Dried milk products

There are a great number of products manufactured in the milk powder industry including milk powders, modified milk powders, powdered milk by-products, and dried milk based products containing either milk solids only or also other foodstuff components. Therefore milk powder technology is a broad subject. The target of industrial production is to produce products which fulfil all the qualitative requirements. The influence of various technological parameters on powder properties is discussed in detail in section 10. Achieving product properties. This chapter presents just a survey of products with the required qualitative properties and technological guidelines for operation on various installations.

Milk powders are defined as follows:

a) Dehydrated products based on non-fat milk solids and milk fat, i.e. natural milk, the fat content of which has been adjusted by centrifugation or addition of cream or skimmed milk to achieve the fat content required in the final product. This is between 0.5 and 30% (expressed on total solids). Besides fat standardization, it is becoming more and more usual to standardize the protein content, i.e. to adjust it to a level of Frisian cows’ milk, i.e. 37-39% (total protein in non-fat solids) by means of the addition of lactose or permeate if the natural protein content of the milk supply is too high. Products involved in this group are skim milk and whole milk powders (designated often also as full cream milk powder) defined as milk powder with max. 1% fat and min. 26% fat respectively. Occasionally there are produced also powders with other fat content such as half cream milk powder with 14% fat or even others with a fat content inbetween these figures. Another product of this group is cream powder with a fat content 35-80%. Permitted additives are the vitamins in various forms and some minerals.

b) Fat filled milk powders based on skim milk and vegetable or animal fat, possibly a mix of those, with a fat content 10-80%. Beside the components and additives mentioned above, other functional additives are used such as emulsifiers, stabilizers, flavouring and colouring agents etc.

The inlet air temperatures as stated for the individual products in the subsequent text are valid at normal ambient conditions, i.e. with absolute air humidity max. 7 g/kg dry air. Higher air humidity will require reducing the inlet air temperature, (equal to reducing the evaporative capacity of a given dryer) or dehumidifying the air.

Similarly, the feed concentrations are valid for milk of normal composition with max. 39% proteins in non-fat solids. Higher protein content requires reducing the concentration or standardizing with lactose or permeate to keep the viscosity below 100 cP at 40°C.

8.1.

Regular milk powders

Regular or ordinary is the usual designation for non-agglomerated products. Practically all types of drying installations can be applied. Generally, two stage and three stage drying processes bring many benefits to powder quality and economy. On the other hand they have adverse effects on bulk density which for regular milk powders is required to be as high as possible and is in fact one of the most important properties. This is due to some unavoidable agglomeration. A blow line transport to the silo or bagging-off point is normally sufficient to break down the agglomerates of two-stage process dried powders and to achieve bulk density higher than with corresponding single stage process. Three stage drying systems however result in low bulk density and a mill may be needed to get the desired bulk density. As to bulk density the use of pressure nozzles gives higher values than the use of a wheel atomizer.

8.1.1.

Regular skim milk powder

Regular skim milk powder is a very important raw material in the food industry for further processing, and each application has some specific requirements to powder properties. The main users of skim milk powder are the bakery industry, recombining industry (for sweetened condensed and evaporated milks) and also the milk powder industry for fat filled powders for feeding calves. This involves dry mixing of regular skim milk powder with 40-60% fat concentrates to get a fat content corresponding to that of whole milk powder and to handle peak season surplus. Smaller amounts of skim milk powder, again produced during the peak season, go to the cheese industry for increasing the cheese production in winter. While the bakery industry requires powder of good water binding properties, achieved by denaturing of whey proteins, the cheese industry is asking for a powder with good rennet ability i.e. having whey proteins non-denatured. Therefore regular skim milk powder is produced in numerous varieties to satisfy the wishes of each user. The process step which influences the degree of whey proteins denaturation is pasteurization, and the conditions will be discussed in chapter 10: Whey Protein Nitrogen Index. Bulk density is another important property from the economical point of view (also discussed in chapter 10).

The process involves separation, clarification, pasteurization in the evaporator to 70-120°C with 15-600s holding time, evaporation and spray drying. Pre-heating of the concentrate up to 80°C prior to atomization is strongly recommended.

Concentrate properties

Solids content: 48-50% TS

Viscosity: max. 100 cP at 40°C measured on the feed to be atomized. Measurements on

concentrate not older than 15 min. and kept under vacuum between evaporator and sampling

for viscosity measurements. Method Brookfield viscometer model LVT with spindle 2, rotation

60 rpm, measured at 40°C.

Protein denaturation: For high bulk density powders the WPNI should be max. 1.0 mg.

Solubility index: no measurable amount.

Sieving test: no visible insoluble (cheesy flakes) on 250 micron mesh after passing 1 litre of

concentrate through the mesh and washing with water.

Scorched particles: no measurable amount.

The obtainable bulk densities together with main operation conditions as given in table 8.1. (see also micro photo on Fig. 8.1.) anticipate that the powder is high-heat, the insolubility index 0.1-0.2 ml, and that the final powder is transported by air to a silo. The table indicates also the relative heat consumption based on the given conditions. The usual requirements for a good quality regular skim milk powder are: residual moisture of max. 4.0%, bulk density min. 0.72 g/ cm³ tapped 1250 times, insolubility index max. 0.2 ml and scorched particles disc A.

**Tablo. 8.1.** Bulk density of ordinary skim milk powder
Process or system	Atomization	Inlet temperature °C	Concentration %TS	Bulk density g/cm³	Relative heat consumption
Single stage Single stage Two stage Two stage Compact Compact MSD	nozzle wheel nozzle wheel nozzle wheel nozzle	180 180 200 200 220 220 240	45 48 47 50 47 50 47	0.75 0.68 0.78 0.72 0.78 0.72 0.66	1.00 0.97 0.77 0.74 0.84 0.79 0.79

8.1.2.

