BY-PRODUCTS TECHNOLOGY

Module 3. Processing and utilization of whey

Lesson 34
MANUFACTURE OF CRUDE LACTOSE

34.1 Introduction

Lactose is a characteristic carbohydrate of milk and is the only sugar of animal origin. It is white, water soluble crystalline powder in its pure form and moderately sweet in taste. Crystalline lactose occurs in two forms: α-hydrate and β-anhydride lactose or a mixture of both forms. The most common form of commercial lactose is α-hydrate, very little lactose is in the form of β-anhydride. Lactose crystallises as α-hydrate from saturated solution at temperature below 93.5°C. The crystals contain one molecule of water per molecule of lactose. The β-anhydride which contains no crystalline water is formed when the crystallisation takes place at temperature higher than 93.5°C. The crystallisation of lactose from saturated solution is the α-form which is less soluble.

Lactose can be manufactured both from sweet whey and acid whey. Generally, unfermented whey is preferred because of its high lactose and low ash content. Acid whey if neutralised, changes the whey characteristics and increases the cost of manufacture. Lactose can be isolated on a commercial scale from whole whey or from deproteinized whey.

The deproteinized whey at the same total solid concentration contains more lactose and is thus more supersaturated than concentrated whey. Moreover, due to the decreased viscosity, a higher concentration can be obtained in evaporator equipped with a suitable finisher. If the whey were not to be deproteinized then due to the greatly increased viscosity of concentrated whey, the separation of the crystallized lactose would be exceedingly difficult or in extreme cases the lactose may not crystallize out at all. On the other hand, adding processing steps of material to clarify the whey has limitations. It may result in loss of lactose through occlusion with the extraneous matter being removed, add to the cost of materials used and yield less useful mother liquor because of the materials added. Thus, purity may be improved at the expense of reduced yield of lactose and less desirable mother liquor for food or feed use. Lactose yields varying from 65 to 76 % have been reported. However both methods, after the recovery of the crystallized lactose, result in a by-product, the mother liquor. This may be either dried, as a fodder product, or can be improved either by demineralization or by protein enrichment with a cheap vegetable protein.

34.2 Manufacturing Process

The conventional process for lactose manufacture consists of following steps:

34.2.1 Clarification of whey

Clarification is necessary to remove fat, suspended curd particles and other impurities (dust, dirt, microbes) from whey (Fig. 34.1: Flow diagram for production of crude lactose). Microfiltration is a pressure-driven membrane separation process using porous membranes with cut off pore size in the region of 10^-6 m allowing passage of proteins. Microfiltration is done at 55°C and can be used to remove large particles, casein fines, micro-organisms or microbial spores, fat globules, somatic cells, phospholipoproteins etc. from whey.

34.2.2 Deproteinisation and demineralisation of whey

The main drawbacks of the traditional process of lactose manufacture are with respect to low yield and purity of recovered lactose and the high cost of manufacture and energy consumption. Whey contains about 20% of the total protein of milk. Degree to which the proteins and salts are removed from whey prior to concentration and crystallisation, determines the purity of lactose. The presence of protein and salts in whey increases the viscosity of concentrated whey and hinder the lactose crystal separation, and in extreme case even prevent the crystallisation. Lower amounts of whey protein denaturation have been found to improve the lactose quality mainly due to lesser inclusion of protein and minerals in the crystallised lactose. Some processing steps have to be included so as to increase the purity but these lead to increased loss of lactose and add to the cost. Subsequently, it yields mother liquor, which is less useful for food or feed. Thus a compromise has to be made between the yield and purity aspects in the process to be adopted for lactose manufacture.

Cheese whey on heat treatment to 85-87°C at pH 4.8, is reported to remove maximum whey proteins cum salts on filtration, while in case of paneer whey, higher deproteinization could only be obtained by heating to 90 to 92°C for 10 min at pH 6.6. In another study, heating whey to 95°C, addition of different coagulants, holding for 15 to 20 minutes, centrifuging at 1500 RPM for 5 minutes has been suggested for maximum deproteinisation.

Lately, ultrafiltration has also successfully been applied industrially for the deproteinisation of whey . The membrane technology has an edge over the conventional technology because of improved product yield, improved product consistency, continuous processing operation, minimum man power and energy requirement, greater efficiency due to decreased processing time and over and above, the whey proteins can be recovered in their natural form. Using UF permeate as the starting material would have the associated advantage of shorter crystallisation time. At present, nearly 12% of the total whey utilised is processed by ultrafiltration. The protein free lactose solution is ideal for producing lactose syrup except for its high mineral content.

The UF permeate, particularly the acid whey permeate has a very high calcium content. During lactose crystallisation, the insoluble calcium salts may contaminate the lactose crystals, and because of their low solubility, they are not readily removed by washing with water. Concentration by evaporation causes precipitation of calcium salts and can result in rapid fouling or scaling of heat exchanger surfaces. Therefore, UF permeate must be pre-treated prior to or during evaporation. Removal of approximately 50 % calcium is sufficient to avoid difficulties during evaporation.

