Evaporated Milk

CHAPTER 13 Evaporated E v a p t e d milk is c

and Human .%rvim,

d by thc Foal and Drug Administmtion, Dgartment of Health Co& of F h l Regulations 21 CFR 131.130 Evapomted Milk.

Introduction The evaporated milk industry has its roots in the early 1800s in England; the idea of preserving and concentrating milk by heating it in sealed cans was first developed in Switzerland. In the United States, canned evaporated milk gained its popularity in the early 1900s and later played an important role during wartime due to its nutritional value and convenience. Figure 13-1 (following page) shows the production of evaporated and condensed milk in different areas in the world from 1979 to 1994. The annual production of Europe, South America and Africa decreased, while Asia, Oceania, North and Central America increased production in the past decade. For many developing countries, evaporated milk is commonly consumed for its nutritional value. In developed countries, it is often used as a component in various food preparations. Different food industries such as infant food producers, confectioneriesand dairies use evaporated and condensed milk as a concentrated source of milk in many of their products. In the United States, evaporated and condensed milk are the third largest ingredient in volume exceeded only by sugar and baking mixes in the baking industry. Definition Evaporated milk is the liquid food obtained by partial removal of water from cows’ milk. It includes regular, skim, low fat, and filled evaporated m i k and all types of evaporated milk require the removal of approximatelyhalf the naturally occurring water in each of these products to a desired total solids and ratio of fat to solids-not-fat(FDA, 21 CFR 131.130). According to the FDA Standard of Identity amended inJune 1994, the minimum milk fat content was reduced from 7.5 to 6.5% by weight, minimum total milk solids content from 25 to 23% by weight, and established minimum milk solids-not-fatcontent requirements of 16.5% by weight. The new standard has 9% fewer calories which can meet nutritionists and consumer demand for a

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product with less fat and calories. In addition, it brings evaporated milk closer to consumer expectations of a fluid milk when reconstituted with an equal portion of water.

FIGURE 13.1 - World Production of Evaporated and Condensed Milk, 1979 to 1994 (Reference:FA0 Yearbook, 1994).

Evaporated milk contains added vitamin D as required by FDA, and is homogenized, sealed in cans and processed by heat to prevent spoilage. Three can sizes are usually used in the evaporated milk industry: 5 fl. oz. (150 g) can, 12 fl. oz. tall can (360 g), and No. 10 (97 fl. oz., or 2.9 kg) can. The major difference between manufacturing evaporated milk (unsweetened condensed milk) and sweetened condensed milk is the method employed to preserve the milk. Sterilization is the intensive heat treatment used for preserving evaporated milk, while the shelf life of sweetened condensed milk is extended by the addition of sugar. General Description of Operations To produce evaporated milk (Figure 13.2), raw whole cows’ milk is generally weighed, inspected and sampled when it is still in delivery tank trucks. It is then filtered, cooled and pumped into holding tanks where it is standardized

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to the desired ratio of fat to solids-not-fat.From holding tanks,it is pumped into a preheater with a temperature-controlledholding tank. Hot milk is drawn into an evaporator with the subsequent condensed milk then pumped into a homogenizer. The homogenized milk is passed through a homogenizer head to a cooler to a filling machine. From there, filled and sealed cans are conveyed through a leak detector to a sterilizer where they are subjected to steam under pressure, destroying microorganismsand giving the evaporated milk the desired shelf life, consistency, and flavor. Cans are then coded, labeled, cased, and stacked for storage in a warehouse and shipped to distribution channels.

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FIGURE 13.2- General Operations for the Manufacture of Evaporated Milk

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MANUFACTURE OF EVAPORATED MILK

hiilk Supply Since evaporated milk contains concentrated milk solids and is destined for longer storage, it is necessary to control the production from the beginning; the quality of the raw material is the same as for the manufacture of fresh dairy products. Fresh milk is obtained from local dairy farms. As soon as the milk leaves the cow, it is cooled, pumped into insulated stainless steel tank-trucksand delivered to the processing plant on a daily or twice-a-day basis (Figure 13.3). As milk arrives at the plant, it is checked for weight, temperature (145°For 7”C),color,

FIGURE 13.3 - Block Diagram for the Manufacture of Evaporated Milk

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odor, antibiotics, titratable acidity, and microbiological activity. The efficient inspection of milk in the receiving room is an integral and necessary step in the successful quality control of evaporated milk After inspection, the milk is pumped into a storage silo where temperature is maintained 545.F (7'C) and kept there no more than 72 hours before processing.

