The kinetics of the precipitation of chemically modified alpha S1-casein by calcium

Calcium-induced precipitation of αs1-casein: effect of inclusion of citrate or phosphate

The calcium-induced aggregation of αs1-casein at pH 7·0 has been studied in the presence of citrate and phosphate anions. The effects of the citrate can be reconciled with the suggestion that the citrate, acting as a Ca chelator, reduces the Ca available for the aggregation reaction, but in no other way interferes with the mechanism of that process. In the presence of phosphate, precipitation is induced below the normal critical Ca concentration. This reaction proceeds to completion at low and high concentrations of phosphate. There is, however, an intermediate range of phosphate concentrations, whose limits depend on the protein concentration, where a colloidally stable precipitate is produced. The significance of these observations is discussed in terms of the currently held theories of micellar structure and formation.

Use of light scattering techniques to study the kinetics of precipitation of mineral acid casein from skim milk

Two light scattering methods for monitoring the precipitation of casein from skim milk (9% w/w) with mineral acid are presented. Experiments were carried out at precipitation temperatures in the range 25–53 °C and precipitation pH values in the range 3·9–5·1. The initial stages of precipitation were explored using a light fibre optics system attached to a closed-loop acid/milk mixing system. The later stages of particle formation and growth were examined using a laser doppler anemometer attached to an open-pipe flow system. It was found that there was a lag initially, followed by a period of rapid particle growth. During a third phase a dynamic equilibrium between particle growth, break-up and shrinkage was established. A model for isoelectric casein precipitation based on these observations is proposed and discussed.

Particle size distribution and calcium content of batch-precipitated acid casein curd: effect of precipitation temperature and pH

The first stage in acid casein manufacture is the isoelectric precipitation of casein from skim milk. This stage controls the particle size of the casein curd which is important in the later washing, dewatering and drying stages. Casein was precipitated batchwise from 9% total solids skim milk using 0·3 M-H2SO4 at temperatures from 25 to 53 °C and at pH (measured in the whey) from 3·6 to 5·1. A wet sieving method has been developed for measuring the particle size distribution of the casein curd. Ferric ammonium sulphate was added to the curd/whey mixture to toughen the curd particles and to reduce their stickiness. For given precipitation conditions the particle size distribution could generally be modelled by a normal distribution. The mean particle size generally increased with increasing temperature and pH. The increase in particle size with pH was pronounced at 53 °C, while at 25 and 35 °C particle size varied little with pH. The calcium contents of washed casein curd samples were low at all precipitation pH at a precipitation temperature of 35 °C, but increased markedly above pH 4·4 at precipitation temperatures of 45 and 53 °C.

Effect of precipitation temperature and pH on the mechanical strength of batch precipitated acid casein curd

The precipitation stage in acid casein manufacture controls the initial strength of the casein curd, which is an important factor in determining fines losses during processing. A plate extrusion method was used to determine the mechanical strength of casein curd, precipitated batchwise at temperatures from 25 to 53 °C over a pH range of 3·9 to 5·1. Curd strength was determined either immediately after acidulation or after cooling for 2 h at 25 °C. Curd was stronger immediately after acidulation than after cooling and increased in strength as precipitation temperature and pH were increased. Curd precipitated at high temperature and high pH was mechanically strong, had a large particle size, and had high total solids and Ca contents. The relationship between these curd properties is discussed, and it is concluded that Ca retention at high precipitation temperature and pH is largely responsible for the characteristic curd properties under these precipitation conditions.