( Originally Published 1939 )
It is necessary to supervise the quality of the raw-milk supply to make a satisfactory condensed milk. Excessive bacterial contamination with its concomitant acidity, off-flavors from any source, and sediment are harmful factors in the production of a good marketable product.
The incoming milk, after it has been examined on the receiving platform, is heated in a hot-well or forewarmer at a temperature of about 170 to 180° F., in order to inactivate the enzymes which may later cause harmful changes in the finished product, kill the germ life, and facilitate the solution of the sugar. The hot milk is dropped into the sugar mixer, where the sugar is added, although some manufacturers add the sugar to the milk in the hot-well, and others at the end of the condensing treatment. Beet, cane, or corn (cerelose) sugars are used in amount to give a concentration of about 62.5 percent content of total sugars (the added sugar plus the original lactose content in the milk) in the finished product. This requires an addition of about 15 pounds of sugar to every 100 pounds of fresh milk.
After thorough mixing, this sweetened milk is drawn into a vacuum pan. Here the batch is heated in a vacuum of about 25 inches, where it will boil at a temperature of about 130° F. The operator observes the boiling and determines when the concentration has been completed by withdrawing samples for examination.
The batch is then ready for cooling. Lactose is so relatively slightly soluble that at the concentration of condensed milk the solution is supersaturated with lactose. On cooling, this sugar crystallizes out and may impart a quality which is described by the layman as granular or sandy. Good commercial practice handles the cooling in such a way that the lactose goes into very fine crystals which remain suspended fairly uniformly throughout the milk.
For the retail trade, condensed milk is packed into 14-ounce tin cans. This milk is not sterile, but the microbic growth is prevented by the high content of solids. Usually the cans are hermetically sealed in order to reduce the availability of air for yeast fermentation, and prevent the entrance of molds.
RELATION TO THE PUBLIC HEALTH
Composition and standards. Condensed milk is defined by the U. S. Department of Agriculture 1 as follows:
Sweetened Condensed Milk. The product resulting from the evaporation of a considerable portion of the water from milk to which sugar and/or dextrose has been added. It contains not less than 28 percent of total milk solids, and not less than 8 percent of milk fat.
The figures in Table XI give the analyses of 31 samples of condensed milk reported from the Connecticut Experiment Station at New Haven:
The same considerations prevail regarding sanitary standards in the condensed-milk as in the evaporated-milk industry. A good quality of milk is essential from the organoleptic and chemical stand-points in order to manufacture a proper-tasting and marketable product. Inasmuch as these properties are dependent on good sanitation, commercial expediency requires care in the production and` handling of the raw-milk supply. There is good official supervision of sanitary quality for these parts of the milk supply because they serve as the reserve to be drawn upon by the municipalities when their regular milk supplies run short.
Microbiology. Condensed milk (sweetened) has not been found to have been the incriminated agent in any milk epidemiology.
Savage and Hunwicke reviewed the earlier work and extensively studied the microbiological condition of the product on the British market. They found that sweetened condensed milk is not sterile but that its bacterial content decreases in storage, although they caution that opened cans should not be kept in warm closets. The main cause of unsound cans was yeast fermentation. They report that other workers had found coliform organisms, streptococci, staphylococci, molds, yeasts, and spore-formers in irregular amount. The darkening and thickening of sweetened condensed milk with age were not found to be associated with any increase in bacterial content but seemed to be a function of time and temperature. They suggest that standards be established for limiting the allowable number of lactose fermenters of the coliform type, or of fermenting yeasts, as a guide to the sanitary control of the manufacturing operations. Even if sweetened condensed milk should become infected, it is probable that the composition of the product itself would constitute an effective protection against its becoming an etiologic agent.
Schoenholz, Esty and Meyer 3 experimentally inoculated detoxified spores and soil into cans of sweetened condensed milk, incubated at room temperature and 37° C. When opened and examined after 5½ months, the product seemed normal in appearance. Viable Clostridium botulinum organisms were demonstrated but the product was non-toxic. They state that the available evidence indicates that spores of Cl. botulinum cannot grow in sweetened milk.
Tanner has reviewed the work by numerous investigators on the bacteriological condition of sweetened condensed milk and concludes that many cans contain viable non-pathogenic organisms. Improvements in sanitation and manufacture have resulted in marked improvement in microbiological quality.
Nutritional value. Stringfield and Tobey 5 have reviewed the subject of the food and nutritional value of condensed milk, and they give formulas for use in feeding it to infants. They discuss the popular belief that babies fed on this product develop flabbiness and pallor. These authors attribute such a condition of infants to overfeeding and to a faulty regimen because even certified milk will do this. They quote the experience of medical practitioners that condensed milk does not develop overly fat babies and that this has been confirmed at the Post Graduate Hospital in New York. They report that this milk has been used satisfactorily in the tropics and in South America, and gives good health and development.
