Lesson 42. DEFECTS IN DRIED MILK DURING MANUFACTURE AND STORAGE, THEIR CAUSES AND PREVENTION-I
DEFECTS IN DRIED MILK DURING MANUFACTURE AND STORAGE, THEIR CAUSES AND PREVENTION-I
Shelf-life of milk powder is dependent on quality of milk used, method of manufacture, composition of final product, efficiency of packaging and temperature of storage. Hygienic quality of the final product is very important from consumer’s safety point of view. Powder must be free from all the pathogens including those which can cause food poisoning. The physico-chemical changes that occur on storage of dried milks determine the storage stability and the nutritional characteristics and are of paramount importance from economical, commercial and functional view points.
Micro-organisms do not proliferate in dried milk; storage defects are purely chemical in nature. These tend to develop slowly, and normally the product may be stored without deterioration for many months. The useful storage life is much influenced by the temperature of storage and the difficulties are more pronounced in hot climates, where the product is potentially of greatest value.
42.2 The Physico-Chemical Changes During Storage of Milk Powders
It has been observed that the life of roller dried whole milk powder from cow's milk is about 10 months; the spray dried product however cannot be stored more than 6 months at 17°C and half of this time at 27°C.
Although the milk powders are not sterile products, the low water activity does not permit microbial growth and microbial activity. The same is true of enzymic activity. The changes occurring in milk powders on storage are of physical, chemical and physicochemical in nature. Both the lipids and the non-lipid constituents of milk undergo such changes and normally the rates of such reactions are accelerated with the elevation of storage temperature and levels of moisture and oxygen in the product.
The quality of milk and the processing parameters adopted while manufacture of milk powders are also the major factors determining their storage stability.
42.2.1 Water activity (aw)
The most important variable determining the rate of undesirable changes in milk powder is the water content . When comparing different types of powder, it is probably easiest to consider water activity ( a w ). The relationship depends on the composition of the product.
- The higher aw of whole milk powder as compared to skim milk powder of the same water content is caused by the fat not affecting aw .
- Whey powder has aw slightly different from that of skim milk powder because in a dry product the soluble constituents (especially sugar and salts) decrease a w somewhat less than casein. This is only true, however, as long as all lactose is amorphous, which often does not apply to whey powder.
- The aw is considerably reduced if lactose crystallizes without absorption of water by the powder, at least if aw is less than about 0.5.
- Crystalline lactose binds water very strongly, and that is also why the usual oven-drying methods to estimate the water content do not include the bulk of the water of crystallization. If the water content excluding the water of crystallization is taken as a basis, then a w is even higher for the powder with crystallized lactose.
- It is thus advisable to make milk powder sufficiently dry and to keep it in that condition. If it is not hermetically sealed from the outside air, it will attract water in most climates. The higher the temperature, the higher the water activity. Because several reactions are faster at a higher aw , this implies that a temperature increase may well cause an extra acceleration of deterioration.
- The deterioration effect may be especially strong if the powder loses its glassy state. A lactose–water mixture will be at most ambient temperatures in the glassy state if its water activity is below 0.3. Because lactose is the dominant component of the amorphous material in a powder particle, about the same relation is supposed to hold for the powder. This means that most dairy powders are in the glassy state (i.e., the nonfat part of the material), except if the water content is high and the temperature is also high. A change in conditions leading to a glass–liquid transition will strongly accelerate most reactions and physical changes occurring in a powder.
42.2.2 Colour, flavor and off flavor characteristics
The user of dried milk is concerned mainly with the flavour defects which arise during storage. Following aspects are important which affect the colour and flavour of dry milks:
- The use of poor quality milk is not desirable for manufacture of good quality dry milk.
- Some feed taints may be removed during the course of vacuum evaporation or drying, but not always.
- Acidity already developed in the milk will be retained in the powder, which acquires an unpleasant flavour.
- Neutralization of acid milk has been practiced, but the flavour of the resulting powder remains unsatisfactory and the colour may darken. Very acid milk cannot be dried by the roller process. First quality powder should not exceed an acidity of 0.15 (as lactic acid), and no powder should exceed 0.17%.
- Assuming that good quality milk has been used, the most likely defect of freshly made powder is some degree of burnt flavour. A distinct burnt flavour is only likely to occur in spray powder following considerable overheating in the drying chamber, but may arise much more readily in roller powder owing to overheating on the rollers or faulty adjustment of the scraper knives. In any case, roller powders always possess a distinct cooked flavour. The flavour of spray powder is influenced considerably by the milk preheating temperature used. Thus "low-heat" powder or powder from "ultra-high temperature" processed milk gives reconstituted milk having a flavour close to that of pasteurized milk. "High-heat" powder when reconstituted possesses a more cooked flavour, which however, is not unpleasant and indeed is preferred by some consumers. In practice, storage changes produce various flavour defects which mask other factors and after 2-3 months the "low-heat" powder is commonly inferior if special precautions have not been taken.
