Lesson 43. DEFECTS IN DRIED MILK DURING MANUFACTURE AND STORAGE, THEIR CAUSES AND PREVENTION-II
DEFECTS IN DRIED MILK DURING MANUFACTURE AND STORAGE, THEIR CAUSES AND PREVENTION-II
Apart from normal defects observed in dried milks, the defects related to reconstitution and nutritional properties of dried milks are also important.
43.2.1 Roller dried powder
Small errors of operation during roller-drying can readily result in dried milk having a darker shade of colour. More serious failures or acid milk may produce a distinctly brown product. Discolouration often associated with a heated flavour, results from overheating due to excessive time of contact between the milk and the hot roller surfaces, too high a steam temperature / pressure or prolonged heating of partly concentrated milk between the upper surfaces of the rollers.
A more common defect is the presence of brown scorched particles, sometimes sufficiently numerous to impart a visible speckled appearance. Even high quality roller powder usually contains a few specks of scorched material. The major cause is incomplete removal of the dried milk film from points on the rollers. Any milk solids which pass the scraper knives remain on the hot roller surface and eventually become burnt. Frequent grinding and sharpening of the knives, accurate alignment across the rollers, and careful adjustment of knife pressures at all points is one of the most essential and skilled operation in the production of roller powder.
43.2.2 Spray dried powder
A general brown colour and burnt flavour is not associated with spray powder unless serious overheating has occurred in the drier. Particles of foreign matter may, however, be present and although they are not usually noticeable in the powder, they may be clearly apparent as sediment in the reconstituted milk. Such sediment may be of several kinds, and is best collected by filtering the reconstituted milk through a cotton pad; the specks may then be examined with a lens. Inadequate filtration or clarification of the milk supply may result in the presence of typical foreign matter from the raw milk.
The air supply to the drier should be filtered to prevent the admission of out side dust and particular care should be taken to avoid the entry of minute particles of coal or ashes when coal fired boilers are used. The air supply is some times drawn from inside the building, because of its higher temperature, but this involves the risk of introducing powder dust which may become charred when heated.
However, scorched particles arise mainly from powder deposits which have been held for long periods in ducts or at other points in the drier or from drying chamber due to inadequate mixing and directional control of the milk and air streams. The moist powder sticks to the hot metal surfaces and eventually becomes brown or even black. Subsequently large pieces of discoloured material may become detached and mix with the powder bulk. Some drying chambers are provided with vibrators or internal rotating mechanical devices to prevent such accumulations.
Dried milk absorbs moisture very readily from atmosphere, and if packed in materials which are not impermeable to moisture such as plain paper sacks, cardboard cartons, etc., it may develop large hard lumps or severe caking during storage. High quality powder may be ruined by poor packaging. Low initial moisture content is also highly desirable. With modern properly managed plant, lumpy powder is more likely to arise from storage defects rather than from faulty manufacturing processes.
Crystallization of lactose is responsible for caking, as it causes the powder particles, largely consisting of lactose, to grow together (to sinter). Because water is needed for crystallization of α -lactose, caking does not occur at low a w , say, below 0.4. At a higher temperature, crystallization can occur far more readily, a w being higher; moreover, the viscosity of the highly concentrated lactose solution (essentially the continuous phase of the powder particles) is lower, causing nucleation, hence crystallization, to be faster. Lactose crystallization also leads to protein insolubility, de-emulsification of fat and accelerated flavour deterioration.
The susceptibility to caking, especially high in whey powder, is considerably reduced if most of the lactose is crystallized before the drying (in the concentrate). Such precrystallized powder is usually called ‘nonhygroscopic,’ which may be a misnomer because the powder concerned does not attract less water (this is determined by its a w in relation to that of the air), but the consequences are less noticeable.
To avoid caking, it is necessary that suitable packaging materials providing barrier to passage of moisture are selected and the product is stored at low humidity levels.
43.2.4 Bulk density
It has been observed that on storage, density of milk powders increases suggesting that on storage, the proportion of larger particles increase in the product.
43.3 Properties Related to Reconstitution of Dry Milks
The major defects in liquid milks prepared from milk powder are lumps, insoluble sediment, floating foam, charred particles, extraneous dirt and churned fat.
43.3.1 Insolubility index and white flecks
The insolubles in milk powders are a lesser problem than poor reconstitution, although in practice the two terms are sometimes taken as synonymous. The term "insolubility index" instead of "solubility index" was introduced when the IDF modification of the original ADMI method was issued. Insolubility index is often considered as one of the most important quality criteria. There are two types of insolubility:
- The one caused by heating powder in the dry state, which decreases with increasing water temperature, as shown in Fig. 43.1 and other caused by heating in the liquid (or concentrate) state, which does not improve with increasing water temperature.
