The precipitation step in the commercial production of casein

Abstract

Casein has been manufactured in large quantities for many years. Despite this, however, there is little fundamental information available on the effects of process variables on the physical properties of casein precipitates. Methods were developed for determining the physical properties of casein curd. A wet sieving method was used for determining curd particle size distributions. Ferric ammonium sulphate was added to the curd/whey mixture before sieving to toughen the curd particles and to reduce their stickiness. A number of techniques were investigated for objectively evaluating curd strength. A plate extrusion method was found to require the least curd preparation and also gave fast, reproducible results. These methods were applied to casein curd precipitated, batchwise in the laboratory, and continuously in pilot and industrial plants. In the laboratory mineral acid casein was prepared from skim milk powder, reconstituted to 9% total solids, at precipitation temperatures from 25 to 53°C and over a pH range of 3.9 to 5.1. Mineral acid casein was prepared in the pilot plant at precipitation temperatures of 40°C and 50°C, between pH 4.0 and 5.0. Industrial curd samples were taken from lactic, mineral acid and rennet casein plants. In the laboratory and pilot plant it was found that curd precipitated at high temperature and high pH was mechanically strong, had a large particle size, and high total solids and calcium contents. Decreasing precipitation temperature or pH caused weaker, smaller curd particles with lower total solids and calcium contents, to be formed. It was concluded that calcium retention at high precipitation temperature and pH was largely responsible for the characteristic curd properties under these precipitation conditions. Curd particles precipitated batchwise were normally distributed, while continuously precipitated curd contained fewer fine particles than would be predicted by a normal distribution. Curd precipitated continuously in the pilot and industrial plants was larger and stronger than that prepared batchwise in the laboratory. The different distribution and increased strength was attributed to the better mixing conditions in the continuous plants. The use of fresh rather than reconstituted skim milk was found to result in larger curd particles. A study of the effects of precipitation pH and temperature on the kinetics of the iso-electric precipitation of casein was conducted; Light scattering techniques were used to follow the formation and growth of precipitates. It was found that an initial lag phase was followed by a period of rapid growth. During a third phase an equilibrium between particle growth, breakup and shrinkage was established. A model for iso-electric casein precipitation based on these observations was proposed and discussed.