Lesson 26. GRADING AND QUALITY OF RAW MILK FOR DRIED MILKS
GRADING AND QUALITY OF RAW MILK FOR DRIED MILKS
The grading and quality of milk for dried milks is based on the bacterial quality of original milk, acidity of the milk and properties of milk concentrate used for drying which affect the final dried product.
26.2 Bacterial Quality of the Original Milk
In deep-cooled milk, psychrotrophic Gram-negative rods can develop during prolonged storage (e.g., Pseudomonas spp.). These bacteria do not survive even a mild heat treatment. Proteinases and lipases formed by these rods may survive and become incorporated into the powder. Prevention of the growth of these bacteria is possible using refrigeration, limiting the storage time, and thermalization process. Contamination and growth during storage of the thermalized milk should be avoided.
Heat-resistant bacteria and bacterial spores are of great importance. They survive low pasteurization (72°C for 15 s), and most are not killed during evaporation and drying. Due to concentrating, the powder contains about ten times as many bacteria per gram as the milk immediately after preheating. A more intense pasteurization will kill the heat-resistant cocci (e.g., Enterococcus faecalis , S . thermophilus ) and in a high-quality medium-heat or high-heat milk powder only bacterial spores and Microbacterium lacticum can originate from the original milk.
Among the aerobic and anaerobic spore-forming bacteria, Bacillus cereus and Clostridium perfringens are especially important to the powder quality. If the reconstituted milk is to be used for cheese making, a very low count of gas forming anaerobic spore formers ( C . tyrobutyricum and C . butyricum ) may be essential. All of these bacteria are likely to originate mainly from contamination during milking (dung, soil, and dust). A low count of anaerobic spore formers points to a good-quality silage. But the pathogenic C . perfringens C . perfringens . Likewise, the total count of aerobic spore formers is not always an indication of the spore number of B . cereus . This probably is because (1) contamination with B . cereus is heavier on pasture, and (2) at higher environmental temperatures B . cereus can develop and sporulate in imperfectly cleaned and disinfected equipment outside the operating periods. To kill bacterial spores, heat treatment at 90 to 110°C for 10 to 20 s is insufficient; hence, UHT treatment should be used. usually does not originate from the silage, though it may from the dung. Hence, a low count of anaerobic spore formers need not be an indication of the absence of
26.3 Neutralization of Acidity of Milk
Fresh, sweet milk contains no lactic acid although mixed milk may have a titratable acidity ranging from 0.10 to 0.15% due to milk proteins, carbon dioxide, citrates, and phosphates. Neutralization refers to the practice of reducing the acidity of the fluid products. If skim milk has become acid, neutralization slightly decreases the titratable acidity of the reconstituted product.
In countries where neutralization of milk is necessary, and for the neutralization of a high acid product, such as whey, food grade alkalis are used. The acidity in these cases should be reduced to a pH of 6.8 (range of 6.6 to 7.0). The specific titratable acidity in this pH is dependent upon the composition of the product and type of alkali used.
Temperature used for neutralization should be 35°C or less and at least 15 min is allowed for the reaction before heating. The selection of alkali may be influenced by intended use of the dry product and the processing method. Sodium bicarbonate causes foaming on a drum so should not be used for the drum drying. A sodium type such as sodium hydroxide is preferred for products requiring a maximum solubility. Care should be exercised to minimize the development of defects in the milk; otherwise adverse effects of the chemical may result.
26.4 Properties of Milk Concentrate for Drying
It is essential to evaporate milk correctly to achieve the required feed characteristics. Inappropriate evaporation can cause increased viscosity and product instability that may make further processes such as drying more difficult or even damage the product irreversibly. Many functional powder properties, such as solubility, heat stability and the WPNI index are set at the evaporation stage. Some properties such as the denaturization of whey proteins, and colour and flavour development, are irreversible.
The correct properties are achieved by managing all the heating and handling processes within the evaporator plant to ensure the desired result. These properties can include: low heat powder for minimum product damage, medium heat powder (e.g. for baking applications), high heat (heat stable) powder with the correct properties to accept UHT treatment after reconstitution, and whole milk powder with the desired WPNI levels for easy reconstitution. Also, air in the concentrate should be avoided by any means, as it upsets the whole concentrate pre-treatment and drying.
