19 Technology of Dried milk products – I: Skimmed and whole milk powders, Instant milk powders

Amit Patel

INTRODUCTION

Dried milk or milk powder is a product obtained by removal of water from milk by heat or other suitable means to produce a solid containing 5.0 percent or less moisture. Whole milk, partially skimmed milk or skimmed milk may be used for drying. Today, the production of dried milks and milk products has become important segment of dairy industry due to their longer shelf life, requiring lesser storage space and lower shipping costs. Dry milk provides a means of preserving the excess milk supply during the flush season for use during the lean season.

CLASSIFICATION OF DRIED MILKS

Dried milks may be classified in a number of ways viz: Based on type of direr used, fat content of milk, heat applied, method of manufacture, special dried product made from milk.

Table 1: Classification of dried milks

Sr.No.	Way of Classification	Dried Milk
1	Based or the type of drier	Freeze dried powder
		Spray dried powder
		Roller dried powder
2	Based on fat content of milk power	Skim milk powder
		Partially skim milk powder
		Whole milk powder
3	Heat applied for making of milk powder	Low heat powder
		Medium heat powder
		High heat powder
4	Method of manufacture	Instantized powder
		Foam dried powder
5	Special dried milk product	Infant milk food
		Dairy whitener
		Malted milk food
		Weaning food
		Buttermilk powder
		Whey powder
		Ice cream mix powder

COMPOSITION

The average composition of milk powders is given in Table 2.

Table 2: Average composition of dried milks

Composition (%)	Type of Dried Milk
	Whole Milk Powder	Skim Milk Powder
Moisture	3.0	4.0
Fat	27.0	1.0
Protein	26.0	35.5
Lactose	38.0	51.5
Ash	6.0	8.0

FSSA STANDARDS

MILK POWDER – means the product obtained by partial removal of water from milk of cow and/or buffalo milk. The fat and/or protein content of the milk may be adjusted by addition and/or withdrawal of milk constituents in such a way as not to alter the whey protein to casein ratio of the milk being adjusted. It shall be of uniform colour and shall have pleasant taste and flavour free from off flavour and rancidity. It shall also be free from vegetable oil/fat, mineral oil, thickening agents, added flavour and sweetening agent. It may contain food additives permitted in Appendix C. It shall conform to the microbiological requirements prescribed in Appendix D. It shall conform to the following requirements

Table 3: FSSA Standards for Milk Powder

Sr.	Constituent	Whole Milk	Partly Skimmed	Skimmed Milk
No.		Powder	Milk Powder	Powder
1	Moisture (%, m/m)	NMT 4	NMT 5	NMT 5
2	Milk Fat (%, m/m)	NLT 26	NLT 1.5 & NMT 26	NMT 1.5
3	Milk protein (%, m/m)	NLT 34	NLT 34	NLT 34
4	Titratable acidity (ml 0.1N  NaOH/10 g solids)	NMT 18	NMT 18	NMT 18
5	Solubility, %	NLT 99	NLT 99	NLT 99
6	Total ash on dry weight, %	NMT 7.3	NMT 8.2	NMT 7.3

NLT- Not Less Than, NMT- Not More Than, m/m – mass/mass

PRINCIPLES OF DRYING

Though a number of drying systems are available, practically only spray drying and roller drying are commercially used in dairy industry. Since the product quality and process economy are much better and are being constantly improved, spray drying has the highest potential today and in the foreseeable future.

Roller Drying

The principle of the roller drying process is that the pre-concentrated milk is applied in a thin film upon the smooth surface of internally steam heated and continuously rotating cylindrical metal drum. As the water evaporates, a thin layer of dried milk is formed on the drum that is continuously scraped off by a stationary knife/doctor blade/scraper located opposite the point of application of the milk. The product is in the form of thin film, which is later pulverized and packed.

Spray Drying

The basic principle of the spray drying consists of atomizing the concentrated milk into sprays of desired size (fog like mist) in a large-size drying chamber, where it is mixed with a current of hot air. Owing to their large surface area, milk sprays surrender their moisture practically instantaneously and dry into a fine powder. Dried powder is continuously removed after separation from the air by device such as cyclone separator or bag filter.

PRODUCTION OF MILK POWDER

Milk destined for the production of powder must be of high chemical, organoleptic, and bacteriological quality. Acidity of milk must be below 0.15% (expressed as lactic acid), otherwise the solubility of the milk powder is reduced. Stored raw milk is not good even when it is kept under refrigerated conditions because it may lead to an increase in free fatty acidy in the resultant dried milk causing many fold increase in lipolysis in the powder produced from such milk. High bacterial counts increase the susceptibility of fat to oxidation during storage of the powder. Oxidation of milk fat reduces the shelf life of the resulting milk powder. Fat oxidation is accelerated by the presence of metals such as copper or iron. Care must be taken that all equipments, pipelines and accessories coming into contact with milk and cleaning solutions are made of stainless steel exclusively.

The basic operations in milk powder production consists of receiving and selecting milk, filtration/clarification, cream separation/standardization, preheat treatment, condensing, homogenization, drying, packaging and storing of powder. Production of skim milk powder differs slightly from whole milk powder production.

