In this article we will discuss about:- 1. Introduction to Lactose 2. Grades of Lactose 3. Method of Manufacture 4. Yield 5. Uses.
Introduction to Lactose:
Lactose normally occurs in either one of its two crystalline forms or as a non-crystalline glass. The more common crystalline form is that of a-lactose hydrate, which is regarded as the lactose of commerce. By far the most important source of lactose in the major producing countries is cheese-whey, although hydrochloric-acid-casein-whey is still considered to be the best source for its production.
In India, chhana-whey is produced in larger quantities than any other whey, and should, therefore, be the greatest source of lactose in the country. However, its collection from small-scale and scattered producers needs to be systematically organized.
Grades of Lactose:
The common grades of lactose are- crude, edible and USP. Crude lactose is the product obtained in the first stage of the common process of manufacture and contains many impurities; it is ordinarily used for refining lactose to the USP grade.
The edible grade contains less impurities than crude, but is below the standards for USP; it is suitable for use in infant foods. The USP grade is obtained by refining edible-grade lactose and meets the highest standards of purity; it is used in pharmaceutical preparations and for infant feeding. The specifications for the above grades of lactose are given in Table 12.9.
Method of Manufacture of Lactose:
Principle:
The lactose is separated from the whey (rennet or acid) by the simple process of concentration and crystallization.
The whey is pre-treated for the removal of impurities, viz., whey proteins, salts and acid, primarily for two reasons:
(i) To reduce the viscosity of the concentrated whey so as to make separation of lactose crystals possible, and
(ii) To increase the purity of the recovered lactose. The lactose crystals are then removed from the concentrate in a centrifuge, while most of the whey proteins and salts pass on to the remaining liquid called the mother liquor, which is used for animal feed or cooked food.
Flow Diagram of Manufacture:
Details of Manufacture:
(Adapted from Webb and Whittier). The whey (rennet or acid) is run into an iron tank/vat and boiled by means of live steam supplied by perforated pipes set in the tank. During the heating, milk of lime may be added gradually in the case of acid-whey until the acidity has been reduced to about 0.05 per cent (pH 6.2). When the milk proteins have completely coagulated, the steam is stopped and the whey allowed to stand undisturbed for a few minutes to enable the impurities to settle as a sludge.
The clear whey is drawn to a storage vat that feeds it to a double or triple-effect evaporator, the sludge being left in the vat to be filtered later. The whey is concentrated to about 20° Baume (equivalent to a lactose content of 30 per cent). The impurities that separate from the whey during the first stage of concentration are removed by filtering the whey in a filter press.
Subsequently the sludge from the coagulating at is passed through the same press. The partially purified whey may be combined with the earlier whey before proceeding with the second stage of evaporation, while the press-cake may be used for poultry feeding.
The partially concentrated and filtered whey is drawn into a single-effect vacuum evaporator for the second stage of evaporation, which is generally done up to 40° Baume. Part of the lactose crystallizes during its concentration in the pan and the operator observes the progress of crystal formation by looking through the sight glass. When graining has progressed sufficiently, it is ‘struck’.
The hot mass of crystals and syrup (51-60°C/125-140°F) is dropped into crystallizing vats, which are rectangular tanks with round bases. These vats are provided with slow-speed spiral screw agitators and with jackets in which cold water can be circulated.
It is essential to keep the crystals in motion so as to prevent them from ‘setting up’ into a solid mass. The speed of agitation and the rate of the cooling can be regulated to obtain uniform crystals of the size desired. Prolonged holding and reduced temperatures where the lactose is less soluble increase the yield.
Centrifugal force is employed to separate the lactose crystals from the mother liquor. The soft, wet mass of crystals and mother liquor are gravity-fed to a basket-type centrifuge, in which the lactose is freed from the mother liquor and washed. During the initial separation and washing, the basket is spun at about 600 rpm.
When the mother liquor ceases to run freely from the sugar crystals, the remainder is washed off with a small stream of cold water played on the crystals. The speed of the basket is then increased to 1200 rpm to remove as much of the wash water as possible. The crude sugar which emerges from the centrifuge is either refined promptly or dried at 79°C (175°F) in a rotary drum drier.
The refining of crude sugar to produce a sugar of USP grade requires that a solution of the lactose be treated to remove colour, protein and salts. The crude sugar is dissolved in water or washings from a previous batch, in a stainless steel tank, by means of live steam, the proportions of sugar and water being adjusted so that the Baume reading is 20° (which indicates an approximate 30 per cent lactose solution).
For each 100 kg. of sugar in the batch, one-fourth kg. of a filtering aid and 1 kg. of decolorizing paste are added. The decolorizing paste consists of 3 parts bone black, 1 part activated carbon, 1 part 36 per cent hydrochloric acid and enough water to make a paste that is easy to handle.
The treated solution is boiled and its acidity adjusted to 0.09 per cent (lactic) with hydrochloric acid. Before filtering, this acidity is lowered to 0.05 per cent with milk of lime. Over-neutralization should be avoided, as this will cause the sugar to caramelize and become discoloured. After the lime treatment, the solution is boiled vigorously for a few minutes and then allowed to stand until the flocculated mixture of carbon, protein and insoluble salts have settled.
It is then filtered through cloth in a filter press, using first a filter aid such as diatomaceous earth and then some fine-textured rag paper. The clear solution is drawn into a single effect evaporator and concentrated to about 40° Be. This is followed by crystallization (initiated by seeding), centrifuging, washing, drying, milling, packaging and storage on similar lines as for crude sugar. However, particular attention must to paid to details so as to meet the standards for USP grade.
Yield of Lactose:
The first crop of crude lactose should contain about 70 per cent of the sugar present in the whey and between 85 to 90 per cent lactose. (The second crop of crystals is difficult to separate and the yield is variable.) The conversion of crude to refined sugar gives an approximately 90 per cent yield, but overall yields amount to nearly 100 per cent, depending upon the lactose content of the crude, since the mother liquor and washings are re-processed.
Uses of Lactose:
(i) In the preparation of humanized milk;
(ii) In infant food;
(iii) In caramels or fudges;
(iv) In solid pharmaceutical preparations such as pills and tablets;
(v) In silvering mirrors;
(vi) In bakery products.