The primary difference between condensed and sweetened condensed milks is addition of sugar. Sweetened condensed milk is preserved by addition of sugar, which reduces water activity to a point inhibitory to most microorganisms. The increased milk solids content also decreases the water activity. The sugar-in-water concentration of sweetened condensed milk is called the sugar ratio, which is calculated as follows:
% Sugar in condensed milk -6-X 100 = Sugar ratio
100 — Total milk solids in condensed milk
Like condensed milk, sweetened condensed milk may be as whole milk or skim milk and be used either in bulk or consumer (canned) form. Most sweetened condensed milk is whole and is used in bulk in bakery and confectionery industries. With modern processing, storage, and handling practices, spoilage seldom is encountered. If the bulk product is improperly handled or held for extended periods before use, surface growth of yeasts or molds may occur. These microorganisms are the most common cause of spoilage of sweetened condensed milks. Their presence is indicative of unsanitary postpasteurization conditions. The consumer (canned) product has been thermally processed and is commercially sterile (see Sec. IV).
Evaporated milk, like other processed canned foods, originated with the experiments of the French scientist Nicholas Appert (Clark, 2000a). Appert, whose work on food preservation began in 1795, was the first person to evaporate milk by boiling it in an open container and then preserving it by heating the product in a sealed container. Fifty years later, another French scientist, Louis Pasteur, laid the scientific foundation for heat preservation through demonstrations that food spoilage could be caused by bacteria and other microorganisms.
Patents dealing with preservation of milk after evaporation in a vacuum were granted to Gail Borden by the United States and England in 1856. These patents applied to concentrating milk without addition of sugar. In 1884, U.S. patent number 308,421 was issued for ''an apparatus for preserving milk'' and, in 1885, the first commercial evaporated milk plant in the world was opened in a converted wool factory in Highland, IL, where ''evaporated cream'' was manufactured and sold (Clark, 2000a).
Evaporated milk is a canned whole milk concentrate to which a specified quantity of vitamin D has been added and to which vitamin A may be added. It conforms to the U.S. Food and Drug Administration (FDA) Standard of Identity 21 CFR 131.130 (U.S. Department of Health and Human Services, 1999a), having a minimum of 6.5% milkfat, 16.5% milk solids-not-fat, 23% total milk solids, and 25 IU vitamin D per fluid ounce. Related evaporated milk products are evaporated skim milk, evaporated low-fat milk, evaporated filled milk, and evaporated goat's milk. Evaporated skim milk contains not less than 20% of total milk solids, not more than 0.5% milkfat, with added vitamins of 25 IU vitamin D and 125 IU vitamin A per fluid ounce. Typical compositions for other evaporated milk products are as follows:
Evaporated low-fat milk: 2% milk fat, 18% nonfat milk solids, vitamins A and D added
Evaporated filled milk: 6% vegetable fat, 17.5% nonfat milk solids, vitamins A and D added
Evaporated goat's milk: not less than 7% milkfat and 15% nonfat milk solids, vitamin D added
A typical processing scheme for evaporated milk (Fig. 2) begins with high-quality, fresh whole milk to which vitamins, emulsifiers, and stabilizers are added. The product is then pasteurized, concentrated under reduced pressure in an evaporator, homogenized, cooled, and standardized to the composition desired in the final product. After cans are filled and sealed, they are sterilized in a three-phase continuous system consisting of preheater, retort, and cooler and then labeled and packed for shipment. In the United States, evaporated milk is packed in 5-, 12-, and 97-fl oz lead-free cans. In 1999, production of evaporated milk and related products (evaporated skim milk, evaporated low-fat milk, and evaporated filled milk) was slightly more than 477 million pounds (American Dairy Products Institute, 2000).
Evaporated milk processing is covered by FDA regulations dealing with thermally processed low-acid foods packaged in hermetically sealed containers (U.S. Department of Health and Human Services, 1999b). Therefore, manufacturers of evaporated milk and related products must comply with stringent pro-
cessing regulations, including establishment and filing of scheduled processes with the FDA and maintenance of strict processing records.