Standardization

The objective of standardization is to adjust the ratio of milk fat and total solids to the level required in the final product. Production of skim milk powder differs slightly from whole milk powder production. For skim milk powder, it is necessary to reduce fat in skim milk to less than 0.1%.

Preheat Treatment

The preheating of milk before condensing in the manufacture of dried milks is done for the production of safe and better stable milk powders and also for inducing in it other desirable attributes. Pasteurization at 72°C for 15 s is sufficient to address the safety requirements associated with processing of raw milk, though, higher temperatures and longer holding times may be necessary to meet requirements relating to thermoduric pathogens and specific food spoilage organisms. For milk powder solubility, which mostly depends on the state of the milk protein system, the high temperature short time (HTST) regime is more convenient when compared to prolonged thermal treatment at low temperatures, and has the same microbiological effect. The temperature most frequently used is in the range of 88-95°C for 15-30 sec. HTST regimes with direct or indirect heating are also used with temperatures ranging up to 130°C. Preheat treatment contributes significantly to the shelf life of dried milks (e.g. whole milk powder) primarily through development of antioxidant or reducing substances.

The formation of reactive or free sulphydryl groups, resulting from the heat treatment of milk, are responsible for preventing oxidized flavour development by acting as free radical scavengers, and thus as antioxidants. Products of Maillard reactions also contribute to the overall antioxidant effect in milk powders.

Milk may be heat-treated at temperatures to achieve various levels and types of protein denaturation, according to final product requirements. Skim milk powder is commonly graded according to the amount of undenatured whey protein nitrogen present in non-fat milk solid  expressed by the whey protein nitrogen index (WPNI), which is defined as the amount of undenatured whey protein nitrogen measured in mg/g powder. Table 4 shows the heat classifications for skim milk powders as was introduced by the American Dry Milk Institute (ADMI) in 1971.

Table 4 Heat classification of skim milk powders and associated heat treatments

	WPN Index	Typical heat treatment
Class of powder
	(mg/g powder)	conditions for milk
Low heat	Not less than 6.0	75°C, 20 s
Medium heat	Above 1.5, but below 6	85 -105°C, 1-2 min
High heat	Not more than 1.5	120-135°C, 2-3min
Condensing

For spray drying, the milk is concentrated in multiple effect evaporators to 40-50 % total solids. For roller drying, the milk is concentrated to 18% total solids. Further concentration for spray drying would increase viscosity and cause difficulties during atomization of the milk. During roller drying, a higher concentration of milk would form a thicker layer on the rollers, followed by inhibited drying and intensive irreversible changes to proteins, lactose and fat. Evaporators use much lesser thermal energy than driers for removal of equivalent amount of moisture from milk.

Homogenization

Homogenization is not an obligatory operation in milk powder manufacture, but is usually applied to decrease free fat content. Higher free fat content in powder is, however, demanded in certain confections. Homogenization also helps to prevent clumping of fat during reconstitution and improves the keeping quality of powder.

Homogenization is conducted after evaporation, or in partly concentrated milk, the concentration rate being not more than 3:1. At higher concentration, homogenization destabilizes milk proteins,thus decreasing powder solubility. The customary homogenization is 2500 to 3000 psi at 62.8 to 76.7°C.

Drying

Though a number of drying systems are available, practically only spray drying and roller drying are commercially used in dairy industry. Since the product quality and process economy are much better and are being constantly improved, spray drying has the highest potential today and in the foreseeable future.

MANUFACTURE OF ROLLER DRIED MILK POWDER

The most used construction of roller drier in the dairy industry is an atmospheric double drum drier made of cast iron. The product at 74-85°C is pumped into the reservoir between the two drums. Milk is continuously brought to the heated surface of the rollers where it is dried. Since the drum speed, on an average at 14-19 rpm, is normally constant, the principal adjustment is the steam pressure during operation.

This should be changed with care to maintain desired 3.0-4.0 % moisture in the powder. After being dried, milk is continuously scraped off the roller by sharp knives in the form of a thin film. Dry film, scraped off by the knives, falls on a spiral conveyer that is along the side of each roller. In this procedure, the film is broken into smaller particles and transported to the hammer milk for pulverizing. The powder is usually sifted through a system of sieves to be classified according to particle size. The product is usually packed in 25 kg kraft bags with a plastic liner reasonable impervious to moisture vapours.

The quality of roller dried milk powder depends on many factors. Higher drum temperature or slower speed will cause scorching of the product despite insufficient drying. In addition to the temperature and duration of drying, the temperature and concentration of milk feed, the thickness of the milk layer over the rollers, and the uniformity of the milk feed play a major role.