Regular whole milk powder

The process is identical to that used for skim milk powder production plus standardization to the required fat content and homogenization of the concentrate. Whole milk is more heat sensitive than skim milk; therefore heating of the feed prior to atomization is even more important. The homogenization is expected to be not higher than 80 bar total pressure with 20-30 bar in the second stage. The pasteurization in the evaporator is usually 95-110°C with 15- 150s holding time. Table 8.2. gives the obtainable bulk density for the various dryer systems. The quality of a good regular whole milk powder is characterized by moisture content max. 3.0%, insolubility index max. 0.2 ml, bulk density min. 0.60 g/cm3 tapped 1250 times, free fat content max. 1.5% and scorched particles disc A.

Concentrate properties

Solids content: 48-50% TS

Viscosity: Max. 60 cP at 40°C measured on the feed to be atomized. Measurements on

concentrate not older than 15 min. and kept under vacuum between evaporator and sampling

for viscosity measurements. Method Brookfield viscometer model LVT with spindle 2, rotation

60 rpm, measured at 40°C.

Protein denaturation: For high bulk density powders the WPNI should be max. 1.0 mg, based

on SNF.

Solubility index: no measurable amount.

Sieving test: no visible insoluble (cheesy flakes) on 250 micron mesh after passing 1 litre of

concentrate through the mesh and washing with water.

Scorched particles: no measurable amount.

**Tablo. 8.2.** Bulk density of ordinary whole milk powder
Process or system	Atomization	Inlet temperature °C	Concentration %TS	Bulk density g/ cm³(1250)	Relative heat consumption
Single stage Single stage Two stage Two stage Compact Compact MSD	nozzle wheel nozzle wheel nozzle wheel nozzle	160 160 200 200 220 220 240	45 48 47 50 47 50 47	0.64 0.60 0.66 0.64 0.66 0.64 0.62	1.00 0.97 0.74 0.71 0.88 0.83 0.83

8.1.3.

Whole milk powder with high free fat content

The traditional raw material for chocolate industry has always been a drum dried whole milk powder mainly because almost the total fat content in this product appears as free fat. In the milk powder industry, however, the spray drying process is, for many reasons, much more lenient than drum drying. Therefore efforts were made to produce a whole milk powder suitable for chocolate manufacture by spray drying. And successful results have been made.

In a normal whole milk powder the continuous phase of a particle is the amorphous lactose which forms a very tight membrane protecting the globular fat against extraction. Thus an important condition for achieving high free fat content is to transform a substantial part of lactose into -lactose-monohydrate. Besides removing part of the lactose from the protective shell, the crystals formed create a net with craters and channels in between, through which the solvent for free fat determination can penetrate into the particles. The crystal formation as such exhibits also a positive effect on creation of free fat, affecting the fat globules with sharp edges. Thus crystallization of lactose is an essential part of the process.

The industrial production is based on simultaneous atomization of two feeds, one of them a pre-crystallized skim milk concentrate of 45-48% total solids and temperature 30-35°C and the other, cream with fat content 50-90% (on total solids basis and possibly standardized with precrystallized skim milk concentrate to 45-48% total solids) in such proportion as to get a powder with 26-28% fat. This is done by a dual feed system supplying either two separate nozzle systems or a twin wheel (a double-deck wheel being able to atomize two feeds separately). The production can be done on any type of single stage dryer with cooling bed (see Fig. 5.3.) or Multi Stage Dryer (Fig. 5.9.). Obviously, because of high free fat content the biggest problem of this process is deposits in the dryer and especially in the cyclones. Therefore the cyclones are replaced by a bag filter. The deposit problems can also be reduced by introducing skim milk powder through a fines return system, i.e. to use the so-called powdering techniques, again in such rate as to get the required fat content of the final product. The use of the Multi Stage Dryer has the advantage that the crystallization of lactose can continue on the static fluid bed, if this is operated with an air temperature allowing high moisture content in the powder, contributing furthermore to the creation of free fat.

The most important property is the free fat content usually required to be 85-95% of the total fat and final moisture max. 2.5%. There are no requirements as to insolubility index which is often quite high.

8.1.4.

Butter milk powder

Butter milk powder is a not too interesting or marketable product and is produced if other possibilities to get rid of butter milk are not available. Sweet butter milk in an amount up to about 25% of the total evaporator feed is sometimes used to standardize whole milk for making whole milk powder. However, the addition of butter milk increases the free fat content of whole milk powder and therefore degrades quality, especially of instant whole milk powder. Butter milk powder, both sweet and acid is used for cattle feeding blends.

8.1.4.1.

Sweet butter milk powder

The drying technology as to feed concentration and drying temperatures is identical to that of regular skim milk powder.

8.1.4.2.

Acid butter milk powder

Acid butter milk is difficult to evaporate due to the high acidity and therefore the highest concentration is about 30% total solids. Drying is also difficult because of the high content of lactic acid. The inlet air temperature should not be higher than 160°C in order to avoid excessive deposits.

8.1.5.

Fat filled milk powder

Fat filled milk powder is a common name for fat containing milk powder in which, instead of natural butter fat, animal fat or vegetable oil or even other types of emulsified fat-like material as lecithin, glycerin-monostearate etc. are used. The fat content can be up to 80% - provided that the carrier is maltodextrin - and products are used for further industrial processing in various food industries, mainly bakery, for dry mixing during preparation of mixtures for feeding calves and special kinds for human consumption i.e. so-called whole milk replacers.

