There is now a legal standard of 5% moisture in the U.K., and this must be carefully controlled as otherwise the quality will deteriorate rapidly. Satisfac-tory limits are2-5-3-5 %. A moisture content of over 4 % indicates a bad product.
A very low moisture content (less than 1 %) may accelerate chemical oxidation but this effect does not operate in practice.
The legal standard of 26 % fat for full cream powders in the U.K. corres-ponds to about 3-3 % fat in the milk. Milk for powder is always standardized by adding the required amount of skim milk; only freak conditions would require the use of cream for this purpose. In the U.K. The Dried Milk Regula-tions 1965!oi (S.I. No. 857) lay down the standards given in Table 32.
Έ. Technical Control L General
Technological factors controlling quality are summarized in Tables 33 and 34 and quality control methods in Table 35.
TABLE 33. Milk powder (roller dried)
Poor solubility, dark colour, burnt particles
High moisture content Lo\V|fat content
Defects and causes Bad condition of knives
Bad setting of knives Excessive steam pressure
Excessive concentra-tion of milk Failure to adjust pH of milk
Low steam pressure Faulty standardization
General references 413 452
453
Fat and moisture tests are routinely applied to milk powders, as to most dairy products. Ether extraction methods such as the Röse-Gottlieb, Schmid, etc. are generally used for the fat. Infra-red oven methods are now often used for the moisture, especially for rapid control purposes.458 These give a result in about 5 min.
From the nutritional point of view, the most important feature of milk powder is its protein content; skim milk powder is as valuable a food as any, especially in regions of under-nourishment (cf. 51, 52, 459).
The Amido black 10 B methods for protein have been applied to milk, milk powder and condensed milk.460
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2. The Solubility Index of Milk Powders
This property can be simply defined as the percentage of the dry matter in a powder which can "dissolve" under defined conditions. It is intimately related to the intensity of heat-treatment and so to the method of drying, including preheating. Many methods have been described over the past 40 years. Elaborate methods are used for investigational purposes, but in
TABLE 34. Milk powder (spray dried) Low solubility
High moisture Poor flavour
Browning Low fat content
Excessive heat treatment 413 in drying—faulty
temperature control
Inadequate cooling 452 Inadequate heat 415 treatment
High count milk 454 Stale milk 455 Growth of Streptococcus
thermophilus in hot well
456, 457 Faulty standardization
TABLE 35. Milk powder: Quality control methods Flavour
Colour Solubility
Chemical Bacteriological Biochemical
Odour and taste (reconstituted) Visual
Gravimetric or volumetric method ; examination of sediment
Nitrogen distribution Fat* and moisture Peroxide value Colony count Coliforms Microscopic count Peroxidase test
* Test of legal significance.
industry simple and rapid methods are commonly employed. Spray dried powders have a high solubility (about 99-9 %) and so a gravimetric method is necessary for accuracy, Roller-dried powders have relatively low solubility values (about 85%) and a centrifugal volumetric method is commonly used.
See published discussions and descriptions of methods.46*-4«^
The solubility of roller-dried powders can be increased by neutralization of the milk with soda ash (Na2C03) to an acidity of about 0-11 % (normal milk 0-14%). Over-neutralization leads to a flat, soapy taste. A review has been given of the use of skim milk powder in the bakery industries466 and a standard has now been issued for spray-dried skim milk powder for use in canteens.467
3. Reconstitution
The ease with which a powder can be reconstituted is of major importance and Radema and Dijk46^ have described a method for determining the reconstitution index. The dispersibility and ease of reconstitution of milk powders have been reviewed by King.469 Reconstitution with ordinary tap water at 24°C has little effect on flavour or bacterial counts.470
4. Free-flowing Properties
These are important aspects for dispensing and manipulating aspects, particularly for vending and preparation of milk drinks.471 Milk powders vary considerably in flowing properties, the order being for spray-dried powders—full cream, skim, instant skim. It is possible to improve this pro-perty by incorporation of a "free-flowing agent", but legal considerations may be involved. Substances used for this purpose include calcium phosphate and silicate, and sodium aluminium silicate. For methods of measurement see Sjollema.472
5. Bulk Density
This is simply the density of the powder in bulk and is a measure of the air incorporated in it or the porosity.
V— V P = —y^ x 100
where P is the porosity, Fis the bulk volume and Vp is the particle volume.472 F. "Instant" Milk Powders
These powders dissolve (skim in cold and full cream in warm water) in a few seconds without vigorous agitation. The original method involved rewetting or humidifying up to about 15% moisture and the redrying down to about 5 %. This treatment changes the structure of the powder51» 52,413 to a pumice-stone type of conglomerate having a honey comb or capillary structure which exerts a "blotting paper" effect. Instantizers are usually attached to the drier and may involve three stages. This type of powder has a low bulk density (e.g. 0-35 instead of 0-7).
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Attempts have been made to prepare "instant" powders by a straight-through process, but this type is usually not so "instant", and has a bulk density only slightly less than that of normal powder. All instant powders are relatively free-flowing and have less "dust" due to the conglomerating effect.
The "instant property" is controlled by "wettability" (establishment of physical contact between water and powder), dispeisibility (penetration of water into conglomerate and disintegration of the particle) and solubility (change from suspension to colloidal suspension or "emulsion"). The final solubility is not necessarily higher than that of the normal powder. Instant-izing naturally costs money, and the economics of the process depend entirely on the method of use by the food manufacturer or consumer and are not related to the quality of the final product. If the powder is in any case mixed with other ingredients in a mechanical mixer for some time, there may not be justification for the extra cost of an instant powder, although the absence of dust is always an advantage. Storage of instant powders for more than 2 or 3 months is inadvisable because of their shorter keeping quality due to their higher moisture content.
The rate of solution of a good instant powder is so rapid that there is no point in measuring it.
Schulz455 gives a full, well-illustrated discussion of methods for making instantly wetable and soluble dried milk products. Methods have been given of examining instant powders.473»474 Electron microscope studies can give more detailed information on the structure of milk powders.475