- The colour and flavor changes will progress during prolonged storage if conditions like elevated temperature in combination with increased moisture content prevail. Although much research remains to be done, two major types of deterioration are well recognized, affecting fat in one case and protein plus lactose in the other. Ultimately these affect the flavour, solubility, colour and nutritional value of the powder. Two major categories of reactions are:
(a) The oxidation of lipids and
(b) The non enzymatic browning commonly known as the Millard reaction.
The former is associated with the tallowiness whereas presumably both these reactions contribute to stale and allied flavours. With the advancement of storage, the discolouration will also progress and fluorescence substances are also produced.
42.2.3 Fat decomposition
Oxidation of the fat leading to the production of flat, and finally marked tallowy off-flavours is a major storage defect of full cream powder and some times occurs to a lesser extent in the residual fat content of separated milk powder. Some of the observations made by different workers are
- The chemical changes result from the addition of oxygen to the double bonds of unsaturated glycerides, giving peroxides which can be estimated by chemical means.
- In the oxidation of linoleic acid, the C-11 methylene group is preferential point of attack. Many other volatile aliphatic compounds such as alcohols, lactones, esters, acids and hydrocarbons arise from oxidation of unsaturated fatty acids.
- At first, the peroxides accumulate but later they decompose to various aldehydes, ketones and acids which impart the unpleasant flavours.
- Aldehydes are detectable with colour reagents but the sense of taste is much more sensitive than chemical tests.
- The reaction commences with a slow linear induction phase, followed by a period of rapid exponential change and combinationwith oxygen, large quantity of O2 can be absorbed. The break point at which the oxidation changes from first to second phase is generally found to be at 37 weeks in whole milk powder.
- Peroxides are formed when the container contains an excess of free O2, but later decomposition of peroxides exceeds the rate of their formation.
- The keeping quality of individual "low heat" powders varies considerably - from 4-12 months in temperate climates and perhaps half this time in hot climates.
- Though the development of poor flavour is parallel to O2 absorption, the ratio is not constant and the most stable powders require the most O2 in order to deteriorate to a given flavour value.
- The odour and flavour profiles of dried milk with addition of antioxidants (BHA/BHT & Maillard reaction mixtures from histidine and glucose) and stored under air and nitrogen are as follows
- The Maillard reaction products are as effective as BHA/BHT in retarding the development of oxidative off flavours and off odours.
- Samples to which antioxidants are added or samples stored under nitrogen retain a higher acceptance for storage times as long as 84 weeks.
- The 2, 3 butadione has a sweet and buttery odour and flavour at low concentrations and the branched chain aldehydes, the Strecker aldehydes are generally described as having fruity odours and flavours at low concentrations.
- The sulphur compounds are known to impart cooked odours and flavours.
- Some compounds, 1-octen-3-one and 1-octene-3-ol that are known to give rise to metallic and mushroom odour and flavour respectively in oxidatively deteriorated milk products.
- Factors affecting oxidative stability of lipids in dried milks include oxygen, moisture, temperature, light and metals like copper and iron. These influence the system depending upon their fat content, presence or absence of antioxidant and packaging under the nitrogen atmosphere.
- The use of antioxidants instead of nitrogen gas packaging, derived from various materials showing promise as practical antioxidants are: wheat germ oil, gum guaiac, rice bran concentrate mixed tocopherols, oat flour and special preparations made from ethyl gallate, ascorbic acid, nordihydroguaiaretic acid (NDGA), thiourea, hydroquinone monobenzyl ether, flavones, querectin, ascorbyl, palmitate, histide, tryptophan, phosphoric acid either alone or in combination of a few.
- For addition of any antioxidant legal acceptance is must and for the present in India, no antioxidant other than lecithin, ascorbic acid and tocopherol is permitted accepting BHA not more than 0.01% by weight in milk powder. In infant milk no antioxidant is permitted.
- Inspite of some fairly promising indications that antioxidants retard tallowiness in dry milks, packaging in inert gas is the most efficient method at present available for preventing fat deterioration.
True rancidity results from the hydrolysis of fat to free fatty acids such as butyric acid. The lipolytic enzymes involved can be derived from the milk itself or they may be of microbiological origin. This defect has become unusual since strict attention is now given to both milk production and plant hygiene and higher preheating temperatures which inactivate lipases.
