CHAPTER VI
THE PREPARATION OF SOURED MILK IN THE HOUSE
There is no great difficulty in making soured milk at home: the necessary operations are quite simple, but at the same time they must be conducted with precision and care, otherwise the results may be unsatisfactory and disagreeable; there may even sometimes be danger in badly prepared sour milk. It is always an advantage in such matters to understand the reason of things, and a few notes on the surrounding conditions, and what has to be accomplished, may be of assistance to the would-be experimenter.
The majority of intelligent people are now acquainted with the fact that the germs of bacteria are to be found everywhere on the surface of the earth, in air, and in water, and that they are the sole cause of the decay of all manner of perishable articles.
The distribution is unequal—bacteria are much more plentiful where there is decaying matter—in dirty houses, sewage, or other contaminated water, etc. Milk is a splendid food for bacteria, and numerous varieties multiply in it exceedingly, and many of these are injurious, producing putrefactive changes which render the milk unwholesome, even poisonous in some cases. Others are beneficial, and are absolutely necessary for the souring of milk for making butter or cheese and for the ripening of the latter. The soured milk which is the subject of this book is the work of certain lactic-acid-producing bacteria, and the problem we have before us is to encourage the growth of the latter to the uttermost and to exclude the others.
As bacterial germs are present in the air and readily sow themselves into any medium with which they come in contact, the first consideration is to get good fresh milk which has been as little exposed to the air as possible. The second is to conduct the experiment where the germs are fewest, and in cleanly surroundings, far removed from decaying matter and free from taints and smells.
Fig. 38.—Photo-micrograph of smear of one-month culture of Bacillus bulgaricus. In spite of its age, the culture is perfectly free from any foreign organisms, which would otherwise lower its value for the preparation of soured milk, and might, indeed, make it directly injurious.
Fig. 39.—Photo-micrograph of culture of Bacillus bulgaricus in malt. In cases of acute enteritis, where milk cannot be supported, the lactic bacilli may be cultivated in malt, and administered to the patient in this form, with occasional doses of syrup of malt, in order to induce a vigorous growth of the lactic ferments in the body.
Fig. 40.—Photo-micrograph of smear from milk that had been allowed to sour spontaneously. Ordinary market milk is always subject to infection from the air, milking vessels, manure, etc., and from these sources a mixed bacterial flora arises, with the result that the milk exhibits curdling, acidity, gaseous fermentation, or mould growth, after being stored for a short time. This is due to the action of sush bacilli, diplococci, yeasts, and moulds as are shown in this illustration.
However fresh the milk may be, it will contain useless or injurious bacteria, and we must get rid of these before attempting to introduce those whose growth we wish to encourage. This is effected by heat. All the living bacteria and most of the germs are killed at temperatures somewhat under the boiling point. Having sterilised the milk in this way, it is necessary, as far as possible, to prevent the entrance of fresh germs from the atmosphere, and we therefore let the milk cool down in covered dishes. When the temperature descends to about 100° F. the culture of the special bacteria is introduced, the covers are replaced, and the milk vessels maintained at or near this temperature for twelve hours, when the soured milk is ready for use. It is not necessary to use fresh culture every time—a little of the soured milk will take its place, and this may be repeated as many as fourteen times before it is necessary to start off again with a fresh culture. A great deal depends on the care exercised and the freedom of the surroundings from bacterial germs. Under the best conditions wild germs will gradually accumulate in the soured milk, but their increase may be greatly delayed by attention to the precautions mentioned. The ordinary souring of milk for butter and cheese making is conducted in cool surroundings, as already stated, because in such conditions the lactic-acid-producing germs increase relatively faster than the wild germs, and so gain the upper hand, but in the case of our special soured milk we kill out, practically, all wild bacteria and germs, and the pure culture having the field to itself, we can conduct the operation at a higher temperature where the action of the bacteria is at its maximum, and so obtain the necessary lactification in the minimum of time.
The appliances for the souring of milk on the domestic scale require some consideration. We propose to describe the principal forms of apparatus which have been put on the market for the purpose, and then to give such suggestions as may assist the ingenious in making apparatus for themselves. Those who wish for information on the subject of larger apparatus will find it in the following chapter.
Fig. 41—Photograph of Agar Culture, inoculated with a lactic powder offered to consumers under a fancy name. Working on the assumption that the presence of lactic bacteria is inimical to the growth of septic organisms, this preparation has been placed on the market.
The plate shows, however an abundance of colonies of foreign organisms—sporogenous bacilli, Staphylococcus pyogenes albus, and Staphylococcus pyogenes aureus. These organisms are shown in the appended photo-micrographs, and constitute impurities in the preparation.
| Spore forming bacillus | Staphylococcus pyogenes albus |
Fig. 42—Photograph of Test Tubes of Sterile Milk, inoculated with a tablet preparation said to contain pure cultures. The darker liquefied portion of the tube-contents is due to digestion of the curd by proteolytic bacteria, thus indicating impure culture. Pure cultures of lactic acid bacteria coagulate the milk uniformly, but do not produce any subsequent change in the appearance of the culture, even after several months.