The production technology is based on mixing skim milk concentrate with a fat blend at the temperature well above the melting point of the fat, usually 50-60°C. The concentration of skim milk must be such as to get, after the addition of fat, a concentration of the feed for spray drying of 45-55%. Fat blends usually contain emulsifiers and stabilizers such as glycerinmonostearate, lecithin etc. Also other additives such as vitamins and minerals are often applied.

For the calculation of total solids content of skim milk concentrate (TS-skim) and the amount of fat to be added to each 100 kg of that concentrate (kg fat/100 kg) to obtain the required total solids (TSR) and required fat content (FR), the following equations are used:

[8,1]

…

[8,2]

…

Example:

Required fat content (FR) 52%

Required total solids of the mix 50%

[8,3]

…

For quick orientation, the results can be found using graph in Fig. 8.2. in which the above example is shown (the amount of fat in kg to be added to each 100 kg of skim milk concentrate is found using steps 1-2-3 and concentration of skim milk by steps 1-3-4).

**Şek.8.1. Required fat content in total solid**

The drying technology and type of spray dryer depends very much on the required total fat content of the product. For fat contents up to 35%, the same spray dryer design is used as for whole milk powder. For higher fat contents up to 60%, designs with cooling bed are required with falling tube transport of the fines to the bed from the cyclones which have to be designed for a low pressure drop. Powders with higher fat content than 60% require either tall form, multi stage or integrated belt dryer designs. Generally the higher the fat content, the lower the air inlet temperature and homogenization pressure and the higher the feed concentration.

8.2.

Agglomerated milk powders

The process technology is identical to that of regular powders, but the drying process requires separation and return of the fines to the atomizing device as described in chapter 4. Fines return system. The conditions of agglomeration are discussed in chapter 10. Agglomeration and instant properties. Contrary to regular powders, any kind of air transport of the final powder has an influence on the quality. However, new developed dense-phase powder transport systems with low conveying velocity and low air-to-powder ratio treats the agglomerated powder gently. However, the mean particle size after transport is always lower and the amount of fines and the bulk density are higher than before transport. Consequently, the functional properties are affected too. If the best possible quality of agglomerated powder is requested then use of any form of air transport is undesirable. The best way to handle agglomerated powders between the spray dryer and packing is a Tote-Bin system or possibly Big bag system. Agglomerated powders from the Multi Stage Dryer will however exhibit acceptable functional properties also after having been conveyed in modern conveying systems as mentioned in chapter 4.

8.2.1.

Agglomerated skim milk powder

Drying conditions:

Inlet air temperature for single stage processing in a conventional and TALL FORM DRYER™ with cooling bed 180°C, for two stage drying in conventional, TALL FORM DRYER™ and COMPACT DRYER™ 200°C, for Multi Stage Dryer, MSD™ up to 240°C,
Feed concentration for pressure nozzles and wheel atomizer 48-50%. All other requirements to the skim milk concentrate similar to what is mentioned in 8.1.1., however the WPNI should be between 2.5 and 3.5 mg.

Agglomerated skim milk powder has instant properties, if the mean particle size is higher than 180 μm and bulk density not higher than 0.48 g/cm3, the amount of particles smaller than 125 μm less than 20% and fat content not higher and preferably well below 1%. Even traces of free fat can be detrimental to wettability. The multi stage dryer equipped with fines return to the nozzles achieves bulk densities as low as 0.40 g/cm3. See also micro photo on Fig. 8.2.

The usual quality requirements for instant skim milk powder are: moisture content max. 4.0%, bulk density max. 0.48 g/cm3, wettability max. 30 s, IDF-dispensability min 90%, insolubility index max 0.2 ml and scorched particles disc A.

8.2.2.

Agglomerated whole milk powder

Drying conditions:

Inlet air temperature for single stage process in conventional and TALL FORM DRYER™ with cooling bed 180°C, for two stage drying in conventional, TALL FORM DRYER™ and COMPACT DRYER™ 200°C, for Multi Stage Dryer, MSD™ up to 220°C,
Feed concentration for pressure nozzles and wheel atomizer 48-50%, homogenization,mpreferably two stages at 80 bars for first stage and 30 bars for second stage.

Agglomerated whole milk powder, in spite of better appearance, flowability and reconstitubility than regular powder is still not instant in cold water, only in water of temperature higher than

40°C.

8.2.3.

Instant whole milk powder

The basic technology is identical to that incorporated in the production of agglomerated whole milk powder. However, the final product must fulfil many qualitative requirements which are, to a great extent, influenced by the individual processing steps. For a top quality product it is necessary to select carefully all decisive parameters, as follows:

standardization of fat,
standardization of protein content by addition of lactose solution or milk permeate to 37-39% (on non-fat-solids),
fortification with vitamins. Addition can take place as batch dosing in the standardization tank, or continuous dosing at the inlet to the evaporator or after pasteurization prior to the first evaporator stage. However, vitamin C in the form of ascorbic acid solution has to be added to the cold milk. In batch processing addition has to be carried out slowly under good agitation. In continuous processing, dosing is done preferably into the milk pipeline of good flow capacity. This is important, because it is an acid and poor mixing can cause local over-acidification and consequently precipitation. Another way to overcome these problems is buffering of ascorbic acid solution by sodium citrate or using ascorbic acid palmitate.
Pasteurization in the evaporator at 85-95°C with 0-180s holding, resulting in WPNI 2.5-3.5 (consult 10.7.4. Heat stability),
Homogenization two stage using 80 bar in first stage and 30 bar for the second stage,
Inlet air temperature for two stage drying in Conventional, TALL FORM DRYER™ and COMPACT DRYER™ 180°C, for Multi Stage Dryer, MSD™ up to 220°C,
Feed concentration for pressure nozzles and wheel atomization 48-50%,
Lecithin treatment using powdered lecithin dissolved in butter oil in 25-50% solution and dosing rate to get 0.15-0.25 lecithin on powder. The temperature of lecithin solution must be 60-65°C. The lecithin treatment in two stage drying systems is done usually between the two fluid beds. In the Multi Stage Dryer, MSD™ or the COMPACT DRYER™, the lecithination is done at the outlet from the static fluid bed just above the first section of an external fluid bed,
After lecithination, the powder has to be fluidized using warm air to keep the temperature well above 40°C, preferably 45°C,
The final powder is collected in tote bins or similar containers or conveyed to silos and kept at the above temperature until filling into tins or bulk transport containers,
The lecithinated whole milk powder has to be gas packed under inert gas, usually nitrogen in mixture with carbon dioxide to achieve residual oxygen content less than 2%. Otherwise the technological conditions for the production of instant whole milk powder can be found in Fig. 7.5. and the quality specification in Fig. 7.4.

