The Sugar We Get From Milk

C. H. Fisher.

Milk sugar, or lactose, is one of the naturally occurring sugars. It differs from sucrose, dextrose (or glucose), and other natural sugars in several respects. It is less sweet. It dissolves slowly in water. The amount that can be dissolved is smaller.

Lactose is present in the milk of all mammals to the extent of 2 to 8.5 percent; it is uniquely the sugar of the animal kingdom. Nutritionally it is highly important; because it is in milk, it is consumed in larger quantities than any other sugar except sucrose.

The cow's milk produced annually in the United States contains approximately 6 billion pounds of lactose. Because only a relatively small proportion of it is separated, however, its production in purified crystalline form is far below that of crystalline sucrose and dextrose.

Lactose was isolated and first understood as a separate milk component by the physiologist Fabritius Bartolettus in 1633. Some time later commercial production of lactose was started in Switzerland. For many years that country produced the only sizable quantities, but before the close of the nineteenth century most of the world's supply was made in the United States near St. Charles, Ill. Probably an even greater proportion of the world's current lactose production is in this country.

The quantity of lactose produced annually in the United States increased gradually until it became constant at about 7 million pounds during the several years preceding the development of penicillin. After workers in the Northern Regional Research Laboratory found that lactose is the best sugar for penicillin substrates, the need for this sugar increased at such a rate that production trebled in a few years. As penicillin became more and more important the manufacture of lactose went up from 7.6 million pounds in 1943 to 23 million pounds in 1946.

Lactose is utilized industrially in several ways. It can be used as such; that is, as the purified solid sugar. Lactose can be used also as it occurs in whole milk and in dairy byproducts, such as skim milk, buttermilk, and whey. Lactose can be converted chemically into various derivatives, such as lactose esters and ethers, which have potential industrial importance. A fourth method comprises degrading the large lactose molecule by heat, chemical agents, or microbiological organisms and agents into smaller molecules. Most of the smaller molecules thus produced are chemicals of actual or potential usefulness.

Lactose was important during the Second World War. It was a valuable ingredient of vanilla and chocolate tablets and other concentrated foods distributed to the Armed Forces in all parts of the world. It was an important fuel in pyrotechnics; because it burns slowly and deepens the color of signals, it was used in various military and distress signals and in target identification candles.

WHEY, AN INEXPENSIVE DAIRY BY-PRODUCT containing lactose as the principal organic constituent, is the best source of lactose.

About 10 billion pounds of whey is produced annually in the United States. Approximately 9 billion pounds of whey is obtained as a byproduct in the manufacture of whole-milk cheese; 1 billion pounds is obtained similarly from cottage, pot, and bakers' cheese. A lesser quantity, 600 million pounds, of whey accompanies the production of casein. Casein whey is important, however. because it was the sole source of lactose in this country until about 1944.

The 10 billion pounds of whey produced each year contains about 500 million pounds of lactose. Because only about 4 percent of that lactose is actually separated and refined, there is plenty of whey for the manufacture of much larger quantities of the sugar. Other important constituents in the 10 billion pounds of whey are 50 million pounds of protein, 40 million pounds of nonprotein nitrogenous matter, 30 million pounds of fat, and 12,000 pounds of riboflavin (vitamin B₂).

Much of the whey is produced at widely scattered points; hence not all of it is available under conditions suitable for economic processing and utilization. Considerable quantities of it, however, are available in production centers under favorable conditions. In addition, at many centers of production, skim milk or whey could be concentrated and transported to nearby manufacturing areas at relatively low cost. In the manufacturing centers, where whey is obtained in volume, the utilization and disposal of this perishable and biologically active material present a problem.

Some types of whey are better than others for making lactose. For precipitating casein to obtain casein whey, either hydrochloric (muriatic) acid or sulfuric acid can be used. The casein whey Produced with sulfuric acid is objectionable because of the difficulty of removing certain metal sulfates that impart cloudiness to the lactose solutions. Self-soured casein whey also is not a suitable raw material for making lactose because a considerable quantity of the lactose has been converted by fermentation into lactic acid. The same is true to a less extent for cottage-cheese whey. Muriatic casein whey is largely free of the objections, and so is considered a desirable raw material for making lactose.

