals.

Although acid hydrolysis has been used for preparing casein hydrolysates, the process has some limitations. The acid hydrolysis ordinarily entails the complete or partial destruction of some of the amino acids like tryptophan. This process also possess problem of removing residual acid from the products of hydrolysis. Neutralization of acid employed for hydrolysis result in the formation of salts which in turn becomes another limiting factor in food/dietic applications.

Use of mont morillonite to inhibit discolouration, normally experienced during acid hydrolysis at high temperature, has been successfully tried. The hydrolysis of milk protein is carried out at 80-95°C by non oxidizing acids (e.g. HCl and/or H2SO4) in the presence of (i) orthophosphorus acid and/or its salts and (ii) acid activated mont morillonite. Similarly for preparing colourless, odourless and tasteless acid hydrolysate, suitable for pharmaceutical industry, use of methanol has been reported.

14.2.2 Alkaline hydrolysis

This is another process of manufacturing casein hydrolysates. However, it is not effective method as it leads to the destruction of some and the recemisation of most amino acids. It also results in partial chemical hydrolysis, oligomerisation and destruction of functional groups of amino acid residues.

14.2.3 Enzymatic hydrolysis

Enzymatic hydrolysis of protein is preferred over acid or alkali hydrolysis as the former causes no destruction or racemization of the amino acids and nutritional quality of the original protein is retained. This process of hydrolysis of casein results in product with high content of water soluble nitrogen and low salt. Production of casein hydrolysates by enzymatic hydrolysis is given in detail in next chapter.

Selected references

Dave, R.I., Joshi, N.S., Patel, J.R. and Thakar, P.N. 1991. Protein hydrolysates- A review. Indian J. Dairy Sci., 44: 9.
Deeslie, W.D. and Cheryan, M. 1988. Functional properties of soy protein hydrolysates from a continuous ultrafiltration reactor. J. Agric. Food Chem., 36: 26.

Last modified: Wednesday, 3 October 2012, 6:46 AM