The instant whole milk powder is, as it has been emphasized before, the most important dried milk product and therefore the final quality is evaluated by many properties.

The properties of a top quality instant whole milk powder are as follows:

Moisture content max. 3.0%

Bulk density max. 0.48 g/cm3

Scorched particles disc A

Wettability 25°Cmax. 15 s

IDF-dispersibility min. 95%

NZ-dispersibility max. Disc 2

Sludge 25°C max. 0.1 g

SDP 25°C max. B

Sludge 85°C max. 0.1 g

SDP 85°C max. B

Free fat content max.1.5%

Flowability max. 50 s

**Şek.8.2. Micro photo of an MSD™-produced instant whole milk powder**

8.2.4.

Agglomerated fat filled milk powder

The technology for manufacturing of agglomerated fat filled milk powders is identical to that of agglomerated whole milk powder (see 8.2.2.).

8.2.5.

Instant fat filled milk powder

The technology for manufacturing of instant fat filled milk powders is identical to that of instant whole milk powder (see 8.2.3.).

8.3.

Whey and whey related products

Whey, both sweet and acid, can be dehydrated as such and also used as a raw material for a number of products. Modern processes such as demineralization, ultrafiltration and enzyme hydrolyzation have further expanded the product spectrum to modified whey powders, i.e. demineralized and hydrolysed products, whey protein powders and dried permeate. Furthermore, whey can be used as a carrier for fat during the production of fat filled whey powders. Most of these products are difficult to dry requiring special technologies and special equipment.

Whey is a valuable raw material requiring the same treatment and care as given to milk. The recommended procedure is cooling down below 10°C just after it is drained from the cheese vats to slow the bacterial activity. It is also strongly recommended to remove the so-called cheese dust by clarification and excess of fat by centrifugation, as residues of both will affect further processing. Lack of treatment resulting in developed acidity degrades the quality of final products and causes difficulties during drying. The composition of sweet and acid whey can vary very much, but average values for both liquid (after clarification and fat-centrifugation), solids and dry powder are given in Table 8.3.

**Tablo. 8.3.** The percentage composition of whey.
Whey	Sweet			Acid
Component	Liquid	Solids	Powder	Liquid	Solids	Powder
Water Lactose Protein Minerals Fat	94,00 4,550 0,80 0,60 0,05	0,00 75,84 13,33 10,00 0,83	2,50 73,94 13,00 9,75 0,81	94,10 4,00 1,10 0,75 0,05	0,00 67,80 18,64 12,71 0,85	1,50 66,79 18,36 12,52 0,83
Total	100,00	100,00	100,00	100,00	100,00	100,00

8.3.1.

Ordinary sweet whey powder

Sweet whey is a by-product from the manufacture of rennet fermented cheese and has usually a pH higher than 6.4. Lower pH is an indication of a developed acidity. Ordinary sweet whey powder can be obtained either by drying the whey concentrate directly from the evaporator or as a pre-crystallized whey concentrate.

The operating conditions for ordinary whey powder without pre-crystallization are: concentrate total solids 42-45%, inlet drying air temperature 180°C and an outlet drying air temperature (around 90oC) resulting in a moisture content of below 2% in the powder.

For pre-crystallized ordinary whey powder, the concentration is about 55%, inlet drying air temperature 180-200°C and outlet drying air temperature (around 92°C) resulting in a “free” moisture content of below 2%. In both cases a single stage drying plant is used.

The pre-crystallization process involves flash cooling of the concentrate from the evaporator to 30-35°C and transfer to crystallization tanks. These must be equipped with efficient agitation and cooling jackets. When the level in the tank is high enough so that the contents are rigorously agitated a seeding material is added. This is lactose in the form of finely ground -lactose-monohydrate, in the amount of 1 kg per each 1000 kg of the concentrate. The full crystallization tank is gradually cooled by the rate 2-3°C/hour down below 20°C. The whole crystallization process takes, inclusive tank filling, 24 hours, so that drying of the tank batch starts 24 hours after commencement of filling. The control of the pre-crystallization process is conducted by means of refractometrical readings before and during crystallization process.

A good quality pre-crystallized whey concentrate should have the average crystal size of 30- 50 μm with single crystals not larger than 100 μm and the crystallization degree should not be lower than 70%. This powder will however still exhibit caking and hygroscopic tendencies if the powder is exposed to humid air. If a 100% non-caking and non-hygroscopic product is aimed at, then it is necessary to dry the pre-crystallized product in a plant with an after crystallization belt see Fig. 5.13. or use the TIXOTHERM™ plant see Fig 5.14. Chapter 5

8.3.2.