Clarification Although not essential, clarification is a common practice for processing good quality evaporated milk, the main purpose of clarification is to remove impurities from raw milk. While this process separates particles by different size and specifies gravity such as sediment, body cells from the udder and some bacteria, clarification is not designed to completely remove bacteria. In the dairy plant, clarification is usually carried out by a continuous centrifugal clarifieror separatorwhich continuouslyseparates solid particles from a liquid. Balance Tank A constant level of milk must be maintained in the balance tank to ensure sufficient milk flow in the heat exchanger during heating operation. The level of milk in the balance tank is controlled by an inlet valve which regulates the incoming milk flow to prevent milk from burning on the heating equipment. The ability of milk to withstand intensive heat treatment is very important for evaporated milk processing and depends largely on its salt balance; stabilizing salts are added to milk to increase heat stability. These salts include carbonates and bicarbonates (Ca2+,K+,Na+),citric acid and citrates (K+,Na+), phosphates (K+),and others. The pH value of milk influences the effect of stabilizing salts; a wellchosen combination of thermal treatment and adequate stabilizing salts can significantly increase the thermal stability of evaporated milk. The amount of stabilizing salts to add to any given batch of evaporated milk is determinedby trial tests. Ready-madecommercial mixtures are available. The addition of vitamin D is mandatory and must be documented for each tank. It must be present in such quantity that each fluid ounce (28ml) of the product contains 25 I.U.(International Units) within limits of good manufacturing practice. The addition of vitamin A and carriers for vitamins A and D is optional. Carrageenan or other emulsifiers are optionally added for the purpose of retarding milk fat separation during storage. Preheating (Forewarming) Standardized milk must undergo a preheating treatment before evaporation in order to stabilize the milk protein. Preheating helps the milk to withstand the high heat of subsequent sterilization without coagulation taking place in

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the sterilizer or later during storage. In addition, preheating also builds the desired product body, flavor, and viscosity. Preheating is carried out in continuous heat exchangers of either plate or tubular type with the capability of longer holding time. Hot water and steam are the heat medium commonly used. All evaporated milk must be preheated to a minimum of 190°F(88'C) and held in flow-throughholding tanks with a temperaturecontrolling device for the minimum calculated hold of 25 minutes. Two time-temperature regimes of preheating are normally used 190-121°F (88-lOO0C)for 10 to 25 minutes, or 240-262'F (115-128'C) for 1to 6 minutes. For the usual type milk supply, the former temperatures below the boiling point are considered adequate to produce desired heat stability in the sterilization process. For low heat stability milk, such as high solids content milk, quickly preheating at higher temperatures (the latter one) provides a satisfactory product. Since each milk supply has its own peculiarities and is subject to seasonal change regarding optimum preheating treatment, the adjustment should be made for each milk supply. Standardization Market milk requires a specified constant fat content. For evaporated regular milk, the minimum fat content is 6.5% and the minimum total solids is 23% as stated earlier. The fat content of raw milk varies with every incoming batch. If raw milk contains a higher fat content than the standard, this standardization process will remove some of the milk fat. Surplus milk fat produced during processing is later used in butter making. Batch standardization and in-line standardization are the most common methods used in dairy plants to obtain the desired fat content. The first step for both methods is to separate whole milk into skim milk and cream. For batch standardization, the milk is then mixed with appropriate amounts of skim milk or cream in proportions that establish the required fat content. This method is labor-intensiveand requires large tanks for adjustment and analysis. It can be done before pasteurization (pre-standardization)or after pasteurization (post-standardization),but the latter method, post-standardization, has some risks involving reinfection of pasteurized milk. For in-line standardization, a regulated amount of cream is remixed with the skim milk in an in-line system immediately after the separator to achieve the desired fat content. For precision in the direct in-line standardization, some variable parameters such as measuring of the fluctuations in the fat content of the incoming milk must be considered. A practical way of standardization is to ensure sufficient fat in the evaporated milk so that product composition can be controlled by the addition of only

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water or milk solidsnonfat. In this way, extra milk fat will not effect the emulsion system after homogenization. In addition, since vitamins and carrageenan are added based on a theoretical finished weight, if additional quantities of standardizingagents are needed later, the final volume will be affected, resulting in insufficient amounts of vitamins or carrageenan.