Fig. 43— Photograph of Test Tubes of Sterile Milk, each tube having been inoculated with a tablet of a preparation said to contain pure cultures. Here, again, there is evident peptonisation of the curd, thus indicating infection by proteolytic ferments. Cultures from these tubes demonstrated the presence of a foreign organism. Microscopical examination failed to disclose the presence of Basillus Bulgaricus.
Fig. 44—Photograph of Test Tubes of Sterile Milk, inoculated with a tablet of "Lactobacilline." In contra-distinction to those shown in Figs. 42 and 43, these tubes exhibit homogeneous curdling of the milk without any subsequent digestion or peptonisation of the coagulum. This is indicative of the purity of the culture.
The Society "Le Ferment" of Paris, which has been authorised by Professor Metchnikoff to prepare and supply to the public his sour milk culture, provides an apparatus for the treatment of the milk. It is shown in Fig. 45, which consists of a double box having the intervening space packed with a non-conducting material. It is provided with a tight lid. Inside, there is accommodation for two milk vessels, each with a capacity of about two thirds of a pint. The most difficult thing in the souring of milk is to maintain the temperature as nearly at 100° F. as possible while the culture is in action. This result is attained by filling the vessel in the middle with boiling water. The insulated walls hinder the escape of heat, and the quantity of boiling water used is calculated to maintain the temperature steady for the twelve hours of cultivation; but in cold, frosty weather it is necessary to refill the central vessel with boiling water in the middle of the period.
Fig. 45
The milk jars are washed with hot boiled water and turned upside down to dry. They should not be wiped with a cloth. Boil the milk to be treated for ten minutes, stirring it to promote evaporation, as it is advantageous to have it in concentrated form. Cool rapidly to 100° F. by placing the boiling vessel in cold water, add to each bowl one third of a tube of the culture in powder form, fill up with the boiled and cooled milk, stir well and cover. Place the jars in the box and fill the central vessel with boiling water, shut the lid tight, and do not open it (unless a fresh charge of boiling water is needed) for ten or twelve hours, when it will be ready for use. If the liquid culture is used (one small phial for each bowl) the milk should be cooled to 86° F. instead of 100° F., as with the powder. The culture is also supplied in tabloid form. The powder and tabloids keep well, but the liquid can only be relied on for about two months. The milk prepared as above should be stored in a cool place, the lids being kept on the bowls. It is good for about two days, after which it becomes too sour. It can be eaten with sugar, which not only sweetens it but is beneficial in affording additional suitable food for the acid-producing bacilli. If fresh milk cannot be had, condensed, sterilised, or pasteurised milk may be used, but, of course, fresh milk is best. Condensed milk should be diluted with two parts of boiling water and then treated like ordinary milk.
An apparatus on similar principles is sold by the Maya Bulgare Company, Ltd., and is illustrated in Fig. 46.
Fig. 46—Soured Milk Apparatus of the Maya Bulgare Company, Limited, consisting of an insulated box, hot-water vessel, and covered vessels containing the milk and culture. The apparatus consists of cabinet in deal, Maya Bulgare ferment in 20-dose bottles, Maya Bulgare ferment in 100-dose bottles, compressed Maya Bulgare tablets in boxes of 8 tubes, Maya Bulgare caramels in boxes of 40, reduced milk, china funnels, and thermometers.
The box is insulated, but the door is on the side, the hot water vessel is underneath, and the covered vessels containing the milk and culture are placed on a shelf above. In the front of the illustration are shown the various packages in which the liquid and powder cultures are put up. The procedure is exactly the same as with "Le Ferment" apparatus.
Lactic Ferments, Limited, make use of a different principle to maintain the temperature steady during the lactifying period. Their apparatus (Fig. 47) consists
Fig. 47 of a water vessel mounted on a stand. The milk vessels (tumblers) are placed in the water, and the temperature maintained at the proper figure by a small night light burning underneath. It is recommended that the milk should be placed in an earthenware jar or jug, which is stood in a pot of water kept boiling for an hour. After cooling add from three to six previously crushed tabloids of culture, and stir well with a glass rod which has been sterilised in boiling water. The milk is then transferred to three tumblers, which it should fill, and these are put into the water vessel, the water in which should be at about 100° F., and the night light started.
Messrs. Allen & Hanbury, Limited, also make use of the night light to maintain the proper temperature during incubation in their "Sauerin" apparatus (Fig. 48).
No water, however, is placed in the metal container. The procedure is the same as that already described, and both tablet and liquid pure cultures are supplied. For children it is recommended that the incubation should occupy from three to four hours only, in other cases eight to ten hours. Grated nutmeg, ground cinnamon, or other flavouring, and cream may be used with the soured milk.
The night light is also employed in the "Veronelle" apparatus of Messrs. Clay, Paget and Company, Limited (Fig. 49).