Ordinary acid whey powder

There are several types of acid whey. All have pH 4.0-4.5, but depending on the type of acid they are very different as to behaviour during processing. The most difficult whey is lactic acid whey originating from the production of cottage cheese or quark. On the other hand, the hydrochloric acid whey from the production of casein is almost as easy to dry as sweet whey. Thus it is not the pH which causes problems, but the amount of acid. Lactic acid is a weak acid and to obtain the pH 4.6, which is the isoelectric point of casein, requires much higher amount of acid than in case of hydrochloric acid, which is a strong acid.

Just like sweet whey, it is possible to dry acid whey either with or without pre-crystallization. The operating conditions for hydrochloric acid whey for both alternatives are identical to those for processing sweet whey. The hydrochloric acid whey can under certain circumstances cause pit corrosion of stainless steel and therefore it is advisable to use acid resistant steel in equipment fabrication. However, this corrosion does not take place during operation but usually when the plant stands. Therefore a spray dryer processing hydrochloric acid whey, irrespective of which material is used, should never be left in a shutdown mode for a longer period with powder on the walls. Cottage cheese and quark whey, especially without pre-crystallization, are very difficult products to dry. Neutralization by means of calcium hydroxide to transform the lactic acid to calcium lactate makes it easier, but this increases the amount of minerals significantly and thus is not very popular. However when used, it is very essential to add the lime (water suspension of calcium hydroxide) to the concentrate very slowly and under vigorous agitation to avoid local over neutralization. This will result in an almost unmanageable concentrate to dry. Using magnesium hydroxide to form Mg-lactate which is a dry salt is preferred by many end-users, and it does not result in dark colouring of the meat of the animals, typically calves eating the product.

Sodium hydroxide for neutralization is not at all recommendable as Na-lactate is very hygroscopic and thermoplastic, and means just asking for more troubles.

Single stage drying of both pre-crystallized and non-pre-crystallized concentrate requires 42 and 48% solids respectively, inlet air temperature 160°C and outlet air temperature (around 92°C) to secure a powder with a residual moisture content of less than 2%. Furthermore this process is very sensitive to ambient humidity and therefore it is almost impossible to operate during high humidity periods. These occur normally in the Northern hemisphere during late summer, i.e. in the period when most cottage- or quark whey is produced. Thus for factories specializing in drying lactic acid whey the FILTERMAT™ type spray drying plant is much more recommendable.

Lactic acid whey concentrates are very thixotropic. Their viscosity, especially during precrystallization, can be so high that the concentrates almost solidify when cooled down below 20°C. Therefore the maximum concentration is 48% (maybe 50% but generally even 48% TS is too high) with the final temperature not lower than 20°C.

8.3.3.

Non-caking sweet whey powder

The production of non-caking sweet whey powder requires a pre-crystallization as described in 8.3.1. Concentration can be up to 62%. During crystallization, the viscosity increases whereby the higher the pasteurization temperature, the higher the viscosity. Thus, in order to control the viscosity within manageable limits, the pasteurization temperature should not be higher than 82°C with 15 s holding.

Efficient pre-crystallization, which is 70% with 50% TS and 80% with 60% TS is very essential for this process. The drying is conducted in a straight-through drying system with 180°C inlet air and 80°C outlet air temperatures. The powder leaves the chamber with about 4.5% moisture content. This is reduced to 2% in the attached fluid bed. Even better non-caking quality and much higher drying economy is achieved on the so-called belt dryer. The transport belt, socalled crystallization or timing belt, provides the residence time of about 8 minutes between the drying chamber and the fluid bed after-dryer. The flow sheet of a belt dryer is shown on Fig. 5.13. An alternative to the crystallization belt is the successfully proven rotating disk. This is completely enclosed in a stainless steel housing and hence suitable for CIP, ideal from the hygienic point of view.

The process using a belt dryer can operate with inlet air temperature up to 160°C and outlet air temperature 55°C. The powder leaves the chamber with 10-14% moisture. The residence time of 8 minutes on the crystallization belt creates conditions favourable for further crystallization, i.e. after-crystallization of lactose. The final product from this process has almost 100% of the lactose as -lactose-monohydrate.

CDI and MSD™ plants can also be used. Here the static fluid bed functions like the crystallization belt, however the moisture content is not high enough and the powder residence time in the static fluid bed is not long enough to secure that 100% of the lactose is pre-crystallized. The best quality product is produced in a spray dryer with an external crystallization belt followed by a fluid bed after-dryer. (Fig. 5.13.). The same quality product can be obtained from the TIXOTHERM™ plant (Fig. 5.14.). The Integrated Belt Dryer, the FILTERMAT™, Fig 5.12. is also very recommendable.

8.3.4.

Non-caking acid whey powder

The drying plant described in the previous section can be used also for drying acid whey concentrates. As regards to lactic acid whey, all the problems and limitations discussed in section 8.3.2 can be expected here. Hydrochloric acid whey is processed under the same conditions as sweet whey.

The drying can also be conducted in a straight-through plant (without the crystallization belt) with the following conditions: 48-50% total solids, inlet and outlet air temperatures 160 and 82°C respectively.

The operating conditions for drying lactic acid whey with the belt process are: 48-50% total solids, inlet air temperature 160°C and outlet air temperature 56-60°C. The after crystallization process proceeds better, and the moist powder on the belt is not that sticky if the whey proteins are completely denatured by pasteurization at 90°C with 10 minutes holding.

As a general rule, a straight-through plant with or without the crystallization belt has the advantage that other milk powders, both ordinary and agglomerated, can also be processed in the same installation (the dryer can be designed in such a way that the belt can be by-passed). If, however, producing lactic acid whey powder is the main duty, then the FILTERMAT™ dryer (see 5.3.5.) is preferable.

