The cost of pasteurization2 is estimated at a little over 0.3 of a cent a gallon for milk and a little over 0.6 of a cent a gallon for cream. The tests conducted by the U. S. Department of Agriculture, also show that the "flash heat" process is more expensive than the "holder" or intermittent process. The cost of pasteurization may in many cases be still further reduced by utilizing the heat contained in the exhaust steam from the engine. This heat would be sufficient, in many cases, for all the pasteurizing done in the plant, if it were properly utilized instead of being permitted to go to waste. 2 Office of Information, U. S. Department of Agriculture. CHAPTER IV CONDENSED MILK, MILK POWDER, AND HOMOGENIZED CREAM Condensed milk and milk powder are very frequently used in the ice cream mixture, where they serve the double purpose of making up a part of the batch and have a stabilizing effect as well. Because of the high percentage of milk solids which they contain, their chief value in ice cream making, aside from their food value, is in giving better body and texture to the frozen product. In the manufacture of condensed milk and milk powder, the water or a large part of it is driven off, leaving the solids in a highly concentrated form. Because of this high concentration it is very important that the milk used in the manufacture of these substances be of good quality. The quality of the milk used in the manufacture of condensed milk must be of the best obtainable. The condensers learned, early in their experience, that when the milk is brought into a concentrated form, the acids that developed, and any undesirable odors or contaminations which the milk may contain are also concentrated. To make good condensed milk it is necessary first of all to secure a supply of milk that is pure, fresh, and as free from contamination as possible. The process of condensing this milk consists in removing from it a considerable portion of the water by evaporation. According to the pure food standard, "Condensed milk, evaporated milk, is milk from which a considerable portion of water has been evaporated and which contains not less than 28 per cent. of milk solids of which not less than 27.5 per cent. is milk fat.”1 The process of evaporating and condensing the water contained in the milk is accomplished by heating it under reduced pressure. The purpose of evaporating under reduced pressure is to avoid giving the milk a cooked flavor and also to avoid, as far as possible, all those changes ordinarily brought about in milk by high heating. Since the boiling point of milk is a little higher than that of water, it would be impossible to condense the milk at that temperature without caramelizing part of the sugar and imparting a pronounced cooked flavor to the finished product. To avoid these and other changes in the milk it becomes necessary to condense it at a lower temperature. Since the boiling point varies with the atmospheric pressure, it is possible to bring milk to a boil at a temperature of 130° F. to 135° F. by reducing the pressure. The apparatus used for this purpose is known as a vacuum pan. The vacuum pan is a large kettle containing a steam coil or steam jacket to supply the necessary heat. The pressure within the pan is reduced by means of a vacuum pump and a 26-inch vacuum maintained until the milk has reached the proper degree of concentration. Under these conditions the water is evaporated quite rapidly and passes into the condenser in the form of vapor. The condenser is located at the top of the vacuum pan and is directly connected with it. As the milk boils the vapor rises and passes out of the vacuum pan into the condenser, which is simply a horizontal extension of the vacuum 1 Bulletin 143, Indiana station, page 484. pan. In the condenser the vapor comes into contact with a spray of cold water which causes it to condense. The condensed vapor and the condensing water are carried off by means of a pump. This process goes on until the milk is concentrated to the proper density. From this point the process is somewhat different for canned condensed milk and the condensed bulk milk. Condensed bulk milk. The condensed milk used in ice cream making is made in much the same way, except that it is not usually put up in cans and is not sterilized. The first part of the process is the same. When condensed to the desired point, the steam is shut off and the vacuum broken but the milk is allowed to stand in the pan, receiving the heat only from the jacket or by blowing steam directly into the milk in the pan. This is called super-heating and is done to prevent the precipitation of the sugar. In the super-heating the milk becomes very thick and heavy. It is then drawn and cooled as quickly as possible and put up in ordinary milk cans. The sterilizing is omitted because the milk is intended for immediate use and should not be held for any length of time. It is used quite extensively in ice cream making both as a part of the batch and as a means of improving the body of the ice cream. Its effect upon the body of ice cream is due to the high percentage of milk solids which it contains. Milk powder is also used as a stabilizer and as a part of the batch. It consists of the solids of milk reduced to a dry state by evaporation of the moisture contained in the milk. Milk powder is a comparatively recent product but has been steadily growing in favor on account of its convenient form and the various uses to which it may be put. It is used in various ways in the manufacture of ice cream. |