BY-PRODUCTS TECHNOLOGY

Module 2. Skim milk and its by-products

Lesson 12
MANUFACTURE OF SODIUM CASEINATE

12.1 Introduction

The soluble form of casein, caseinates may be prepared from freshly precipitated acid casein curd or from dry acid casein by reaction with dilute solution of alkali (such as sodium, potassium, calcium or ammonium hydroxide). For the manufacture of caseinates, fresh acid casein curd is preferred over dried casein as raw material, since the former yields caseinates with blander flavour than does the latter. Caseinates prepared from dry casein will also incur the additional manufacturing costs associated with drying, dry processing, bagging and storage of the casein. However, in countries that import casein, buyers may still prefer to purchase casein and produce their own sodium caseinate. Casein should have a low calcium content (< 0.15% dry basis) in order to produce a caseinate solution with a low viscosity, and a low lactose content (< 0.2% dry basis) to produce sodium caseinate with the best colour, flavour and nutritional value. Control of the curd characteristics is also important to ensure rapid dissolution.

Sodium caseinate is the most commonly used water soluble form of casein and is used in wide range of processed food products as a source of protein, and for their physico-chemical, nutritional and functional properties.

12.2 Manufacture of Sodium Caseinate

Irrespective of the starting material used, the manufacture of sodium caseinate consists of: formation of a casein suspension, solubilization of casein using sodium hydroxide, and drying the sodium caseinate produced (Fig. 12.1).

12.2.1 Casein suspension and solubilization

The fresh casein curd passed from a de-watering device (about 45% solids) is minced and the finely-divided curd mixed with water at 40°C to give a solid content of 25-30% solids. If dried casein is used, it is ground and held in water for sometime so that it absorbs water and becomes softened. Casein suspension is then passed through the colloid mill. The temperature of the emerging slurry, which may have the consistency of 'toothpaste', should be below 45°C, since it has been observed that milled curd can re-agglomerate at higher temperatures.

The main difficulties experienced in the conversion of acid casein to sodium caseinate are:

(a) Very high viscosity of sodium caseinate solutions of moderate concentration, which limits the solids content for spray drying to 20%.

(b) Formation of a relatively impervious, jelly-like, viscous coating on the surface of casein particles which impedes their dissolution on addition of alkali.

To overcome the former difficulty, it is essential that the pH and temperature are controlled during conversion as these influence viscosity, while the latter can be overcome by reducing the particle size by passing a curd-water mixture through a colloid mill prior to addition of alkali.

12.2.2 Addition of alkali and pH control

Sodium hydroxide solution with strength of 2.5 M is pumped into the casein slurry, emerging with the consistency of ‘toothpaste’ from the colloid mill at >45^oC. The quantity of sodium hydroxide required is generally 1.7-2.2% by weight of the casein solids. Other alkalis such as sodium bicarbonate or sodium phosphates may be used, but the amounts required and their cost is both greater than those of sodium hydroxide. Hence, they would generally be used only for specific purposes such as in the manufacture of citrated caseinates. The addition of the dilute alkali must be carefully controlled with the aim of reaching a final caseinate pH of 6.6-7.0 (generally about 6.7). The recommended technique for achieving the correct caseinate pH is to add sufficient alkali to bring the pH close to, but below, the specified value and then add the additional alkali needed towards the end of the dissolving operation. This technique is used for following main reasons

● Firstly, because reduction of the pH of a sodium caseinate solution by addition of acid is likely to cause localised precipitation of casein, and

● Secondly, the development of any off-flavours associated with localised conditions of high alkalinity is minimized.

● A third reason is the potential for formation of lysinolamine when the pH is excessively high (e.g. > 10).

12.2.3 Dissolving

The casein alkali mixture is transferred to first dissolving vat, where it is recirculated and/or pumped to a second dissolving vat where solubilisation is completed as the solution temperature is raised to about 75°C. An in-line pH meter is used to monitor and regulate the addition of the NaOH solution. The viscosity of sodium caseinate solutions is a logarithmic function of the total solid concentration. Each dissolving vat, therefore, must be equipped with a powerful agitator and a high speed recirculating pump (Fig. 12.2: F₁, F₂). In addition to concentration, other factors which affect viscosity of sodium caseinate solutions are temperature (semi-logarithmic), pH, calcium content of the curd, type of alkali used and seasonal and genetic factors. Once the solution is prepared, it is important to raise the temperature as quickly as possible to 60-75°C to reduce the viscosity. However, care should be taken to avoid holding the hot (>70°C) concentrated sodium caseinate solution for extended periods prior to drying, since it is possible for brown colour to develop in the solution due to reaction between the protein and residual lactose. During the dissolving operation, the incorporation of air should be kept to a minimum since caseinate solutions form very stable foams. For this, all joints on pipes, especially on suction side of pumps, must be airtight and recirculation line must discharge below surface of liquid in dissolving vat. In view of the many variables which can affect the viscosity of sodium caseinate solutions, it is considered desirable to standardize them to a constant viscosity, rather than to a constant concentration, prior to drying.