Several methods have been developed for recovering lactose from muriatic casein whey. In one of them the whey is heated to boiling in iron tanks with live steam. Lime is added during the heating until the acidity is about 0.5 percent or the pH value is 6.2. The coagulum is allowed to settle and the clear whey is evaporated in a multiple-effect evaporator to a concentration of 30 percent lactose or 20 Baume. After being passed through a filter press, the sirup is concentrated further to about 40 Baume by evaporation. The hot mass is dropped into crystallizing vats, where it is cooled and agitated slowly. The solid lactose thus obtained is freed from mother liquor by centrifugation and then washed with cold water. A second crop of crystals can be obtained by concentrating the mother liquor. The wet crude lactose should be either refined or dried promptly to prevent spoilage.

The recovery of crude lactose is usually 3 to 3.5 pounds per 100 pounds of whey. Further crystallization is required to produce refined, or United States Pharmacopoeia, lactose, the yield of which is 2.5 to 3 pounds per 100 pounds of whey. The less costly lactose of crude or technical grade is satisfactory for many purposes. At one time manufacturers usually attempted to obtain the maximum yield. Some manufacturers now find it profitable to make only partial recovery of about 2.5 pounds and to use the remaining mother liquor to make poultry feed.

The increased demands for lactose imposed in 1944 by the penicillin development were met principally by increasing production from cheese whey. Lactose can be made from cheese whey by concentrating it in a vacuum evaporator to 55 to 60 percent content of solids, cooling the concentrate with occasional stirring in a vat, centrifuging to separate the solid lactose, washing with cold water, and drying.

Cheese whey ordinarily must be gathered from several cheese factories; and,. unless properly handled, it often ferments. So procurement is more costly, and the average yield is lower than that from casein whey.

The three grades of commercial lactose are crude, technical, and refined. The refined lactose is a white, odorless powder, at least 99.7 percent pure, as determined by the polariscope. The crude and refined grades sold for about 16 and 26 cents a pound, respectively, in December 1949. As sucrose and dextrose are usually available at less than 8 cents a pound, lactose is at a disadvantage for applications that can be met equally well by the other sugars.

Because B-lactose is more soluble than cc-lactose, the normal form, and gives the impression of being sweeter, B-lactose is in demand for some uses. To meet the demand, methods for making B-lactose have been studied. Drying lactose solutions by the spray-drying process produces a mixture of the two forms in approximately the equilibrium ratio of 1.65 parts beta to 1 part alpha. The product dissolves much more rapidly than a-lactose, but it is hygroscopic and has poor wetting properties. The product made by drying lactose solutions on a drum drier contains as much as 90 percent of B-lactose if the most favorable drying conditions are used. Such a product has good wetting properties and is less hygroscopic than the spray-dried product and slightly more soluble initial]) than pure B-lactose. Other methods fol making B-lactose have been developed, and its production in purified form has become an established industrial operation.

MUCH ATTENTION has been directed toward the mother liquor, or molasses, from lactose production to achieve maximum utilization of the byproducts. Early observations revealed that the material possesses marked growth-promoting properties that are accentuated by the addition of traces of crude rice polishings. At the time of the observations, vitamins were differentiated merely as fat-soluble or water-soluble. The evolution of vitamin technology gradually disclosed that lactose molasses contains numerous water-soluble vitamins, mainly riboflavin. Methods for the commercial recovery of natural crystalline riboflavin from this product were perfected about 1935, and for a short time this crystalline material was the only pure riboflavin commercially available. The natural product, however, did not long enjoy this status because of the persistence of research chemists, who rapidly synthesized riboflavin and initiated its mass production.

The byproducts of lactose manufacture, which contain valuable vitamins, minerals, and other food factors, are generally concentrated and used in poultry and animal feeds.