Harrison studied the nutrition of five infants 4 to 7 months of age fed on sweetened condensed milk. All were supplied with orange juice and cod-liver oil. The milk was diluted so that 10 and 15 percent of the calorie requirement was provided by the proteins. The retention of calcium and phosphorus was less than when the minerals were increased in the diet but was sufficient to permit normal growth.
The Commission on Milk Standards recommended that condensed milk be "so labeled that the product prepared from fluid milk of a good quality may be distinguished from that prepared from inferior milk." They urge that the product be prepared from Grade A milk and that the manufacturer be protected in claiming it on the label.
Types of spoilage. Condensed milk deteriorates in quality on long standing. It gradually thickens in consistency, and develops a liverish or custardlike body. It may become too dense to pour. Storage at warm temperatures intensifies this condition.
A dark color develops, also, under similar conditions. This cannot be an ordinary caramelization because it takes place progressively. It is thought that the color is due to a reaction between dextrose and some amino acids. Storage at low temperature reduces the rate of color development, and at high temperatures increases it.
Bacterial spoilage is sometimes encountered, but a yeasty fermentation is of more frequent occurrence.
Chemical analysis. The sample is warmed to a temperature of about 90° to 100° F. (32° to 38° C.), and the entire contents, including that adhering to the sides of the can, are thoroughly mixed. A sample of 100 grams is diluted with water up to a volume of 500 milliliters.
Total solids. The percentage of total solids is determined by measuring out 10 milliliters of the above solution and proceeding as directed for milk by drying in a flat-bottom dish on either sand or asbestos fiber.
Fat. Fat is determined by weighing 4 to 5 grams of the above dilution into a Röhrig tube or a similar apparatus, diluting with water to about 10.5 milliliters, and proceeding as for milk.
Sucrose. Sucrose is determined (after clarifying the solution) by first precipitating the lactose of the milk with cuprous oxide, then inverting the sucrose in an aliquot of the filtrate, and again precipitating with cuprous oxide,° or by polarizing according to the official method.
Bacteriological examination 10 The same procedure as outlined for evaporated milk may be followed for sampling, except that extremely viscous samples should first be warmed at a temperature not above 37.5° C.
Plating. Dilutions for plating should be made by weighing the sample. The rest of the technic is the same as for evaporated milk.
Microscopic examination. The direct microscopic examination is useful to supplement the other determinations but should not be relied upon as an adequate procedure for the determination of the bacterial content. The technic is the same as for evaporated milk.
Hemolytic streptococci. Examination for hemolytic streptococci is made on blood agar plates incubated at 37° C. for 2 days.
Thermophilic organisms. Thermophilic organisms are determined in the reconstituted sample as in evaporated milk.
Thickening caused by bacteria. To determine whether the thickening was caused by bacteria, the sample is diluted and incubated at 30° C. for 1 month. If such organisms are present, the diluted milk will be thick, and the titratable acidity will be materially increased.
Cultures of these organisms can be obtained by plating on nutrient agar and incubating for 2 days at 37° C.
Gas-forming organisms. Gas-forming organisms may be determined by inoculating 1 milliliter of milk into deep agar medium of special composition.
Yeast. The presence of yeast can be ascertained as a routine check by incubating the samples at 37° C. If the cans swell, the milk should be plated on standard agar containing 30 percent sucrose. These organisms can be determined by the method followed in the micro-biological analysis of butter (see page 180).
Control procedure. The same conditions prevail in the regulatory procedure for condensed milk as for evaporated milk. The product is not given a special heat treatment to sterilize it, and therefore sterilization is not claimed or expected. However, the product should be clean, and should possess no flavor foreign to that of good milk, except the sweet taste of the sugar and the cooked flavor due to the condensing operation.
1. Service and Regulatory Announcements, Food and Drug 2, fifth revision, November, 1936.
2. W. G. SAVAGE and R. F. HUNWICKE, Food Inspection Board, Special Rept. 13, 1923.
3. P. SCIIOENHOLZ, J. R. EsrY, and K. F. MEYER, J. Infectious Dis., 33, 289 (1923).
4. F. W. TANNER, The Microbiology of Foods, Twin City Printing Co., Champaign, Ill., 1932, pp. 273 et seq.
5. O. L. STRINGFIELD and J. A. TOBEY, Arch. Pediat., 77, 769 (1930).
6. H. E. HARRISON, J. Pediat., 8, 415 (1936).
7. Commission on Milk Standards, Condensed Milk, U. S. Pub. Health Repts., Reprint 634, 1921.
8. Methods of Analysis, 4th ed., Association of Official Agricultural Chemists, Washington, 1935, p. 280.
9. T. MOJONNIER and H. C. TROY, The Technical Control of Dairy Products, 2nd ed., Mojonnier Bros. Co., Chicago, Ill., 1925, pp. 591-596.
10. Subcommittee on Bacteriological Methods of American Dairy Science Association, J. Dairy Sci., 18, 652 (1935).
11. Committee report, J. Dairy Sci., 16, 289 (1933).