8.3.5.

Fat filled whey powder

Fat filled whey powder is used as an ingredient for stock food-mixtures. It contains 30-60% fat, usually lard or tallow. The higher the fat content the greater the difficulties with deposits both in the chamber and in the cyclones. Cyclones are therefore replaced by CIP-able bag filters, see chapter 4. With fat content 50% and higher, it is recommendable to use the socalled powdering techniques, in which a dry whey powder is introduced through a fines return system into the atomized spray cloud. In this case, the concentration of fat in the atomized feed and the amount of dry powder must be adjusted to get the required total content of fat in the final powder.

Installations suitable for fat-filled whey include single stage dryers with cooling beds (5.1.3), and both CDI and MSD™ plants with the static fluid beds, operating with cold air. However, if the dryer is just for fat-filled whey powder the FILTERMAT™ concept is the most advantageous.

8.3.6.

Hydrolysed whey powder

The lactose in whey can be enzymatically hydrolysed whereby as much as 90% lactose can be transformed into glucose and galactose. Both these sugars have a high solubility in water and cannot be crystallised. The only drying plant which can successfully handle this duty is the FILTERMAT™ dryer (see 5.3.5.). The product is extremely hygroscopic, and therefore powder handling has to be done in a well air-conditioned room. Whey powder with 90% hydrolysed lactose, when exposed to the air of 50% humidity, can pick up more than 10% moisture within a few minutes. As both glucose and galactose have much higher sweetening power than lactose, the main use for this product is as an ice-cream sweetener etc.

8.3.7.

Whey protein powder

Production of whey protein powder is a good alternative to processing whey, since the product is much sought and has high value on the market. It is used mainly as a component in baby food formulations and also as a protein fortification in various food formulae. Standard products on the market contain 35, 60 and 80% protein. 80% is the most used and valuable product, which originates from an ultrafiltration plant as an approximately 25-30% total solids concentrate, which can be further concentrated in the evaporator up to almost 40%.

Due to the high protein content, the powder tends to be very light and fluffy with a high content of occluded air. To minimize this, pressure nozzle atomization is preferred. The most used plant for this product is a two stage TALL FORM DRYER™ (TFD). Two stage drying is used to protect the proteins against denaturation. Other dryer types such as SDI and CDI operating with pressure nozzles can be used as well. If the final product should be agglomerated, then the MSD™ plant is used. Whichever dryer is selected, a bag filter is necessary.

**Şek.8.3. Relationship between total solids content and density of 80% protein concentrate at 20°C.**

8.3.8.

Permeate powders

A troublesome by-product in the manufacture of whey protein concentrate is the permeate consisting mainly of lactose and salts. It can be dried in similar plants and with similar technology as described for non-caking whey powder. High concentration, up to 66% and pre-crystallization are recommendable, but problems can be expected in the evaporator due to precipitation of calcium phosphate, especially with acid permeate. The drying process is either a single stage dryer with an inlet air temperature of 160oC and outlet air temperature (about 90°C) to result in a powder with a maximum of 2% free moisture, or a two stage process with outlet 60-65°C and after crystallization belt followed by an after dryer. Also the TIXOTHERM™ process as described in chapter 5 can be used.

8.3.9.

Mother liquor

Mother liquor is a by-product from lactose production and is the liquor remaining after separation of the lactose crystals by centrifugation. The technology of lactose manufacture varies considerably and so does the composition of the mother liquor. The protein content can vary between 30-45% and generally the product having protein content higher than 40% is very difficult to dry. The pre-crystallization is, due to high protein content, difficult to conduct and therefore it is usually not applied. The recommended process is single stage drying with a cooling bed with inlet and outlet air temperatures 160-180 and 90°C respectively.

8.4.

Other Dried Milk Products

Almost every liquid milk or milk-containing drink or product existing on the market can be manufactured in form of a powder and therefore there are a huge number of different products which are used both as household consumer products and as raw materials in further industrial processing. The main products of this group are baby food, coffee-whitener, cocoamilk- sugar powder, cheese pow-der, butter powder and caseinate powders.

8.5.

Baby food

The group of products designated generally as baby food or Infant Formulas contains a wide range of products of very different compositions and ingredients requiring diverse processing technology. The common feature of all infant formula products is compliance with high microbiological and hygienic standards together with an ease of reconstitution in lukewarm water without un-dissolved lumps. The basic baby food product is a whole milk powder and modified milk powder. The aim of this modification is to adapt the composition to closely resemble human milk. Such a product is designated as humanized milk powder. The composition of cow’s milk and human milk is shown in Table 8.4.

**Tablo. 8.4.** The percentage composition of cow’s and human milk.
	Cow’s milk		Human milk
Component	Liquid	Solids	Liquid	Solids
Water Fat Lactose Casein Lactalbumin Minerals	87,35 3,75 4,85 2,78 0,47 0,80	0,00 29,64 38,34 21,98 3,72 6,32	87,30 4,10 6,90 0,60 0,90 0,20	0,00 32,38 54,34 4,72 7,09 1,57
Total	100,00	100,00	100,00	100,00

The ideal food for infants is breast milk. The first approach to developing alternative feeding methods was the application of sanitary standards for milk handling together with heat treatment ensuring the required microbiological standards. However, cow’s milk as such is inappropriate for feeding infants.