42.2.5 Other Forms of fat deterioration and their prevention
The other flavours are described as "stale" or most characteristically as like "Coconut". A ‘toffee” flavour may also be a form of the same defect. They are produced only from milk fat and do not occur in dried separated milk. The defect is accelerated by high-temperature storage and by high moisture content, but is independent of O2 and occurs in gas packed powder. This form of deterioration seems to occur at a fairly early stage during storage, it develops before the fat oxidation defect from which it also differs in flavour. There is strong evidence that the change is caused by a rearrangement of monoethenoid fatty acids in milk fat to form lactones and that δ-decalactone in minute quantities is the substance responsible for the coconut flavour. The origin of this lactone is thought to be 9-deconoic acid. These rearrangements involve no O2 and the empirical formulae remain the same. No definite method of prevention has been suggested except low-temperature storage.
In the absence of special precautions, oxidation occurs rather more rapidly in spray than in roller powders, but in recent years much progress has been made in extending the life of spray powders. With full-cream powder the main cause of deterioration is overcome if fat oxidation is prevented, other possible defects being within the control of manufacturer. Hence, to keep the autoxidation within reasonable limits for a long time, following measures should be taken:
1. High Temperature Preheating: The milk should be intensely heated to form antioxidants. The problem is, of course, that the heat treatment also causes a distinct cooked flavor. Compared with pre-heating the milk at 73.8°C, preheating at 82.2°C improves the keeping quality of the powder, whilst at 87.8°C, the increase is 3-5 times. The mechanism is not fully understood, but in general it is associated with the formation of trace of protein decomposition products containing a sulfhydryl group which function as antioxidants and prolong the induction period. Natural milk proteins contain no free -SH groups, but in the denatured protein formed in heated milk, such groups become unmasked and reactive. The first action may be the formation of cysteine followed by further reaction at the exposed sulfur ends of the chains to split off H2S or methyl mercaptan. Traces of H2S are detectable in the heated milk. The action of -SH group is uncertain, but may be due in part to prevention of the catalytic action of heavy metals by removing them as sulfides.
2. High temperature preheating may also prevent changes produced by enzymes derived from the raw milk or from bacteria, thus improves the bacteriological quality of the powder and preserves the vitamin content during storage.
3. The rate of autoxidation strongly increases with decreasing a w ; however, to prevent other types of deterioration (especially Maillard reactions) a w should be as low as possible. The effective Q 10 of the autoxidation reaction in milk powder is relatively low (about 1.5) because a higher temperature also causes higher a w . The most suitable water content is generally 2.5 to 3%.
4. Packing in Inert Gas: This depends upon prevention of the chemical reaction by removing the O2 from the container and replacing it with N2 or CO2 usually the former. It is the only method which completely prevents oxidation for practical purposes and can extend storage life up to 7-10 years even at high temperature. Oxygen should be removed as effectively as possible by gas flushing. A problem is that the vacuoles in the powder particles contain some air, hence, O2. Either the powder should contain hardly any vacuoles or the gas flushing should be repeated after a few days. Equilibrating the gas inside and outside the vacuoles by diffusion takes several days in whole milk powder (several weeks in skim milk powder). Equilibration is faster if the powder particles have a greater number of cracks.
5. Addition of Antioxidants: Of the more successful substances “Ascorbic acid” added @0.03% in the liquid milk prolongs powder keeping quality by several months but disappears during storage. The most effective antioxidants appear to be esters of gallic acid particularly Ethyl or Propyl gallate. The incorporation of as little as 0.07% of ethyl gallate in the powder prolongs the keeping quality to about two years when only low temperature preheating has been used. Ethyl gallate is non-toxic and has no effect on flavour; it does not disappear during storage and appears to remain unchanged. Keeping quality may be extended still further by combining high-temperature preheating with the addition of ethyl gallate. In this way, it is possible to produce powders having a keeping quality of from 3-4 years in temperate climates. Nordihydroguaiaretic acid has also been found effective when added @ 0.04% of the weight of the fat.
6. The rate of deterioration increases rapidly with rise of storage temperature. The keeping quality of an unstable powder may fall as low as 6 weeks in a tropical climate.
7. Oxidation is more rapid in Powder of high acidity.
8. The powder should be packaged in such a way that air and light are kept out. Generally, this implies packaging in cans.
9. Rigorous measures should be employed against contamination of the milk with heavy metals such as copper and iron which act as catalysts and shorten the induction phase. Stainless steel equipment should be used.
10. Other less certain factors are the possible presence of oxidizing enzymes of microbiological origin in the milk and seasonal variations in the glycerides-composition of the fat.
11. Intensive homogenization of the concentrate should be carried out.
42.2.6 Protein - Lactose / Maillard Reactions
Millard reactions between protein and lactose produce a series of storage defects which often occur together in a progressive sequence of stale flavours, increasing loss of solubility, unpleasant "glue" flavour and darkening or brown discoloration. These changes may occur in all types of powder but are associated predominantly with separated dried milk and tend to be most pronounced in roller dried powders. They are greatly accelerated by high-storage temperatures and in addition, the moisture content of the powder is a major factor. Critical moisture levels appear to be about 5% for spray powder and 4% for roller powder. Above these figures, the defects are accentuated.