12.2.4 Drying of sodium caseinate solution

The homogeneous sodium caseinate solution is usually spray dried in a stream of hot air. In order to ensure efficient atomization of the sodium caseinate solution, it must have a constant viscosity as it is fed to the drier. It is common practice to minimise the viscosity by preheating the solution to a temperature of 90-95°C just prior to spray drying. However, care should be taken to minimise the time for which the caseinate solution is at high temperature.

The total solid content of the solution destined for spray drying ranges between 20 and 22% and only occasionally may be as high as 25%. The highest possible caseinate concentration is determined experimentally for every spray drier. At about 20% solid concentration of the sodium caseinate solution, approximately 4 kg of water need to be evaporated to produce 1 kg of powder. Hence, the solids output from a drier used for caseinate is about one quarter of that when used for drying of skim milk (feed concentration usually 45-50% solids). However, it is possible to increase the inlet air temperature in order to increase the water evaporation rate. Thus, while steam radiators may produce inlet air temperatures of up to 170°C, it is possible (with indirect oil heating or direct gas firing) to produce inlet air temperatures of up to 260°C. The warm spray dried sodium caseinate powder is cooled in a fluid bed drier. The low solids content of the feed solution produces a spray-dried powder with a low bulk density. It may vary from 0.25 g/ml to 0.40 g/ml. Generally, pressure nozzle dryers, operating at 100-250 bar, produce caseinate with a higher bulk density than that from disc atomizing dryers. The powder particles produced on disc dryers tend to be in the shape of hollow spheres. Since the powder is so light, the losses from the product recovery cyclones may be rather high, and it is, therefore, considered prudent to install bag filters for improved recovery. The moisture content of spray-dried sodium caseinate should be less than 5% for satisfactory storage and this appears to be consistent with many product specifications.

12.2.4.1 Alternative method of sodium caseinate drying

Other methods used to reduce cost, increase processing rate during caseinate manufacture and control the properties of the resulting powders, include:

● Production of roller dried sodium caseinate by feeding a mixture of curd (50-65% moisture) and an alkaline sodium salt (Na₂CO₃ or NaHCO₃) onto the drying drum of a roller-drier. Sodium caseinate with good flavour and a high bulk density could be produced by using the roller drier at relatively low steam pressure (i.e. low drying temperature).

● Production of granular sodium caseinate by lowering the moisture content of acid casein curd to < 40%, reacting the curd with Na₂CO₃ with agitation for up to 60 min and drying the resultant caseinate in a pneumatic ring drier or a fluidized bed drier. The resulting caseinate has a higher bulk density and improved dispersibility compared to spray and roller dried product.

● Drying a mixture of acid casein curd (45% dry matter) and Na₂CO₃ in an attrition drier to produce a product that looks like spray dried sodium caseinate but which has a much higher bulk density.

● Spray drying sodium caseinate solutions of higher solids content (up to 30%) in a drier fitted with a modified atomizer disc or preparation of concentrated caseinate solutions (33-47% solids) by a modified procedure and drying these solutions in spray or roller dryers or by an extrusion drying method.

● Conversion of casein to caseinate in the presence of a limited amount of water using extrusion technique. Commercially, casein in powder form is introduced and transported with water continuously into an extruder machine. An alkaline reagent is introduced in second step and whole mixture subjected to intense kneading under pressure with a rise in temperature to initiate chemical reaction between casein and alkaline reagent. The mixture is subjected to second intense kneading under pressure and intense shear to finish off chemical reaction with temperature of mixture rising to cause mixture to melt and to obtain a viscous caseinate paste. It is then cooled with a degassing operation to reduce and adjust its temperature and viscosity. Finally caseinate paste is extruded at 70-90°C with 30-40% moisture to form a continuous strand of caseinate paste at outlet from extruder machine. A continuous thin sheet of caseinate paste is formed from strand. The sheet of caseinate is simultaneously cooled to a temperature below 20°C and cut longitudinally into a plurality of parallel strips. Strips are then cut into small sized pieces of caseinate.