Because of the heat processes and packaging used to manufacture evaporated milks, the product is commercially sterile. This means that the product is free of all microorganisms of public health significance and does not show microbial defects during its intended shelf life under normal conditions of handling, storage, and distribution. Whereas vegetative cells do not survive evaporated milk processing, and absolute sterility is obtained in most cans, small numbers of nonpathogenic spores occasionally may survive the heat treatment and, depending on the microorganism and its previous growth and heat exposure, subsequently may germinate (Curran and Evans, 1945). Kalogridou-Vassiliadou (1992) studied 40 strains of bacilli implicated in causing flat sour spoilage in evaporated milk. The microorganisms were identified as Bacillus stearothermophilus (five strains), B. licheniformis (10 strains), B. coagulans (15 strains), B. macerans (five strains), and B. subtilis (five strains). Species of the genus Bacillus (i.e., cereus, coagulans, megatherium, stearothermophilus, and subtilis) earlier were implicated in evaporated milk spoilage (Foster et al., 1957; Hammer and Babel, 1957). Langeveld et al. (1996), in studies of B. cereus naturally present in raw milk, reported no evidence that this organism would cause intoxication in healthy adult humans at levels less than 105/mL. Beard et al. (1999) and Wandling et al. (1999) studied the effects various concentrations of the bacteriocin nisin had on thermal resistance of Bacillus spores in dairy products. They reported that although addition of nisin lowered decimal reduction times (D values) for spores of B. cereus, B. stearothermophilus, and B. licheniformis, it apparently required specific nutrients to sensitize spores to heat. Medium composition, exposure time, and pH also had an effect on the heat sensitivity. Classic studies (Curran and Evans, 1945; Theophi-lus and Hammer, 1938) on the microbiology of evaporated milk have contributed significantly to the knowledge of the microbiology of this product.
Under current continuous processing conditions wherein heat treatments of 117-121°C (242-250°F) for 10-15 min are common, and batch retorting is uncommon, spoilage of evaporated milk is unlikely to be encountered. Specific methods for microbiological examination of evaporated milk are contained in Standard Methods for the Examination of Dairy Products (Marshall, 1992).
Development of the dry milk industry stems from the days of Marco Polo in the 13th century. It is reported that Marco Polo encountered sun-dried milk on his journeys through Mongolia and that, from this beginning, dry milk products evolved (Clark, 2000b). Through early pioneering scientists, such as Appert and Borden, the basic methods were developed for the emergence of processes for drying milk products. Ekenberg and Merrill have been acknowledged as developers of the first commercial roller- and spray-process drying systems, respectively, in the United States (Beardslee, 1948). Since initial development of commercial drying systems, significant technological advances have been made, resulting in the manufacture of a variety of dry milk products.
The primary dry milk products manufactured domestically are nonfat dry milk, dry whole milk, and dry buttermilk. Nonfat dry milk is the product resulting from removal of fat and water from milk. It contains lactose, milk proteins, and milk minerals in the same relative proportions as the fresh milk from which it is made. Nonfat dry milk contains not more than 5% by weight of moisture. The fat content is not more than 1.5% by weight unless otherwise indicated. Dry whole milk is the product resulting from removal of water from milk and contains not less than 26% milkfat and not more than 4% moisture. Dry whole milks with milkfat contents of 26.0 and 28.5% are most commonly produced. Dry buttermilk is the product resulting from removal of water from liquid buttermilk derived from manufacture of butter. It contains not less than 4.5% milkfat and not more than 5% moisture.
