Common Defects of dried milks

Some of the common defects that may develop in dried milk products due to various reasons are described below:

i. Off Flavours

Oxidised/ tallowy flavour: Dry whole milk and other dry high-fat milk products undergo oxidative deterioration (Also called tallowy). Whole milk powder with low to medium preheat treatments (equivalent to a WPNI of about 3-5) has a greater tendency to undergo lipid oxidation, with distinctive tallowy and musty flavours,than powders made with higher heat treatments (equivalent to WPNI values of 2.5).Chemical changes result with the addition of oxygen to the double bonds of unsaturated glycerides, giving at first peroxides and later aldehydes, ketones etc.,which impart the unpleasant flavour. Copper and iron act as catalysts. Higher storage temperature, higher acidity, sunlight and ultra violet irradiation promote faster development of oxidative deterioration.

Rancidity flavour: Rancidity is due to hydrolysis of fat through lipase enzyme leading to production of free fatty acids, like butyric acid.

Stale flavour: Stale flavours, due to carbonyl compounds, can be detected in milk powders almost as soon as they are made. The mechanism of formation of these compounds may be through the Maillard reaction, but many compounds contribute to a stale, cardboard flavour, including oxidation by products. The defect is accelerated by high moisture content and high temperature of storage.

Cooked flavour: Milk powders often have cooked flavour, which results from components formed during preheating and possibly during evaporation. During drying, conditions are mostly not such that off-flavours are induced. On the contrary, a considerable part of the volatile sulphydryl compounds (especially H2S) is removed.A cooked flavour in milk powder mainly results from methyl ketones and lactones formed by heating of the fat (they thus are almost absent in skim milk powder) and form Maillard products.

ii. Discolouration

Milk powder tends to darken during storage, turning to brown due to maillard reaction, which refers to the reaction between free amino group of protein and lactose. This is associated with old or stale off-flavour. High moisture content and high storage temperature enhance browning discolouration. Spray dried milk powder is more susceptible to age darkening and to greater intensity than roller process powders.

iii. Free Fat

Free fat or extractable fat is the portion of the fat content of a milk powder that can be extracted by organic solvents (e.g. carbon tetrachloride). It is expressed as a percentage either of the total fat content or of the total mass of powder. Free fat content of milk powders affect flowability, reconstitutability and stability of milk powder. The factors controlling the level of free fat are:

•  The type of fat: Low melting fats tend to produce higher levels of free fat
•  Total fat content: As the total fat content increases the free fat rises progressively up to 20%.
•  Product composition: The formation of free fat is reduced by a high content of amorphous lactose, and is increased by high protein content.
•  Gentle drying conditions: Controlled inlet air temperature, outlet air temperature and atomizer speed.
•  Gentle powder treatment: Avoiding pneumatic transport, low pressure-drop in cyclones and effective cooling of powder by fluid bed.
•  The powder moisture should not be too low
•  Avoid lactose crystallization
•  Addition of emulsifier

 iv. Poor Solubility

Powders with poor or low solubility cause sediment, which is unsightly. They can also cause processing difficulties and can result in poor economy, as milk solids may be lost as insoluble material. The sediment produced when milk powders are reconstituted is measured in terms of an insolubility index, so, in reality, the solubility of a powder is expressed as its degree of insolubility. Insolubility index is a measure of the extent of denaturation of the proteins in milk powder. Any dryer can in fact be mal-operated resulting in a powder with poor solubility (i.e. insolubility index). The analysis shows the insolubles to be a protein/fat/mineral complex. The protein consist casein and/or denatured whey proteins.

At a given temperature, the rate of denaturation of protein in concentrated milk doubles for every 5% increase in total solids up to about 92%. Most of the denaturation during the drying process occurs when the concentration of total solids is greater than 50%. Further, it is important to note that at or near critical concentration(i.e. 86-88%) even at ambient temperature, 50% insolubility may be produced within 10 h.

v. Slowly Dispersible Particles

These result from the surface wetting of unagglomerated very fine particles. The formation of a gelatinous coat prevents the ingress of water and slows the rate, at which the particles dissolve. If not maintained in suspension by agitation, the particles are likely to form a sludge.

vi. Sludge Formation

Powders with poor dispersion properties can form sludges and can demand greater energy inputs to ensure complete solution. In an industrial or domestic context, the presence of a sludge following the reconstitution of a milk powder is regarded as a product defect.

vii. Scum Formation

Scum is the layer formed on the surface of the reconstituted milk comprising of the foam (upper layer) and the undissolved/ slowly hydrated particles (flecks) (lower layer).

viii. Occurrence of White Flecks

White flecks are slowly hydrated particles in lecithinated whole milk powder and are measured (in mm) as the lower layer of the scum (i.e. foam + flecks) after reconstitution under specified conditions. 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.

ix. Scorched Particles

Scorched particles are generally accepted to be a measure for any deposits in the drying chamber (e.g. deposits in the wheel or around the nozzles or in the air disperser) having been exposed to high temperatures thus getting scorched, discoloured and at the same time insoluble. Contamination of material that subsequently passes through and removes this material by collusion contributes to scorched particles in such product. Other factors outside the dryer which contribute to scorched particles are dirt or sediment of raw milk not efficiently removed by clarification. Deposits (i.e.brown, insoluble jelly lumps) formed in the tubes of evaporator calendria also contribute to the scorched particles. The scorched particles are expressed as disc

A, B, C, and D standard depending on the intensity and colour of the particles left on the filter.

x. Cakiness

It is a tendency of a powder to form hard lumps in bags during storage. Amorphous lactose is responsible for the tendency of milk powders to pick up moisture from surrounding air. Milk powder becomes sticky during the first phase of this moisture uptake and stoned-hard at the end of this process. Final caking is caused by the crystallization of lactose followed by the evaporation of excess moisture.

xi. Feathering

Product intended for use as whiteners in hot beverages such as coffee and tea, must be reasonably thermostable. The lack of thermostability occurs as, so called feathering i.e. particles visible on the surface or throughout the beverage or sediment at the bottom. A temperature of 80-85oC coupled with the low pH of coffee, and often high levels of calcium and magnesium hardness in the water makes for an environment hostile to milk protein stability. Proper protein standardization, appropriate heat treatment of raw milk and incorporation of phosphates and citrates as pH regulators and protein stability enhancer are suggested to improve the stability of milk proteins.