Lesson 4. PRETREATMENTS FOR CONCENTRATED MILKS
PRETREATMENTS FOR CONCENTRATED MILKS
In course of manufacture of concentrated and dried milks, raw milk is subjected to various pretreatments, which in turn determine to a great extent, the physical, chemical, microbiological and organoleptic characteristics as well as the shelf life of the finished product. The technological aspects of various pretreatments and their significance in the manufacture of dried milks are discussed here.
The following pretreatments are given to milk during the processing schedule:
1. Filtration and/or centrifugation
3. Forewarming /Preheating
4. Concentration / Concentrate heating
4.2 Filtration and / or Centrifugation (Separation, Clarification and Bactofugation)
The basis for high quality and fine flavor of the product is "clean milk" in preference to "cleaned milk". However, for sanitary, ethical as well as for technological reasons the milk that is received at the dairy is filtered and/or centrifuged.
Weigh can strainers, strainers and filters in the milk line or a duplicate filter unit with by-pass connection are general filtration techniques. Bactocatch method developed by Alfa-Laval Co., Sweden, involves filtration of the milk through ceramic filters prior to pasteurization. This microfiltration technique removes 99.6% of the bacteria in the milk so that the following normal low pasteurization results in 50% life extension when milk is kept at 8°C. The advantages of this process are:
- Bacillus cereus level is reduced appreciably
- Absence of disagreeable flavour that sometimes is present in overheated milk
- Excellent quality low heat powder for cheesemaking, least adverse effect on rennetability, curd characteristics etc.
- Low heat powder having extended shelf life.
There are three types of centrifugal processes to which milk may be subjected during manufacture of dried milks. These are
- Separation proper i.e. skimming
Centrifugal clarification is done in hot or cold condition in a centrifugal triprocess separator, which serves the purpose of clarification. A clarifier removes leucocytes, cellular debris and particles from earth or fodder gaining entry into milk which might act as a catalyst for chemical reaction during storage of powders.
The Bactofugation process, originated by Professor Simonart is designed to remove bacteria from milk by means of centrifugation. Basically the bactofuge is a clarifier with one inlet and one outlet for the treated liquid. Additionally, there are two nozzles of about 0.4m diameter fitted into the bowl wall for the discharge of skim milk or what is called 'bactofugate' rich in protein and bacteria. The machine is characterized by a much greater efficiency of removal of bacteria than conventional clarifiers. Bactofugation is a selective method of removing bacteria, the size and density of the bacteria being the criteria of their removal. Bactofugation of milk is always applied in combination with heat treatment, a temperature of 55-65°C offering the optimum bactofugation effect. A combination of heat treatment and bactofugation results in a greater reduction of bacteria than is possible by heat treatment alone. Likewise, bactofugation of milk by single pass process is less effective (90% removal of bacteria) than two pass process where two bactofuges are arranged in series (bacterial reduction may be up to 99.9%). To improve the quality of raw milk for dried milk, bactofugation has been suggested by some workers. Though clarification has also some beneficial effects, cream separation has no effect on ultimate quality of powder. However, some losses of constituents, though negligible, do occur while bactofuging the milk. Heating (130 - 140°C the bactofugate and remixing it with treated milk can eliminate the losses due to bactofugation.
Influences of centrifugation (i.e. clarification and bactofugation) of milk on quality of milk powders can be summarized as follows:
1. Centrifugation prior to concentration improves casein dispersion and heat stability.
2. Improved shelf life of powder due to reduced tendency of oxidation and lipolysis due to removal of slime, foreign materials (which may act as chemical reaction catalyst), leucocytes, lipolytic bacteria and enzymes etc.
3. Improved solubility to powder (1.8-2.0%) when clarification is applied at 35- 40°C.
4. Decrease in sediment formation
5. Better organoleptic quality of resultant powder.
6. Improved bacteriological quality of finished product.
- Milk intended for dry milk manufacture is standardized to obtain a product of expected quality, to meet the legal standards and to optimize yield and profit.
- In manufacture of dried milks, the milk fat and SNF are so standardized that the finished product will meet the standards of not less than 26% fat (BIS, IDF and ADPI standards for whole milk powder).
- The fat to SNF ratio adjustment is done by addition of calculated amount of cream or skim milk to the milk. This purpose is accomplished in the present day industry by use of tri-purpose separator which has the standardizing device too. Other computerized systems are also used commercially.
4.4 Preheating / Forewarming of Milk
In manufacture of milk-powders milk is subjected to heat treatment in two stages-
(A) Prior to concentration
(B) After concentration, before atomization.