Measurement of Insolubility Index is usually performed on milk powder. If it is not possible to trace the problems with too high Insolubility Index by changing the drying parameters, it is recommended to perform the test on the concentrate. The amount of concentrate to be used is calculated as follows:
g concentrate = g powder x 100 / %TS of concentrate
g powder = 10 g skim milk, 13 g whole milk, or 6 g whey powder. The rest of the procedure is as described for powder.
- The concentrate leaving the last effect of the evaporator is liquid. The concentrate may however have different viscosity depending upon the composition, heat sensitivity of the proteins, pretreatment, temperature and solids content.
- Whole milk concentrates are generally less viscous than skim milk concentrates, and as a general rule the viscosity should not exceed 60 and 100 cP, respectively, if the atomization should be optimal. Higher viscosities can be handled in the dryer, but may loose the capacity by bad atomization or big droplets and an inferior product will be the result.
- The composition of the concentrate will influence the viscosity, especially on the protein (P) content in relation to the lactose (L) content. When the ratio P:L is high, the concentrate will get a high viscosity. The ratio P:L can be adjusted by adding lactose. As a general rule, a higher fat and lactose content will give lower viscosity. Higher protein content will give higher viscosity.
- When milk is exposed to a high heat treatment, especially in indirect pasteurizing systems, prior to the evaporation, the viscosity of the concentrate will be higher.
- The concentrate temperature will naturally have a direct influence on the viscosity and higher temperature means lower viscosity.
- The solids content of the concentrate will have a very significant influence on the viscosity, and the higher the concentration, the higher the viscosity.
- However, above parameters shows the direct influence on the viscosity. One of the main influences on the viscosity is the time, i.e. the viscosity is a function of time, also known as age thickening. This means that the viscosity will increase if the concentrate is left for some time. The increase is depending on composition, mainly proteins binding to each other, temperature and concentration. The age-thickening is only partly reversible by agitation.
- A temperature increase will result in a viscosity drop; but as the age thickening is more pronounced at higher temperatures, the viscosity will soon increase to the same level and further on as the time passes.
26.4.5 Skim milk
Skim milk having 48-50% TS should have maximum viscosity of 100 cP at 40ºC. For manufacture of instant products, the WPNI should be 2.5-3.5 mg and for high bulk density powders, the WPNI should be max. 1.0 mg. It should not show any measurable amount of scorched particles as well as solubility index and Sieving test should show no visible insoluble particles.
26.4.6 Whole milk
Whole milk having 48-50% TS should have maximum viscosity of 60 cP at 40ºC. For manufacture of instant products, the WPNI should be 2.5-3.5 mg and for high bulk density powders, the WPNI should be max. 1.0 mg. It should not show any measurable amount of scorched particles as well as solubility index and Sieving test should show no visible insoluble particles.
Whey having 52% TS should have maximum viscosity of 100 cP at 40ºC. It should have >75% crystallized lactose with mean crystal size ranging from 30-50 µ. It should have maximum 25% denatured whey proteins and should not show any measurable amount of scorched particles as well as solubility index and Sieving test should show no visible insoluble particles.
26.5 Effect of Pre-Concentration of Milk on the Quality of the Resultant Powder
The chemical and physical properties of a concentrate with high total solids affect essentially the properties of the resultant powder.
- The major effect of concentration is on the viscosity of the milk. The results depicted in Table 26.1 demonstrate the increase in viscosity of skim milk when concentrated. Although viscosity increased with concentration, the effect was not linear, was most marked when the concentration of milk solids exceeded 45%. The large viscosity increases above 45% TS are due to the concentration of casein micelles occupying over 30% of the total volume of the milk.
- Table 26.2 shows stages of product concentration in the manufacture of milk powders. It is evident from the table that with increase in concentration sensitivity to heat denaturation of protein increases which in turn influences the quality of milk powder, especially the solubility.
Tables 26.3, 26.4 and 26.5 demonstrate influence of the dry matter content of the concentrate on the various properties of skim and whole milk powders.
Table 26.1 The effect of concentration on the viscosity of skim milk
Table 26.2 stages of product concentration in the manufacture of milk powders
Table 26.3 Relation between the degree of preconcentration and physical properties of SMP
Table 26.4 Relation between the degree of pre-concentration and the physical properties of WMP
Table 26.5 Influence of the dry matter content of the concentrate on various characteristics of whole milk powder