Lactose

Lactose is carbohydrate of milk and is the only sugar of animal origin. Lactose in its pure form is a white, water-soluble crystalline powder of moderately sweet taste with no odour. Lactose is extensively used in food and pharmaceutical industries due to its nutritional importance and multiple functional properties. Low sweetness,consistency improvement in confectionery and bakery products, as carrier and anticaking agent in powder foods, filler for tablets and capsules, controlled browning in bakery products, flavour enhancer in sauces and dressings, nutritional importance in infant formulas, an additive in culture media etc. are some of the desirable functional properties and applications of crystalline lactose powder. Lactose is also important fermentation substrate for the production of a variety of fermented products. As a rough estimate, the quantity of lactose produced on a world-wide scale amounts to some 50 lakh MT per annum.

Lactose is produced commercially from whole whey or from UF permeate. The protein and mineral contents of the whey are the limiting factors for the crystallization of lactose, and this is the reason why UF permeate of whey is preferred as the starting material in the production of lactose. However, A significant quantity of food and pharmaceutical grade lactose is produced by conventional process. During the process, a protein-mineral precipitate is segregated, which is dried and sold as a by-product for animal feed.

i. Manufacture of crude lactose

Lactose can be manufactured either from sweet whey having a minimum pH of 5.6,usual range being 5.9 to 6.3 or from acid whey with a maximum pH of 5.1 (usual range, 4.4 to 4.6). In Indian context, chhana and paneer whey, whose pH ranges from 5.1 to 5.6, can also be used as a good raw material for lactose manufacture.Generally, sweet whey is preferred because of its high lactose and low ash content.Acid whey can be neutralized, but this will change the whey characteristics and the cost of manufacture will also increase.

i) Clarification and separation of whey: Clarification and separation is necessary to remove the fat, suspended curd particles and other impurities(dust, dirt, microbes) from whey.

ii) Deproteinisation and demineralization of whey: Whey contains about 20% of the total proteins of milk. The presence of protein and salts in whey increases the viscosity of concentrated whey and hinders the crystallization of lactose. The reduction of protein and mineral contents in whey causes reduction in viscosity and thereby permits concentration to higher total solids. Degree to which the proteins and salts are removed from whey prior to concentration and crystallisation, determines the yield and purity of lactose.Cheese whey on heat treatment to 85-87°C at pH 4.8 yields maximum whey solids on filtration, while in case of paneer whey, higher yield could only be obtained by heating to 90 to 92°C for 10 minutes at pH 6.6. Up to 85 % removal of proteins can be achieved by adding 1 % CaCl2 (20% soln.) to whey at 90 to 95°C. Addition of CaO at 93°C and holding for 30 minutes is also practiced where more than 90 % of proteins and minerals are removed.

iii) Concentration: The concentration of whey to 45 - 60 % total solids is very critical, because a high total solids concentrate will be too viscous to pump, while a lower total solid concentrate will result in lesser degree of supersaturation of lactose and so insufficient lactose crystallisation. This is performed either by a pre-concentration through reverse osmosis, followed by evaporation in multi effect evaporators or merely by evaporation. Reverse osmosis has the potential for removing a major portion of the water from whey or permeate more economically than the evaporator process. The concentration process must be conducted in such a way that no lactose crystallisation takes place in evaporator and piping.

iv) Crystallisation: Crystallisation is initiated in the hot concentrated whey. This is a complex process during which lactose molecules diffuse to the crystal surface and simultaneously release and transfer the heat of crystallisation from the crystal to the liquid. The purpose of crystallisation is to secure the formation of crystals that can be separated from the mother liquor. 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 are added in the form of fine particles of á-lactose monohydrate at the rate of 1 Kg per ton of concentrate. In concentrated lactose solution, the crystallisation rate depends on available crystal surface for growth, purity of the solution, degree

of supersaturation, temperature, viscosity and agitation.

Cooling of lactose syrup to a temperature below saturation temperature is necessary for crystallisation of lactose. Higher temperature of crystallization increases the growth rate of the crystals. Therefore, slow cooling of the concentrate is employed. Slow cooling to 10°C in minimum 20 h (up to 40 h) and further holding for 15 h with intensive stirring has been suggested. During crystallization, ß-lactose is converted into -lactose, which is crystallised out.Automatic systems in lactose crystallisation tanks are available to regulate temperature within 0.5oC.

v) Harvesting of lactose crystals: The lactose crystals can be harvested batchwise in basket centrifuges, which have the advantage of permitting complicated wash cycles. 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 %. However, on a commercial scale, continuous decanters with a screw conveyor for crystal discharge are more commonly used. The crystals from first decanter are fed into a second decanter in order to improve washing and removal of mother liquour. The washed crystals recovered in decanter have moisture content of approximately 10% and can be dried directly.

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/hr.

vi) Yield and purity: Lactose yields varying from 65 to 76 % have been reported by different workers. Lactose % in crude lactose obtained by different workers varies between 91 and 97 %.

vii) Drying: The most common crystalline form of lactose i.e. á-lactose crystallizes below 93.5°C and the other form, the ß-lactose, crystallises above 93.5°C. The drying process, therefore, should be limited to a product temperature of 93°C to prevent crystallisation into ß-lactose anhydride. Flash drying can result in the formation of a thin layer of amorphous lactose on alpha hydrate crystals leading to lump formation in the bagged lactose. A fluidized bed drier with a maximum product temperature of 92oC/15-20 min gives good results.Pneumatic transport of lactose from the drier must be carried out by means of dry air at about 30oC. It gives gentle product cooling.

ii. Refining of lactose

For high degree of purity, as in pharmaceutical grades, refining of lactose is done by subjecting crude lactose to treatment for removing colour, residual protein and salts followed by recrystallisation. Crude lactose is dissolved in hot water to a 50-60% concentration depending on its purity. About 1% of decolorizing paste consisting of 3 parts bone black, 1 part activated carbon and 1 part 36% hydrochloric acid is added. Quick dissolution requires heating to 105ºC. Carbon absorb colour and probably removes other impurities to some degree. Hydrochloric acid is added to assist the action of carbon, to solublize salts and to aid in removal of protein. Lime is used to adjust the reaction to that most favourable for the

 

Lactose Commercial - Specifications (IS 1000:1989)

precipitation of protein, and probably aids by combining with the protein to some extent. The liquid is boiled and filtered with the assistance of filter aid to remove carbon and precipitating impurities at high temperature to avoid premature crystallisation. The carbon and the precipitated impurities are removed by filtration.The resultant clear solution is further evaporated to 70% TS and introduced into crystallising tanks. The crystallized lactose is harvested and dried. Edible lactose is normally dried to 0.5% moisture content and pharmaceutical lactose to 0.1% moisture content. The lactose is finally ground to 80-200-mesh size before bagging.

iii. Grades of Lactose

The international trade recognises following commercial forms of lactose.