4.4.1 Heating prior to concentration has the following objectives
Preheating of milk to at least the boiling temperature of the first effect evaporation is essential, so that evaporation can start immediately the milk enters the heating tubes. The most common boiling temperatures of the first effect in the food industry are between 40-100°C. Heating of milk to a temperature exceeding the boiling temperature of the first effect are required for the purpose of:
(a) Enzyme inactivation, e.g. lipoprotein lipase.
(b) Pasteurization effect - improved microbiological quality of resultant product due to microbial inactivation. Heat treatment employed in manufacture of low and high heat powders had profound effect on bacterial profile. Both pasteurization and high heat treatment (90°C, 10 min) almost completely eliminate psychrotrophic bacteria. Pasteurization had little effect on either thermoduric or spore counts whereas high heat treatment brought about significant reduction in bacterial numbers.
(c) To lengthen the running time of evaporators (as most of the whey proteins are denatured when milk is preheated before evaporation).
(d) To generate specific properties in the resultant milk powders, e.g. heat stability, viscosity, etc. by altering the characteristics of some of the components of milk such as casein, whey protein, mineral balance etc.
(e) To improve oxidative stability of whole milk powder during storage. This is due to production of -SH groups and other reducing substances in milk on account of heating.
Table 4.2 Typical time temperature combination for different types of powders
4.4.2 Concentration / Heating after concentration/ concentrate heating
The viscosity of the concentrate has bearing on drying efficiency as well as certain properties of the dried milks. In general, high concentrate viscosity can be the source of many practical problems by increasing the likelihood of blockage of feed lines and in the calandria of evaporators.
Therefore, it is often desirable to reduce the viscosity of concentrate to as low a viscosity as possible. The viscosity of concentrated milk is significantly affected by temperature. To achieve desired low viscosity of the feed, the concentrate has to be heated to 70-75°C prior to atomization. This in turn has the following advantages:
(a) Saving of energy as it is more efficient to heat the concentrate by means of steam or hot water than to use heat from the air supplied to the dryer. The evaporation capacity of the dryer is increased by ~ 5% when the concentrate is supplied to the atomizer at 70°C instead of 50°C.
(b) The average size of the droplets into which the concentrate is atomized is reduced due to the lower viscosity. This facilitates the drying making it possible to reduce the air outlet temperature and still maintain the specified moisture content of the powder. The lower outlet temperature results in increased capacity and gentler drying which results in improved powder quality - especially solubility.
(c) As the temperature conditions in the last stages of the evaporator and in the feed tank for the dryer can be favorable for growth of micro organisms, heat treatment of the concentrate immediately before drying will contribute to safeguard the bacteriological quality of the powder. Heating of the concentrate must be done in such a way that no holding takes place, i.e. the concentrate heater must be placed immediately before the atomizer. This is necessary in order to avoid the so called "age-thickening" which occurs very rapidly at temperatures above 60°C. The time dependent thickening of hot concentrate is irreversible and results in very high viscosity. Table 4.3 shows changes in viscosity of concentrated skim milk (39%TS) held for varying period at 75°C. Table 4.4 shows the effect of storing concentrated skim milk (39%TS) at 50°C and 60°C on powder solubility. Since concentrate is a complex system its heating has to be done carefully to minimize adverse effects of heating on chemical and physical characteristics of the concentrate. Otherwise, such change will reflect on the quality of resultant milk powders. Tables 4.5 and 4.6 show influence of concentrate heating on the various characteristics of whole milk powder.
Table 4.4 The effect on powder solubility of storing concentrated skim milk (39% TS) at 50° and 60°C
Table 4.5 Influence of the heating on the characteristics of whole milk powder concentrated (Non-homogenized, dry matter content: 49.5%)
Table 4.6 Relationship between the heating and the dry matter content of non-homogenized concentrates concerning the free fat content of whole milk powder
- Homogenization is used as one of the pretreatments in manufacture of whole milk powder as well as some specific products.
- The purpose of homogenizing milk/concentrate prior to spray drying is to reduce the size of the fat globules and consequently to change the physical structure in such a way that the dried product obtained has a low free fat content.
- The type of homogenizer, the stage at which product is homogenized, the temperatures and pressures of homogenization employed have profound effect on the various properties of the concentrate and this in turn have influence on the quality of resultant milk powders.
Table 4.8 Influence of the homogenization pressure on various characteristics of the concentrate and whole milk powder
From the above write up it can be seen that the various pretreatments employed during manufacture of dried milks have profound effect on successful manufacture of these products. Therefore, a better understanding of the effects of relevant pretreatments is essential to avoid unexpected difficulties in the course of manufacture and to meet ever increasing stringent product specifications .