Evaporation After preheating, the milk is pumped to an evaporator. The concentration of evaporated milk is carried out by evaporation of a certain amount of water. The major purpose of the evaporation system is to transfer heat from a heat source to the product in order to evaporate water or other volatile liquids from the product. Vacuum evaporation is widely used in the dairy industry in order to reduce boiling temperatures lower than normal at prevailing atmospheric pressure so that undesirable changes of milk components caused by high temperatures can be avoided. Boiling commonly takes place between 122" F and 140'F (50-6O'C) when the milk passes through steam heated tubes under vacuum. With lower evaporation temperatures, there is less heat damage to the products, but at the same time, evaporation temperatures should never be lower than 113'F (45'C) to eliminate the growth of microorganisms such as Staphylococci. The amount of moisture removal from the product is controlled by the determination of total solids with a Baumk hydrometer. A customary practice is to concentrateslightly more than desired in the final product. The dry solids content of the milk increases as the water is boiled off. A continuous check is made on the density. The evaporation system includes a single-effect evaporator and a multiple effect evaporator of two or more vacuum units. In the dairy industry, a singleeffect evaporator is often called a vacuum pan. In multipleeffect evaporators, the units operate at decreasing pressure as the product moves through. Tubes and plates are most often used to achieve indirect heat transfer. Tubular evaporators can be classified by their design: 1) tube position-horizontal or vertical; 2) direction of product flow-rising film or falling film; 3) shape of tube set-in coils or straight. Plate evaporators are built up by units, each unit consisting of four plates; a rising film as an inlet and a falling film to discharge, between which are two steam passages. The falling-film tubular evaporator is the type most often used in the dairy industry (Figure 13.4, following page). The heating medium for both evaporators is usually low pressure steam. For the purpose of minimizing energy consumption, there are two systems of vapor recompression in operation: 1) evaporation with thermal vapor recompression (TVR)- by compressing the vapor to a higher temperature

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through a steam injection from one effect, the vapor generated is the heating medium for the next. The vapor from the last effect is condensed and can be used as boiler, cleaning water, or preheating media; and 2) evaporation with mechanical vapor recompression (MVR) - the vapor is generated by one of the associated effects or in the same effect. Vapor generated from all effects passes to a compressor or high-pressure fan, where the increase in pressure produces a corresponding temperature rise as the preheating medium.

Homogenization The tendency of milk fat to float in milk and form a cream line on the surface makes it possible to separatefat from milk. This property is undesirable in evaporated milk and can be effectively prevented by homogenization. Homogenization decreases the average diameter of milk fat globules by mechanical treatment,breaking them down into smaller globules and uniformly dispersing throughout the milk. The diameter of milk fat globules in nonhomogenized milk varies between 0.1 and 15 pm with a wide variance; after homogenization, about 85% of the fat globules are smaller than 2 pm in diameter and all are under 3 pm. If homogenization does not produce fat globulessmaller than2 Fm diameter and the viscosity is not adequately adjusted by correct sterilization, fat separation will occur during long-term storage. By reducing the fat globule diameter close to that of human milk fat, homogenization also improves the digestibility of evaporated milk. Concentrated milk is pumped from the evaporator to a homogenizer which operates at a pressure of 5-25 MPa (725-3630 psi). Homogenization causes the disintegration of milk fat into smaller globules. At the same time, globule membrane regeneration takes place by absorption of serum protein and casein as additional surface-activematerial. After homogenization, milk fat is in the form of newly created globules, not free fat, but no longer covered with only original membrane material. Two-stage homogenization for evaporated milk is normally recommended because it has been found that products of one-stage homogenization, even under optimum conditions and with a low fat content, occasionally results in fat clusters. Two-stagehomogenization has higher pressure in the range of 1525 MPa for the first stage and lower pressure of 5-10 MPa for the second stage. The first stage in homogenization disperses fat globulesby high pressure, and the second stage provides a constant lower. pressure to stabilize the newly homogenized globules and to prevent subsequent cluster formation. High homogenizationpressure decreases the stability of the protein slightly creating the consequent risk of milk coagulating during sterilization, but

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stability of fat dispersion is greatly improved. It is therefore a case of finding the exact homogenization pressure that will be high enough to produce the required fat dispersion but low enough to eliminate the coagulation risk. The homogenization temperature in the production of evaporated milk usually matches the temperature of the evaporator, usually 149'F (65°C). Homogenization forms a polydispersed emulsion system of milk fat with markedly narrow distribution and contributes to the body and texture of the final product. Other advantages of homogenization on the chemical and physical structure of milk include uniform distribution of fat, no cream line, whiter color, reduced sensitivity to oxidation and a more full-bodied flavor.