The way to humanized milk was long, and was governed both by the level of paediatrician’s knowledge and the available technological possibilities of the dairy industry. The first step was just addition of sugar in the amount of about 20% in the dry matter. This type of baby food, i.e. whole milk powder with sugar is still produced in some Asiatic countries. The second step was the modification of fat by the addition of some vegetable oils with high content of polyunsaturated fatty acids, especially the essential acids as linoleic and arachidonic. The last step, i.e. the modification of the proteins became possible by the processes of demineralization and ultrafiltration. The full humanization step consists of replacing a part of butter fat with vegetable fat to get a similar composition as human milk fat, then increasing the content of lactose, replacing part of casein with lactalbumin, vitamin and mineral fortification. The main raw materials for manufacture are lactose, demineralized whey powder, whey protein concentrate, caseinate, malto-dextrin, lactulose (galactosido-fructose), fractionated coconut oil, sunflower, and corn and soy oil. The mineral additives are tri-calcium-phosphate, sodium and potassium citrates, magnesium and potassium chlorides, calcium carbonate and zinc, ferrous, cupric and manganese sulphates (possibly ferro-lactate or ferro-sacharate). The most common vitamins used include ascorbic acid, alpha-tocopheryl acetate, riboflavin, vitamin A palmitate and vitamin D-3. Mono- and di-glycerol-stearate, lecithin and carrageenan are used as emulsifiers.

However, apart from humanized milk powder, a number of specific nutritional and dietetic products have been formulated in the paediatric nutrition area for normal infants, premature infants and lactose-intolerant infants etc. including various acidified and fermented products. The composition of these products is a result of cooperation between research scientists, dairy chemists, clinical nutritionists, paediatricians and milk powder manufacturers.

An extensive clinical testing and shelf-life evaluation of a new infant formula product are essential prior to marketing.

The processing technology for each specific formula is proprietary to the manufacturer. However, the wet process for the dryer feed preparation employs traditional dairy processing equipment. Either batch or continuous processing is used. In general, the major ingredients are dissolved or dispersed in water or skim milk. The minerals, vitamins and emulsifiers are usually added at the end. After blending but before spray drying, the blend is heat treated and homogenized.

The most important spray dryer requirement for baby food is the possibility of long periods of continuous operation without extensive deposits and maintenance of high hygienic standards. There are two philosophies as to operating a baby food spray dryer:

to operate non-stop for as long as possible, usually one week and then wet wash the installation after each stop of production,
to operate non-stop for as long as possible but to use dry cleaning only, i.e. never water.

To meet these criteria the most suitable types of spray dryer for infant food powders are TALL FORM DRYERS™ (TFD) and Multi Stage Dryer MSD™, both with a fluid bed for after drying and cooling. Use of a conventional spray dryer with either cooling bed or pneumatic transport system is also possible.

From the viewpoint of drying, infant formulae are categorized as being the more difficultto- dry products. This is mainly due to high content of lactose (up to 60%) and other carbohydrates and, possibly also the acid content, either as added lactic acid or as a product of bacterial fermentation using cultures of streptococcus lactis and lactobacillus. Baby food products do not need to be truly instant, i.e. instant in cold water, because they are normally reconstituted in lukewarm water at the human body temperature. However, it is important that they are completely dissolved without even a trace of lumps which can clog the baby bottle teat. Therefore it is advantageous if the powder is slightly agglomerated. Due to the high hygroscopicity and stickiness of the powder, two stage drying is applicable only to a certain degree, and the products have to be spray dried to a moisture content of 3-4% followed by fluid bed drying and cooling to 2.5% moisture . The dry matter content of the feed can vary between 22-25% for fermented products to 55% for high-carbohydrate products. The drying air inlet temperature is in the range of 160-180°C. The feed temperature for high carbohydrate products is 80°C which is one of the conditions to ensure long operation times without bacteriological problems.

8.6.

Caseinate powder

The most important caseinate product is sodium caseinate. The feed for the dryer is produced by dissolving, preferably freshly precipitated, casein curd in sodium hydroxide to obtain a neutral point around pH 7.0. The casein curd is prepared by precipitating skim milk by an acid (to bring the pH slightly below the isoelectric point, i.e. about 4.6), separating the whey and washing with water (usually twice depending on the desired purity). The acid used can be hydrochloric acid or lactic acid created by bacterial fermentation. The solids content of the feed for spray drying should be not much higher than 20% due to viscosity problems. Spray drying is conducted preferably using pressure nozzle atomization in order to get acceptable bulk density (wheel atomizer results in max 0.35 g/ml while pressure nozzles can give 0.5 g/ml). Bag filters are recommended to avoid unacceptable powder stack losses. The feed is spray dried using a feed temperature 90°C and inlet air temperature up to 250°C in conventional dryers but up to 320°C in a Multi Stage Dryer, MSD™. The outlet air temperature to get moisture content of 4-5% is about 90°C.

The manufacture of calcium caseinate is in principle the same as for sodium caseinate. However, for re-dispersion and dissolving calcium hydroxide is used. Also drying conditions are similar. The composition of both sodium and calcium caseinates is given in Table 8.5.

**Tablo. 8.5.** The percentage composition of sodium and calcium caseinate.
Component	Na-caseinate	Ca-caseinate
Protein Ash Lactose FatMoisture pH	85.5 4.5 4.01.5 4.5 6.5-7.2	86.2 3.8 3.5 1.5 5.0 6.6-7.5

Both sodium and calcium caseinate functions as a water binder, emulsifier, whipping agent and filler for food and meat products. It is used as a source of protein in dry cereal products, infant food, dietetic and diabetic products and it is also a useful component in coffee whiteners and toppings. In meat products such as sausages and other processed meat products, it improves the texture, binding the moisture and fat, while inhibiting shrinkage.

8.6.1.