The containing vessel is of tin or aluminium, and has two stands, the high one for hot and the low one for cold weather, as in the latter case greater heat is needed to maintain the incubating temperature. The milk is placed in an earthenware jar and is sterilised by placing it in a saucepan of water and boiling it; continuing the boiling for half an hour. It is allowed to cool to about 98° F., and placed in the incubator, culture added, and the lamp lighted, the cover of the incubator being kept on. The period of incubation is given as six hours. To prepare the next day's supply a tablespoonful of the soured milk is retained and used instead of the culture. This may be continued for fourteen days, when a fresh start with culture is necessary. The soured milk will keep for thirty-six hours. Capacity, one and one half and two pints; also a large size for family use.
For the preparation of soured milk on a small scale, one of the various forms of vacuum flasks now on the market may be used with satisfactory results. A little cold water must be poured into the flasks, and warm water added, until, by means of three to four changes, boiling water can be safely poured in without cracking the flask.
This boiling water must be allowed to remain in for about twenty minutes, and then replaced by freshly boiled milk that has been cooled, so that its temperature in the flask is about 105° F. The culture of lactic organisms should then be added, the opening of the flask plugged with clean cotton-wool, and the cap screwed on.
In an actual test, the temperature of the milk placed in such a flask was 105° F., at 7.30 P.M., and had dropped to 93° F. by 9 A.M., the following day. The milk was curdled, and possessed the normal acid taste of such cultures.
The different types of apparatus are all quite simple, and it would be easy to make something at home. Get two round tins, the one less in diameter by from two to three inches than the other, put one or two pieces of wood across the bottom inside the larger tin, and fill up the space between with cotton-wool, which is an excellent non-conductor of heat. Place the smaller tin centrally inside the larger one. Fix three or four distance pieces of wood in the space between them, fill up with cotton-wool, leaving a little space at the top to permit the lid of the smaller tin to be fixed on. The boiling-water vessel may be a tin saucepan with a lid, but no handle, and its proper place would be on the bottom of the inner tin. A tripod stand made of three pieces of wire bound together, and with legs reaching past the hot-water vessel to the bottom of the tin, would support a false bottom of tin forming the shelf on which the vessels of milk to be treated would rest. These might be tumblers covered with tin lids. To prevent radiation through the lid of the inner tin, a thick pad or hood of cotton-wool packed between cloth would be placed over the top. A little experimenting would be necessary to determine the quantity of boiling water required to maintain the proper temperature.
The soured milk obtained in the above manner is of the consistency of ordinary buttermilk; a separation of whey frequently takes place, and this may be poured off if desired. The taste and flavour should be pleasantly acid and agreeable, and both are distinctive enough to give a good idea of the purity of the product. In many places a more concentrated article is made by boiling down the milk to one half or one third of its bulk, and then fermenting it in the ordinary way. A kind of thick pudding is thus obtained, which is highly recommended both as an agreeable article of food and as a stronger medium for the support of the germs it is the purpose of the sour milk treatment to introduce into the digestive system.
The cultures for making soured milk can now be had from all the leading wholesale and retail chemists, with directions as to the quantities to be used.
It is very necessary that the milk used should be not only fresh and good, but also free from chemical preservatives. The effect of these is to inhibit the growth and development of bacteria, and they have therefore an injurious influence on the special lactifying germs it is our object to cultivate. Preservatives are now not nearly so much used as they were, a few years ago, and there should be no difficulty in obtaining milk free from them.
For keeping both fresh and soured milk good a small ice-chest is an excellent device. The germs which are continually dropping into milk kept in open vessels in the air, even in cleanly surroundings, are mostly kept out in the close ice-chest, and the low temperature prevents in a natural way the development of the bacterial germs already in the milk. For the preservation of all kinds of foods the ice-chest is a capital investment, not to mention the benefit of having cooled drinks, etc., as required. They manage things well in this respect in the United States, where ice is regarded as a necessity in the summer time by even the poorest people, and is delivered with the same regularity as the milk.
It is necessary to utter a word of warning as to certain forms in which the sour milk bacillus is being offered to the public. It is being compounded with sugar, chocolate, and other articles, and sold in the form of sweets, etc. There is no reliable proof that these preparations are valuable. Certainly, sugar is a medium in which the lactifying germs can live, but the quantity so introduced into the system must be very small compared with what is obtained from properly fermented milk. It takes some time, even with strong cultures, to fix the acid-producing germs in the large bowel where they are wanted, and until proper evidence is forthcoming that confectionery preparations are efficacious we would recommend that only the regularly fermented milk be used. It would save trouble, no doubt, to treat oneself with a few chocolate creams containing the necessary germs daily, but if the matter is taken up seriously it will be better to take some pains and stick to methods the efficacy of which has been demonstrated, leaving the others until their bona fides has been proved. There are always enterprising firms who are prepared to simplify things for us, but we must make sure that their simplifications are warranted.