Direct contact of the pre-concentrated milk with the hot drum surface results in some irreversible changes in the milk. Caramelization of lactose, Maillard’s browning reactions, and denaturation of proteins are the most important changes affecting the quality of roller dried milk powder. Products of the Maillard type reactions (interactions of lactose with the amino groups of lysine) may cause a scorched flavour of the powder. Denaturation of proteins decreases the solubility of the powder. In spite of these negative effects, roller drying process is used for the following advantages of this process:

•  Relatively low capital and operating costs

•  Relatively smaller floor space requirement

•  Economical processing, easy operation and maintenance

•  Suitable for processing smaller quantities of milk

•  Production of dried milk with longer storage life

•   Advantageous in some applications: such as in the production of certain confectioneries or bakery products.

Roller drying is most often used for nonfat milk. Roller dried milk is also used in feed blends industry.

MANUFACTURE OF SPRAY DRIED MILK POWDER

The predominant method of drying milk and milk products is spray drying. The condensed milk from the evaporator is pumped to a balance tank and from there passed through a filter and introduced into drying chamber with or without preheating.

The solubility of skim milk powder is not affected up to a preheating temperature of 80°C. Feeding cold concentrated milk to the drier may result in sticking to the chamber causing more burnt or scorched particles in the final powder. Either a pressure nozzle or spinning-disc is used for atomizing the concentrate into fine droplets and exposing to a current of hot air in the spray-drying chamber. Due to increased surface area, and to the high latent heat of water evaporation (2.26 MJ/kg), sprayed particles release their moisture quickly, thereby causing an immediate temperature drop of the incoming air. While the inlet temperature reaches up to 215°C, the temperature in the chamber drops down almost instantly to the temperature of outlet air (about 95°C in one stage drying). Residual moisture is one of the most important properties of milk powder both from a quality and an economic point of view. The outlet temperature is usually used as the parameter by which the final moisture of the product is controlled. The outlet temperature depends on a large number of factors.

    The chamber design, the residence time of milk particles in the chamber, the final desired moisture content in the powder and the design of powder collecting systems controlling the air inlet and outlet temperatures. Dry product is taken away immediately after drying. The powder coming out of the drier is preferably cooled to 30°C by a secondary air stream before packaging and storage. Cooling is done to prevent clumping, sticking and heat-damage to the product. Prolonged heating causes staleness in non-fat dry milk and jeopardizes the flavour and keeping quality of dried milks. On prolonged exposure of the powder to heat, the fat has a tendency to melt and oil off. The fat thus becomes the continuous phase. Some of it covers the surface of the dried milk particles and is exposed to air and light and becomes subjected to oxidation.

In two-stage drying, it is also important to control the intermediate moisture, i.e. the moisture of the powder at exit from the drying chamber because it influences many other properties including solubility index, particle density, bulk density, agglomeration, etc. Because there is some gain in moisture content during pneumatic conveying and blending and to a lesser extent during storage, it is normal to produce powder from the dryer of lower moisture content than that called for by the specification so that the final powder remains within specification.

Spray drying has numerous important advantages compared to other drying techniques: The whole process proceeds very rapidly; air residence time in the chamber is up to 30 seconds. Because of this and because drying is accomplished at lower drying temperatures, the product has excellent properties.

Certain disadvantages of spray drying include very large size of the drying chamber, expensive equipment, high electricity and steam consumption and low bulk density of the dried product. However, the bulk density of milk powder obtained by spray drying is still higher than that of roller-dried powder. Because of high investment cost, Spray drying installation is economically justified only for large capacities, over 100,000 kg of raw milk per day. Today there are specialized drying plants with modern facilities, producing 18 tons/hour of powder.

MANUFACTURE OF INSTANT MILK POWDER

During the last few decades there has been a growing market for powders which are instantly soluble in cold water. Ordinary non-agglomerated powders tend to lump when mixed with water, and if strong mechanical stirring is not applied, it may result in an inhomogeneous mixture which is not attractive to the consumer. The quality requirements to instant milk powders are constantly  getting stricter as an increasing number of properties must be optimized simultaneously and controlled within still narrower limits in order to obtain a high quality product with the highest degree of uniformity, fulfilling the requirements set by costumers, organizations and industrial and/or legislative standards.

Instantization

The historic development started with the pioneering research of Mr. David D. Peebles in the beginning of the fifties, and instantized non-fat dry milk was marketed from 1954. Soon it replaced the regular spray dried products on the retail market.

The principle of instantization is the agglomeration of individual spherical milk particles into clusters and the conversion of lactose from the glass into a microcrystalline form which makes the powder more wettable and less hygroscopic. In the instantization process, the surface of milk powder particles are humidified, or the particles are only partially dried during manufacturing, so that the surface is tacky and partial crystallization of lactose leading to formation of microcrystals occurs before the particles are re-dried. This produces a clustering of the particles in loose spongy aggregates of low density (known as agglomerates / conglomerates / granulates) which flow freely and disperse readily in cold water, as water penetrates the spongy structure of these aggregates and allow them to sink and disperse. Agglomeration is a result of wet and/or semi-dry particle collision with size >125 µm. Control is achieved by returning dry fine powder to the wet spray during different stages of spray drying.

REFERENCES

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• Hall, C.W. and Hedrick, T.I. (1966). Drying of Milk and Milk Products, The AVI Publishing Co. Westport, Connecticut.
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