Cooling and Filling After homogenization, the milk is cooled to about 40-57°F (5-14°C) depending on the waiting time for the followingsterilization. Volumetric filling machines for 2-piece,or %piecesanitary cans are commonly used for evaporated milk. The product is filled and sealed automatically. Filling temperature is selected to give the least amount of foam formation. Sterilization The sterilization of filled, sealed cans is carried out either on a batch basis or by continuous sterilizers. In a batch sterilizer, cans are first stacked in special crates which are then stacked inside the sterilizer. In a continuous sterilizer, an internal spiral backed conveyor carries cans through the sterilization process at a controlled speed. Uniform heat transfer is accomplished by rotating the cans in both types of sterilizers. Appropriate rotation can minimize the heatinduced changes in the product, but excessive agitation is detrimental to optimum viscosity. The sterilization time-temperature regime of evaporated milk ranges from 230-248°F (110-120°C) at 15 to 20 minutes duration. The procedure for aseptic packaging of evaporated milk requires ultrahigh temperature (UHT)sterilization prior to packaging, known as flow sterilization (a continuous process). The sterilization temperature is controlled at 266-284 * F (130-14O'C) for a few seconds, and then the sterilized milk goes through the filling and sealing processes under aseptic conditions. To ensure the required sterilization condition, random samples are kept for six days at 80-86'F (2730'C). Any growth of microorganisms, appearance of gas, or explosion of cans indicates unacceptable sterilization practices. After sterilization, the cans are cooled to storage temperature and dried.

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storage

The sterilized evaporatedmilk cans are then labeled and packed in cardboard cartons for storage. Storage temperature and time are two determining factors for the quality of evaporated milk. Commercial evaporated milk remained acceptable for two years if held at a temperature of 32-60'F (0-16"C), but deteriorated rapidly above 70'F (21'C). Low temperature(32-45"F, or 0-7'C) storage also prevents sediment formation caused by the settlingout of calcium and magnesium salts. The milk color becomes brownish and the pH decreases if the storage temperature is too bigh; discoloration is rapid if the storage temperature is above 90'F (32'C). The body and texture of the product are detrimentally changed if stored at a freezing temperature. Humidity of storage areas should be controlled below 50% to avoid the corrosion of metal cans.Inversion of the cans is a practical way to minimize the separation of fat and other constituents and to prevent the product from forming a surface cream line during extended storage.

Evaporated Skim Milk Evaporated skim milk is produced by the partial removal of water only from skim milk. It contains not less than 20% of total milk solids by weight and not more than 0.5%of milk fat by weight (FDA, 21 CFR 131.132).For the production of evaporated skim milk, raw whole milk is passed through a separator where the milk is separated into its two principal fractions, cream and skim milk, and the skim milk portion goes through a similar manufacturing procedure to that of evaporated whole milk, but without adding milk fat back to the separated skim milk for standardization. Evaporated Filled Milk (no Standard of Identity) Evaporated filled milk is a prepared blend of skimmed milk, vegetable oil, stabilizers, and vitamins. In the past, different names have been used for evaporated filled milk such as uevaporated melloream" and "imitation evaporated milk." The procedure for producing evaporated filled milk basically follows the manufacturing procedure of evaporated skim milk with the addition of vegetable oil to replace milk fat normally found in evaporated milk. Partially hydrogenated soybean oil, the vegetable oil in evaporated filled milk, is usually added before evaporation.

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ACKNOWLEDGMENTS The editor acknowledges the contribution of Robert A. Rindo O-AT-K A Milk Products Cooperative, Inc., for supplying information and reviewing this chapter, and Julia Chia-Day Fu for writing the chapter.

References Rindo, R. A. 1995. O-AT-KA Milk Products Cooperative, Inc. Personal communications. FDA. 1995. "Food and Drug Regulations." Title 21, Code of Federal Regulations, Parts 131.130 and 131.132. The Office of the Federal Register, National Archives and Records Administration, Washington, DC. Caric, Marijana, 1994. Concentrated and Dried Dairy Products. VCH Publishers, Inc., New York, NY. FAO. 1994. FA0 Yearbook: Production. Food and Agricultural Organization of the United Nations. Rome, Italy. Gale Research. 1994. Market Share Reporter. Detroit, MI. Hall, Carl W., and Hedrick, T. I. 1971. Drying of Milk and Milk Products. The AVI Publishing Company, Inc., Westport, CT. Honer, C. "O-AT-KATaps into Process-Control."Dairy Foods 89(2):49-53.