Lesson 35. DRY WHOLE MILK
Module 13. Technology of dried milks
DRY WHOLE MILK
The manufacture of dry whole milk is not much different from nonfat dry milk. Since keeping quality is a restricting factor, care should be taken to obtain the maximum storage life. High quality milk with a low copper and iron content, sanitary plant practices, and a good processing procedure are important influences on keeping quality.
The manufacture of whole milk powder with all necessary stages of manufacture starting from taking milk upto the packaging and storage are shown in Fig. 35.1.
35.2.1 Milk standardization
The milk fat is standardized in ratio to the solids not fat so the dry product will meet the legal standards. Clarification may be conducted either before or after standardization to remove leucocytes and safeguard against the possibility of extraneous material in the product.
The concentrate is not always homogenized, especially if atomization is done by means of a nozzle, because the fat globules are effectively disrupted in the nozzle. Homogenization of highly concentrated milk considerably increases its viscosity because the transfer of large casein micelles to the fat globules gives the latter such an irregular shape as to increase the effective volume fraction of fat globules plus casein micelles. This increase leads to coarser droplets during atomization, with all drawbacks involved. Consequently, if the concentrated milk is not homogenized, evaporation can continue up to higher dry-matter content.
Homogenization of the whole milk is common if direct reconstitution of the dry whole milk is contemplated. Without homogenization the fat may churn during agitation while combining with water. Another advantage of homogenization is the improvement of keeping quality. The fat globules, although smaller, are more thoroughly recovered in the protein membrane. A pressure of 175 to 250 kg/cm2 at 62 to 74°C provides sufficient homogenization.
Storage (buffering) of the concentrate before atomization is not always applied; it is done mainly to overcome differences in capacity between evaporator and drier. However, the concentrate should not be kept warm for more than a short time to prevent the growth of microorganisms. A refrigerated concentrate generally is too viscous to be atomized readily, and it is therefore heated just prior to atomization because otherwise the viscosity increases again. The heating can at the same time serve to kill bacteria that may have recontaminated the concentrate.
1. Intense pasteurization is needed to obtain resistance to autoxidation.
2. Low-heat treatment minimizes the cooked flavour in the product, but does not develop antioxidants for delay of oxidation, one of the principal factors in keeping quality.
3. Usually the primary consideration is given to prolonging the shelf life.
4. Preheating also destroys the enzymes. If the lipase enzymes are not destroyed, hydrolytic rancidity will occur in the dry whole milk.
5. Preheating must accomplish pasteurization, thus reducing the viable microorganisms.
6. A beneficial influence on heat stability of the product may occur from the preheat treatment.
7. Heating of the milk also is necessary before it enters the evaporator.
35.2.5 Manufacturing process
• Tubular heaters are currently used in many plants. They eliminate dilution by the steam condensate and steam impurities that may be toxic and/or cause off-flavors.
• Numerous optimum temperature-time conditions have been used in preheating whole milk. Commercial practice frequently employs the range of 82°C for 15 min to 93°C for 3 min.
• The heating, after concentration has been suggested as preferable possibly on the basis that a higher percentage of the antioxidants retained. These compounds may be removed along with the moisture during vacuum condensing.
• When sufficient solids concentration as Baum’e degrees has been attained (35 to 45%), product is continuously removed from the evaporator.
• Another variation in the procedure is to preheat skim milk to a high temperature necessary for formation of antioxidants and then to condense the skim milk. Homogenized, pasteurized cream is used to standardize the condensed skim milk to the desired ratio of milk fat to solids not fat. Some operators prefer to homogenize the concentrate after adding the cream.
• The temperature of condensed milk after it is pumped from the evaporator is boosted to 62.8 to 73.9°C in a heat exchanger prior to the high pressure pump.
• The condensed milk is dried with inlet air at 148 to 232°C and exit air at 74 to 93°C, depending upon drier characteristics. To reduce heat damage during dehydration, and yet obtain the desired moisture, a low exhaust air temperature is preferred.
• The dry whole milk should be immediately removed from the hot air stream to maintain better "body characteristics', and keeping quality. The higher the temperature and the longer the time the product is above the melting point of the fat the greater the amount of free fat that results.
• The use of refrigerated air to move the dry whole milk to a cyclone after it leaves the drying chamber is one system of decreasing the temperature. Cooling to room temperature (not below) is preferred. When ambient temperature and humidity are high, the air may be reheated just enough to avoid absorption of moisture by the product due to the dew point. However, other methods of cooling whole milk powder have proved to be satisfactory such as augering across a surface cooled by water or brine in jacketed equipment. The system must be sufficiently protected against the entrance of condensate from the ambient conditions.
• A 12-mesh screen is used for sifting dry whole milk. The product should packaged immediately or held under vacuum for 7 to 10 days before gas packaging.
35.3 Foam Spray Process
Milk is standardized, preheated, and homogenized similarly as for regular spray drying. During evaporation, the solids content may be increased to 50% or above. A gas, preferably nitrogen, is injected into the concentrated milk at 14 (or more) kg/cm2 greater than the pump pressure. The gas is distributed into the concentrate by means of a mixing device between the high pressure pump and the nozzle. A regulator and needle valve control the flow of gas into the concentrate coming from the high pressure pump at 105 to 126 kg/cm2. The usage of gas is at the rate of 0.037 m3 per liter of concentrated whole milk with 50% T.S.
Foam spray dried whole milk has improved dispersibility and approximately one-half the bulk density of the regular spray dried product. The occluded air in the particle causes poor sinkability when product is recombined with water.
35.4 Drum Process Dry Whole Milk
Only a small amount of drum dried whole milk has been processed in recent years. The poor keeping quality, unsatisfactory reconstitutability, and scorched flavor are considered to be the chief deterrents. However, for uses demanding a high percentage of free fat such as in certain confections, the drum dried product suffices.
Whole milk which has been standardized at the ratio of 3.5 (fat) to 8.75 (solids not fat) will yield a product with 28% fat if dried to 2 % moisture. The standardized milk is pasteurized and may or may not be homogenized. Unhomogenized milk will give a higher free fat in the dried form. The customary homogenization is 175 to 210 kg/cm2 and the temperature is 62.5 to 76.5°C. The preheat treatment is regulated for the intended use, but normally is desirable only for dry milk intended for baking products. The severe heat treatment during drum drying negates any attempt to produce a low-heat product.