Demineralisation by ion-exchange and electrodialysis and by using an ammonium bicarbonate process has been recommended The main limitations of these processes are, high capital cost, high running cost and the problem of effluents, although, the ash content is reduced to less than 0.5%. Other suggested pre-treatment include reducing pH to eliminate the formation of insoluble salts and addition of food grade calcium chelating agents (e.g., sodium hexametaphosphate) to form insoluble complexes that may be removed prior to crystallisation. Hobman (1984) reported a process to reduce calcium salt levels by upto 80% by treating with alkali and heat. To improve the purity of lactose, it is necessary to reduce the mineral load. (Fig. 34.3: Flow diagram for production of lactose from deproteinated whey)

Partial demineralisation can also be achieved by nanofiltration (NF). NF performs two functions simultaneously. It partially demineralises a solution while concentrating lactose component. It can be used for the concentration and partial demineralisation of whole whey upto 28% of dry matter and also for concentration of the UF permeate. NF removes 75% of the water when operated at a typical 4 fold concentration. Demineralisation of cheese whey by NF shows 20-45 % overall demineralisation while more than 90-95 % of non-ionic species such as lactose. A 2-5 % loss of lactose can be expected in a typical whey demineralisation process (Jelen, 1991).

Table 34.1 Chemical composition of UF permeate stream from cheese and casein whey

34.2.3 Concentration

The concentration of whey to particular total solids is very critical because, a high total solids concentrate will be too viscous to pump, while a lower total solids concentrate will result in insufficient lactose crystallisation. The permeate is concentrated to a solid content of 60% or more (upto 70%). This is performed either by a pre-concentration through reverse osmosis, followed by evaporation or merely by evaporation. Reverse osmosis, when employed as a pre-concentration step, has the potential for removing a major portion of the water from whey or permeate more economically and in more energy efficient way than the evaporator process. Evaporation is carried out in falling film multi effect evaporators for economic reasons. The basic limitations of the these systems are: high cost of operation, problem of foaming, fouling, increase in viscosity and browning due to high residence time. For a higher level of concentration of whey, a combination of reverse osmosis and evaporation is the most energetically favourable way. The concentration process must be conducted in such a way that no lactose crystallisation takes place in evaporator and piping. This is done by keeping the temperature and concentration within metastable area.

34.2.4 Crystallization

The purpose of crystallisation is to secure the formation of crystals that can be separated from the mother liquor. The crystallisation rate depends on available crystal surface for growth, purity of the solution, degree of supersaturation, temperature, viscosity and agitation. Crystallisation is initiated in the hot concentrated whey or UF permeate. The nucleation process is initiated by seeding and agitating the supersaturated solution. Cooling of lactose syrup to a temperature below saturation temperature is necessary for crystallisation of lactose. During crystallisation, ß-lactose is converted into α-lactose which is crystallised out. For easy recovery of lactose crystals, their size must be sufficiently large to ensure quick settling of crystals. Easy recovery is obtained with an average size of 0.2 mm. The number of crystals and their average size can be controlled by seeding the concentrate with a known number of very fine lactose crystals. The seed crystals must be added in the form of fine particles of α-lactose monohydrate (200 mesh) at the rate of 0.1% of concentrate. The cooling of the concentrate should be slow. The entire crystallisation process lasts between 15-24 h under constant slow agitation. Automatic systems in lactose crystallisation tanks are available to regulate temperature within 0.5°C. The system can be supplied pre-programmed since random access memory has a battery buffer to prevent loss of data in case of power failure. Different cooling rates have been employed by different workers for crystallisation of lactose. Slow cooling to 10°C in 20 h and further holding for 15 h has been suggested. Cooling rate of 5°F/h and cooling from 78°C to 10-15°C in 50-60 h with intensive stirring have been recommended. Intermediate cooling temperature of 60°C with a final cooling temperature of 30°C over a 12 h period has also been recommended for effective lactose crystallisation.

34.2.5 Recovery of crude lactose

34.2.5.1 Harvesting of lactose crystals

The lactose crystals can be harvested either in a basket centrifuge batchwise or in a continuous decanter attached with a screw conveyor. Continuous decanters with a screw conveyor for crystal discharge are more commonly used on a commercial scale. The liquid phase overflows and consequently a liquid level is formed. The solid outlet ports are situated higher and crystal mass is discharged with relatively low moisture content. The crystals from decanter are fed into a second decanter in order to improve washing and removal of mother liquor. It is important that the choice of the decanter should not only be decided on the basis of the size of initial investment but also on parameters like loss of lactose and free moisture.

34.2.5.2 Washing of lactose crystals

Wash water is introduced into the centrifuge during the separation of lactose crystals to assist in the removal of the remaining impurities. The use of 10 % wash water can reduce the ash level of the lactose by more than 66 %. Washing with demineralised water or possibly, RO permeate which can be injected into the decanter to wash out as many impurities as possible has also been suggested. A specially designed centrifuge gives a high degree of separation of lactose crystals from condensed cheese whey. Crystals of 40 µm can be recovered with final moisture content of 1.5-2.5%. Another designed centrifuge proved capable of continuously separating crystalline lactose with 2.5-2.9% moisture from concentrated whey at the rate of 250-300 kg/h.

Selected references

Hobman, P.G. 1984. Review of processing for utilization of lactose in deproteinated milk serum. J. Dairy Sci., 67: 2630.
Jelen, P. 1991. Nanofiltration - a new membrane processing application for demineralisation in the dairy industry. J. Inst. Can. Sci. Techno. Aliment., 24 (5) : 200.
Sachdeva, S., Bhattacharjee, P.P. and Singh, S. 1998. Technology of lactose manufacture - A review. Indian J. Dairy Sci., 51 (1): 1-12.