It is a measure of the extent of denaturation of the proteins in milk powders.
- It is determined as the volume of the insoluble sediment after dissolving and centrifuging according to a prescribed procedure. The analysis of insolubles shows these to be a protein/fat/mineral complex, the fat probably being held by relatively weak chemical bonds. The protein itself is casein and/or denatu¬red whey proteins.
- The rate of insolubilization is dependent on both the moisture content and the storage temperature of the milk powders .
- The insolubilization can be inhibited for all practical conditions of storage by maintaining the moisture level below 3%.
- Normally beyond certain period of storage at elevated temperature viz. 40°C, the solubility index increases and such effect is much pronounced in high-heat powders.
- At a given temperature, the rate or denaturation of protein in concentrated milk doubles for every 5% increase in total solids upto about 92%. Most of the denaturation during the drying process occurs when the concentration of total solids is greater than 50%. The effect of concentrate total solids on insolubility is given in Table 43.2 .
The occurrence of white flecks is of similar origin to insoluble sediment. Unlike insolubility, this defect can be detected visually but is difficult to determine quantitatively
43.3.2 pH / Acidity on reconstitution / recombination
This has influence on the chemical and functional characteristics of milk powders such as heat stability, viscosity and rennetability. The decrease in pH and increase in acidity could be attributed to the tying of amino groups of the protein in the Maillard reaction during storage of dried milks. Keeping aside the type of dried milk, higher the temperature of storage, higher the decline in pH of milk powders.
43.3.3 Viscosity on reconstitution
Storage conditions obviously exert an influence on the viscosity. Pre-heat treatment and the temperature of storage affect the viscosity. Low heat powders show faster and higher rise of viscosity at higher temperature of storage due to decrease in pH of the milk powders during storage.
43.3.4 Free fat
The amount of free fat in dried milks varies appreciably depending upon the manufacturing method and storage conditions. It is lowest in spray dried powders and highest in roller dried ones. The figures for free fat as % of the total fat have been reported in the literature ranging from 1-20% in spray dried powder and in roller dried powders 91.6 - 95.8%.
Storage conditions may be important for the free fat content of milk powders. If lactose crystallizes, due to moisture absorption from surroundings, the free fat content increases sharply. Higher temperature of storage has also the similar effect. A steep increase in the free fat of spray dried whole milk at an average moisture content of 5.3% is observed.
The presence of larger quantities of free fat tend to increase the susceptibility of fat oxidation and thus to decrease in keeping quality of dried milk. During storage of spray dried whole milk, the free fat on the particles surface is oxidized first, while the interior fat is protected by the dense mass of SNF. The spray dried whole milk, varying considerably in free fat content, is found to have no effect on the development of oxidation flavour during storage at 30° C in the presence of air. The degree of oxidation of free fat is 5 to 8 times greater than that of the remaining fat in the dried milks containing high levels of polyunsaturated fatty acids. Contradictory reports are available regarding the faster oxidative deterioration of the product containing higher free fat.
43.3.5 Reducing capacity
On storage, the acid ferricyanide reducing capacity of milk powders increases and this results from interaction of lactose with protein (Maillard reaction) and probably also from decomposition of lactose catalysed by the buffer salts of milk. The degree of preheat treatment of milk prior to evaporation and drying has an influence on the ferricyanide reducing values of final powder. On storage the capacity of dried milk powders to reduce acid ferricyanide increases due to enhanced interaction between lactose and proteins.
43.3.6 Production of water and carbon dioxide
On storage of dried milks, both carbon dioxide and water are produced as the Maillard reaction products. Carbon dioxide is produced from non lipid constituents of milk at a rate which depend upon the moisture and temperature of storage. Oxygen accelerates the rate of production of carbon dioxide. Moisture of powders increases during storage under conditions which permit that reaction to occur.
43.3.7 Heat stability on reconstitution
There is a general decline in heat stability and such decrease is higher in powders held at higher temperatures of storage and time. This could be attributed to decrease in pH of the powders during storage and associated marked increase in solubility index, either or both of which may contribute to a reduction in heat stability. It is known that a change of 0.05 of a pH unit may alter heat stability times substantially.
43.3.8 Rennet coagulation time and curd tension on reconstitution
Reconstituted low-heat powders has an initial lower coagulation time as compared with medium-heat powder but the reconstituted high-heat powder do not coagulate when held for an extended period. The coagulation time of low-heat sample increase on storage period and by higher storage temperatures.
