Читать книгу The Story of Milk - Johan Ditlev Frederiksen - Страница 13

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Professor V. Storch, originator of pure cultures for ripening cream and milk

Before 1890 it was supposed that the flavor of fine butter depended upon certain volatile oils and acids peculiar to butter-fat. In the early nineties Professor V. Storch of the Danish Experiment Station showed, however, that it was due rather to the products of bacteria and he isolated the lactic acid bacilli which would produce such exquisite flavor even when perfectly neutral and tasteless butter-fat was churned with milk acidified or ripened with a pure culture of these bacilli. In this country Dr. H. W. Conn of Wesleyan University, Storrs, Conn., did much to advance the theory and practice of ripening cream with a pure culture starter.

“Pure cultures” are produced in the bacteriological laboratory by picking out under the microscope colonies of the desired species of bacteria, planting them in a sterilized medium and allowing them to grow under the most favorable conditions and with the exclusion of all other germs.

Streptococcus lacticus (Storch, No. 18)

Bacterium lactis acidi (from Storch)

When such a culture has reached its maximum growth it is transplanted into a larger quantity of a sterilized medium containing proper nourishment for the particular organism. In the bacteriological laboratory, where alone absolute sterility of utensils and medium, and entire exclusion of foreign infection are possible, the culture may remain pure while this inoculation and propagation are repeated over and over again. But when the propagation is carried on in the house or the dairy, for instance in preparing starters or buttermilk, such absolute cleanliness is impossible and in the long run infections will creep in from the air or from the utensils and after a while it becomes necessary to start with a new “pure culture.” How often such renewal must be resorted to depends largely upon the surroundings and the care of the operator. Usually it must be done after a week’s time, although it is surprising to find milk preparations made by the simplest processes equal in purity to those prepared with the assistance of bacteriological science and technique. This is, for instance, the case in Bulgaria, where the famous Yoghourt sour milk is prepared pure without special care and in Denmark where the country is fairly permeated with the lactic acid bacilli used in ripening the cream for the celebrated Danish butter and where careful buttermakers often maintain their starters for months or even for years without “renewal.”

Typical lactic acid bacteria

(L. A. Rogers)

There are many different varieties of bacteria which convert sugar of milk into lactic acid, at the same time developing flavors more or less agreeable and characteristic for the various products. In the bacteriological laboratory certain species are selected which will produce the results desired for the particular purpose in view.

Starters.—Beginning with a commercial dry culture in the form of a powder as generally used in the creamery or the cheese factory as well as for the preparation of commercial buttermilk, or with buttermilk tablets as used in the ordinary household or the hospital, such culture is added to a small quantity of thoroughly pasteurized milk. If fresh, sweet skim milk is available it is preferable to whole milk as the butter-fat in the latter only interferes with the process; but either can be used.

Milk for starters should be strongly pasteurized by being kept at a temperature near the boiling point—at least 180°—for 40 to 60 minutes, then cooled to the degree at which it is to be set, usually between 65° and 75°, somewhat higher for the first propagation with the pure culture than for the subsequent transplantings when the bacteria, more or less dormant in the dry powder or tablets, have attained full vitality. Some species of bacteria, as the Bacillus Bulgaricus, require higher temperatures—90° to 100° or even 110°—than others. The culture having been thoroughly incorporated in the milk by vigorous and repeated stirring or shaking, the milk is left at rest in an incubator or a waterbath or wrapped in paper or cloth in a warm room where an even temperature can be maintained, until it is curdled, which may take 18 to 24 hours or even longer for the first propagation.

One part of this curdled milk is now added to 5 or 10 parts of fresh pasteurized milk and set to ripen in the same way as described above, possibly at a little lower temperature, and this is repeated every day, thus maintaining the “Mother Starter.” After the second or third propagation the bulk of each batch is used as a starter in the larger lot of material to be ripened, be it cream for butter or milk for cheese or for commercial buttermilk, while a little is taken for maintenance of the mother starter as described above.

The amount of starter to prepare every day depends upon the amount of milk or cream to be ripened and the per cent of starter used in same. For instance, if you have ten gallons of cream to ripen every day in which you wish to use about 10% or 12% starter, or one gallon, take a little less than one pint of the first or second propagation for one gallon of milk; the next day use one pint of this to add to a gallon of fresh starter milk, and the remaining gallon to add to the ten gallons of cream, and so on every day.

If you have 4,000 lbs. of milk in the cheese vat to ripen with 2% or 80 lbs. starter, prepare 88 lbs. of mother starter. If, on the other hand, you wish to make only a quart of buttermilk every day, take, say, two buttermilk tablets, crush them thoroughly in a spoonful of pasteurized milk and stir this into a tumblerful of the same milk; let stand till it is thickened the next day and use a tablespoonful of this thickened milk in a quart of fresh pasteurized milk which when ripened is your buttermilk, from which you take out a tablespoonful for starter in the next batch, and so on. In this case there is no “mother starter” except that perhaps the first tumblerful prepared with the tablets may be called so, but afterwards the starter is taken right out of the finished product every day.

The process may be modified to suit special purposes and local conditions, but the following precautions should be strictly observed: (1) to interrupt the ripening immediately by quick and intense cooling as soon as it has reached the proper point in case the ripened product is not used at once, and (2) to keep it ice-cold until it is used. If this is done, it may be kept for two or three days without deterioration if it is not convenient to make it fresh every day which, however, should be the rule.