Steps in a typical dry milk processing operation include (a) receipt of fresh, high-quality milk delivered in refrigerated, stainless-steel bulk tankers; (b) clarification, and, if nonfat dry milk is to be manufactured, (c) separation. The milkfat removed usually is churned into butter. If dry whole milk is to be manufactured, the separation step is omitted but may be replaced by a standardization procedure. Pasteurization by a continuous high-temperature short-time (HTST) process, whereby every particle of milk is subjected to a heat treatment of at least 72°C (161 °F) for 15 s is accomplished next. Holding the pasteurized milk at an elevated temperature for an extended period (85°C [185°F] for 20-30 min) is used in the manufacture of high-heat nonfat dry milk, which commonly is used as an ingredient in bakery or meat products. Following concentration of milk by removing water in an evaporator until a milk solids content of at least 40% is reached, the product enters the dryer for final moisture removal.
Commercial U.S. drying processes are of two types: spray and roller (drum). Currently, the latter is used to a limited extent and primarily for product intended for other than human consumption. Two basic configurations of spray dryers are in use: horizontal (box) and vertical (tower). In both, the pasteurized and concentrated milk is directed under pressure to a spray nozzle (horizontal dryer) or to either a spray nozzle or an atomizer (vertical dryer) where the dispersed liquid then comes into contact with a current of filtered, heated air. The droplets of milk are dried almost immediately and fall to the bottom of the fully enclosed stainless steel drying chamber. The dry milk product is continuously removed from the drying chamber, transported through a cooling and collecting system, and finally conveyed into a hopper for packaging, usually
Figure 3 Processing scheme for dry milk.
in 50-lb bags or in tote bins. Figure 3 reflects a typical processing scheme for dry milk.
In processing nonfat dry milk, various heat treatments may be applied to give the finished dry milk product desirable functional characteristics. Three heat-treatment classifications, based on the use of the whey protein nitrogen test, are of practical importance in indicating the suitability of spray-process nonfat dry milk for specific purposes (American Dairy Products Institute, 1990). Instant-type dry milks are processed by special methods that result in products with improved solubility. Instant nonfat dry milk is defined by its solubility index value (American Dairy Products Institute, 1990).
The American Dairy Products Institute (1999a) publishes annual census figures that reflect markets of end use for dry milk products, which may be referenced for further information about quantities of dry milks processed and their use. In 1998, U.S. production of nonfat dry milk was 1.1 billion pounds, dry whole milk production was 139 million pounds, and dry buttermilk production was 49 million pounds (American Dairy Products Institute, 1999a).
Industry microbiological standards for dry milk products are established by the American Dairy Products Institute. In addition, government standards for these products also have been generated by the USDA and the FDA (U.S. Public Health Service, 1995). Table 1 shows these standards by source, product, and, as applicable, grade.
Relatively few species of bacteria have been reported as naturally occurring in dry milks. Hammer and Babel (1957) and Foster et al. (1957), in earlier texts covering the microbiology of dry milk products, summarized literature reports indicating microorganisms of the genera Streptococcus, Micrococcus, Bacillus, Clostridium, and Sarcina as comprising the primary microflora of dry milks. Rodriquez and Barrett (1986), based on a study of the microbial population and growth in reconstituted dry milk, confirmed the occurrence of viable cells of the genera Bacillus and Micrococcus in nonfat and dry whole milks.
Since initiation of the requirement that all milk be pasteurized before drying, current heat treatments used to process dry milks destroy all microorganisms of public health significance. Relatively low numbers of microorganisms survive processing, and those heat-resistant organisms (both spore-forming and non-spore-forming types) rarely, if ever, are responsible for finished product deterioration. Because the drying process is accomplished in a completely closed system, postprocessing contamination also is rare. When such occurs, it usually is from an airborne source. Because of low moisture levels in dry milks, those viable organisms that may be present are unable to grow and decrease in number during storage. Specific methods for microbiological examination of dry milks are contained in Standard Methods for the Examination of Dairy Products (Marshall, 1992).
Spray-dried milks have been implicated in outbreaks of staphylococcal food poisoning (Anderson and Stone, 1955; Armijo et al., 1957). In both instances, illness were caused by a preformed enterotoxin that was not inactivated by the drying process. Miller et al. (1972), in a study of the effect of spray drying on survival of Salmonella and Escherichia coli, reported that heat treatments
Was this article helpful?