The curd tension of the reconstituted low-heat powder falls within medium firm range. The medium heat powders give a softer curd, the curd tension of such powders decrease with storage time being adversely influenced by higher storage temperature.
Both rennet coagulation time and curd tension are influenced by decrease in pH and affected by change in calcium sensitivity during storage of milk powders particularly at higher temperatures. It is known that the calcium and magnesium ion concentrations of reconstituted spray dried milk powders are somewhat lower than the levels found in raw milk and it is also established that there is decrease in the sensitivity of casein to calcium ions in case of milk powders stored in excess of 4°C.
43.4 Loss of Nutritional Value
Loss of nutritive value during storage primarily concerns loss of available lysine due to Maillard reactions. Storage at 20°C at normal water content does not cause an appreciable loss; at 30°C, a loss of 12% after storing for 3 years has been reported. Extensive Maillard reactions cause a decrease in protein digestibility and formation of weak mutagens.
Extensive autoxidation results in formation of reaction products between hydroperoxides and amino acid residues and between carbonyl compounds and ε -amino groups; this may cause the biological value of the protein to decrease slightly. Of greater concern is the loss of vitamin A in vitamin-fortified skim milk powder, due to its oxidation. This especially occurs if the vitamin preparation is dissolved in oil and then emulsified into the skim milk before atomization. Usually, dry added preparations are more stable. It is, however, very difficult to homogeneously distribute a minute amount of a powder throughout a bulk mass. Some of the important nutritional characteristics of dried milks as affected on storage are:
43.4.1 Quality of milk proteins
Non fat dry milk (3.3% moisture) when stored at 25 and 37°C for one year it is observed that nitrogen digestibility, protein efficiency ratio (PER) and net protein utilization (NPU) are not affected. The proteolytic digestibility of proteins is significantly reduced in skim milk powders stored at 40 ° C and 0.80 aw for one month.
43.4.2 Available Lysine
- Nonfat dry milk powders stored at 25 and 37°C for one year when incorporated into diets limiting in lysine indicate that relative nutritive value decreases by 8% in samples at 25°C and by 8, 9 and 8% in samples stored for 3, 6 and 12 months at 37°C respectively. Chemically available lysine also shows similar small decreases in samples stored at 37°C .
- The major cause of deterioration in milk powder is the reaction between the free amino groups and the reducing sugars. Powders with less than 5% moisture show no decrease in biological value after storage at 28.5°C for 27 months suggesting that moisture content plays an important role in this reaction.
- Storage of skim milk powder samples at 40°C and at low water activity or under dry conditions show no loss of available lysine. Storage at 40°C at 0.57 and 0.86 aww for one month; the loss of tryptophan and arginine as being about 10%, but others such as proline and tyrosine decrease only slightly. considerably reduce the amount of methionine by 6 and 19% respectively. Losses of other essential amino acids are observed only after storage at 40°C and 0.80 a
43.4.3 Vitamin Content
The effect on vitamins stability at various storage conditions reported in literature are as under:
- Instantized nonfat dry milk stored for 20 years showed vitamins decrease to varying degree with thiamine showing the greatest loss (6%), riboflavin decreased by 50% whereas only a 2% loss occurred for niacin.
- In one case a 33% reduction in the content of vitamin B6 was detected in skim milk powder after storage for 40 months.
- In another study losses of B1 and C were 10% after 2 years of storage.
- The amount of vitamin C loss depends upon the oxygen and water vapour permeability of the packaging material. It is suggested that milk powders should also be protected against light to keep losses of the light sensitive vitamins, in particular riboflavin, as low as possible.
- The vitamin A stability in skim milk powder was measured over a period of 16 weeks at 21, 26 and 32°C . The samples kept in dark lost 20% of the vitamin activity at 21°C, 27% at 26°C and 38% at 32°C .
- Vitamin fortified milk powder stored at ambient temperature for 2 years showed no loss of thiamine, riboflavin, niacinamide, pyridoxine, ascorbic acid and L-tocopherol acetate. So it was concluded that these vitamins will also survive when milk powder is stored at higher temperature for short period i.e. for 6 months. Only vitamin A was substantially degraded.
On storage of milk powders, the digestibility of protein and the available lysine content have been found to decrease. Such decreases are dependent on the moisture content, storage period and storage temperature. Higher moisture and longer periods of storage at elevated temperatures have been shown to accelerate such deteriorations. The same has been found to be applicable to various vitamins in the product. It is suggested that to protect light sensitive vitamins, exposure of powders to such conditions should be avoided.