This reaction in addition to accounting for the major part of browning, the protein-carbohydrate complex or its decomposition products also result in production of reducing substances, fluorescent compounds and disagreeable flavour materials. About 80 different compounds have been isolated and identified which include furanics, lactones, pyrazines, pyridines, acetyl pyrrole, amides, pyrolidinone, succinate, glutarimides, carboxylic acids, acetone, 2-heptanone, maltol etc.
Some of the findings in this regard are
- Maillard reactions increase considerably with water content and with temperature. They lead to browning and to an off flavor. The ‘gluey’ flavor that always develops during storage of dry milk products with too-high water content is usually ascribed to Maillard reactions; the main component appears to be o -aminoacetophenone.
- If extensive Maillard reactions occur, they are always accompanied by insolubilization of the protein. Accordingly, the insolubility index increases when milk powder is stored for long at a high water content and temperature.
- One of the first reaction products detected in Maillard reaction is hydroxymethyl furfural (HMF), the concentration of which increases with storage time, temperature and moisture content. Many of the products of Maillard reaction are aroma substances (aldehydes, reductones, other furfurals etc.) which have an appetizing odour, but high levels of these compounds may cause considerable odours and off flavours. The ε-amino groups of lysine are mainly involved and because the compounds formed during the reaction such as fructoselysine, furosine etc. are resistant to enzymes, the content of available lysine is reduced, but usually only to a small extent. Another result of the reaction is the production of substances which lead to colour changes, but only in a more advanced stage of the reaction.
- Browning reactions of the Maillard type occur if the storage of powdered dairy products at ambient temperature and having the moisture content ≥ 5%. The HMF content is further increased in milk powder with added iron or added Vitamin A.
- With the progress of Maillard reaction, the decrease of amino-N is closely related to increase in combined sugar, increase in oxygen absorption and carbon dioxide production, increase in colour and reducing power and decrease in solubility .
- A number of compounds have been shown to inhibit the browning reaction. In milk products, active -SH groups serve as natural inhibitors in retarding the heat induced browning, but the mechanism is not understood. Sodium bisulphite, sulphur dioxide and fo rmaldehyde also inhibit browning in milk system as well as in simpler amino acid-sugar solutions.
42.2.7 Deterioration of a high moisture powder proceeds in a series of stages
- First the lactose absorbs moisture and changes to crystalline L -lactose monohydrate containing 5% water of crystallization.
- In powders of low moisture content, the lactose is present in the amorphous α and β-forms (1: 1.5) with only 0.5 to 5.0% of lactose hydrate. If the moisture content and the temperature remain low, the lactose preserves its amorphous form and no deterioration occurs. If the moisture content is high, as much as 40-60% of the lactose may be hydrated in a fresh powder and at levels of 6.5 to 7.0% for full-cream powder and 7.5 to 8.0% for separated milk powder crystallization becomes rapid and complete (dried milk can also absorb moisture very readily from the air). Such powders may show an apparent decrease in their moisture content. There is often a loss of free-flowing properties and possibly some definite "Caking" of the powder. The impervious lactose envelope around spray powder particles probably changes to a crystalline lattice.
- The free lactose content of the powder then falls and an insoluble protein-lactose compound is formed which contains lactose equivalent to the quantity of the free soluble lactose which has disappeared. The reaction is presumed to occur between the aldehyde group of the lactose and a protein-amino group. The amino group mainly involved is that of lysine, and about 40% of the original lysine disappears, which considerably reduces the nutritional value of the powder. As the protein - lactose reaction proceeds, the remaining protein also becomes progressively insoluble and eventually the loss of solubility may be pronounced, particularly of roller -dried milk.
- The final stage involves a decomposition of the protein - lactose compound to form products which include substances having an unpleasant glue-like flavour and a brown colour. The reaction involves absorption of O2, and the evolution of CO2, but the exact nature of the breakdown and the identification of the decomposition products are uncertain. In general, the O2 reacts preferentially with the fat in full-cream powders, and tallowy flavours predominates, whilst the stale and gluey flavours are more characteristic of separated milk powder, but both forms of deterioration can proceed simultaneously.
- If the powder is gas packed, the protein lactose reaction and loss of solubility may still occur, but the final changes are much less serious. There may be some development of brown colour and a slight caramalize flavour, but unpleasant flavours are unusual. It would appear that development of the gluey flavour requires a supply of O2. Gas packing is therefore partially effective but is not a real solution.