Coffee whitener

The development of the so-called coffee whiteners or coffee creamers took place almost 40 years ago after experiencing that the milk powders, which were available at that time, when used in hot drinks like coffee and tea, flocculated creating an unpleasant appearance in the cup and sediment at the bottom. It was recognised that the main cause of these phenomena was flocculation of the whey proteins. The pH of coffee or tea is quite low, sometimes well below 5, which together with the temperature, in many circumstances almost 100°C, creates favourable conditions for denaturation of the whey proteins on the surface of the particles before acceptable dispersion and dissolving could take place. The logical consequence was to develop a product without any whey proteins but that will exhibit the whitening power and taste of milk. The protein component of coffee whiteners is sodium caseinate; the carbohydrates are represented mostly by malto-dextrin and the fat by a mixture of vegetable oils. Emulsifiers, stabilizers and colouring agents are also used. The composition of various types of coffee whiteners, as referred to in the literature, is shown on Table 8.6.

Today the industry master the technology for achieving excellent coffee stability of plain milk powders, and both instant skim milk powder and instant whole milk powder can be used for preparing coffee or tea with milk. However, the coffee whiteners were already well introduced on the market and as they are cheaper than milk powders they survive. The disadvantage of coffee whiteners compared to instant milk powders is their poor instant characteristics when the coffee is not that hot (i.e. below 50°C). Therefore further product development was directed to make a cold coffee instant powder, involving a lecithin treatment, conducted in the same way as for instant whole milk powder.

**Tablo. 8.6.**
Component
Corn syrup solids Fat (vegetable) Emulsifier (1) Stabilizer (2) Moisture Sodium caseinate Sucrose Flavour and colour Stabilizing salts (3)	46.0 46.0 5.0 1.0 1.1 - - traces 0.9	13.5 54.0 2.7 0.8 3.7 11.0 13.5 traces 0.8	55-60 35-40 0.3 - 0.5 - - 4.5 - 5.5 - traces 1.2 - 1.8	- 1.0 - 12.0 - - - - 8.0 - 25.0 traces 0.5 - 1.0	54.0 36.0 - - 2.5 5.0 - traces 2.5

(1) Glycerol-mono-stearate, polyoxystearate, sorbitol-mono-stearate, Atmos 150/Span 60/Tween 60/20/20

(2) Carrageenin, alginate, guar gum.

(3) Sodium citrate, sodium phosphate, dipotassiumphosphate.

Coffee whiteners have to be well agglomerated, however without the presence of too large an agglomerate size, which otherwise will create so called floaters, appearing on the surface of the coffee as small lumps. The feed for spray drying is prepared by blending the components. Due to high content of malto-dextrin, the solids content of the feed can be rather high i.e. 64- 67%. Any type of two stage dryers with fines recycling, and spray dryers with integrated fluid beds can be used. The inlet air temperatures are 180°C (for MSD™ 220°C).

8.6.2.

Cocoa-milk-sugar powder

This type of product is a typical household and vending machine product for quick preparation of both hot and cold chocolate drinks. The composition of the final product can vary to a wide extent as shown in table 8.7.

**Tablo. 8.7.** The percentage composition of cocoa-milk-sugar powder.
Component	%
Cocoa Skim milk solids	10-35 25-60

With low amounts of sugar, up to 20%, it is possible to prepare the whole blend with 45-50% total solids and dry it using inlet air temperature of 180°C. With higher amounts of sugar, it is necessary to add a part of the sugar and possibly also cocoa powder in dry form through the fines return system to achieve agglomeration. For use in vending machines the product must possess good free-flowing properties. Most plants with fines return system are suitable. Most recommendable is the Multi Stage Dryer, MSD™.

8.6.3.

Cheese powder

Feed preparation for spray drying is similar to that of processed cheese. Good ripened cheese is first crushed into small pieces and agitated in a jacketed vat with water to obtain slurry of about 40% dry matter. A solution of stabilizing salts i.e. sodium citrate and disodium phosphate is added under vigorous agitation and the mix is gradually heated up to 80°C. Before spray drying the mix is submitted to a two stage homogenization. There may appear viscosity problems requiring adjustment of the total solids content. For spray drying, any type of spray dryer with cooling bed can be used with inlet air temperature 180°C. Cheese powder is a difficult product to dry as it has a tendency to deposit. Therefore chamber types with low possibility to form deposits, i.e. TALL FORM DRYERS™, are preferred. Another problem is the possible unpleasant smell of the exhaust air, requiring treatment by absorbing filters or bioscrubbers, especially if the factory is placed in a populated area. As there is a great variety of cheese types, the technology involved in feed preparation, drying and product composition can vary. However, cheese powder has roughly 50% fat, 40% protein, 3% carbohydrates, 4% minerals and 3% moisture.

Cheese powder is used as an industrial ingredient in cheese biscuits, dressings and dipmixtures. As a consumer product, it serves the same purpose as grated parmesan cheese.

8.6.4.

Butter powder

The product which is called butter powder contains about 80% milk fat, i.e. almost the same content as in normal table butter. Obviously, this product is difficult to dry and handle. Sodium caseinate, non-fat milk solids, emulsifiers and stabilizing salts are used for preparing the mix for spray drying. The final moisture of the product is below 1%. To enable easier further processing, a free-flowing agent is added. The product is used exclusively in bakeries as a source of milk fat in dry form for making croissants. The Multi Stage Dryer, MSD™ and the FILTERMAT™ dryer are the types of dryers which have proved most successful for drying butter powder.

Okumaya devam etmek için lütfen kaydolun

Handbook of Milk Powder Manufacture

Hazırlayan Dr. Ing. Jan Pisecky

Handbook of Milk Powder Manufacture cover

The Handbook of Milk Powder Manufacture is a valuable reference book for dairy processing engineers wanting to take a deeper look at the complex world of milk powder processing.

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