General Aspects

Numerous food powders experience significant changes in their properties during storage, transportation, or processing, which are related to the particle size distribution. Attrition causes reduction in average particle size while aggregation increases it. Fines generated by attrition may either form clusters or coat larger particles (plating). In-terparticle adhesion is decisively influenced by particle size, the ratio between adhesion and weight usually being inversely proportional to the square of the particle size (1). As a result, this ratio is two orders of magnitude higher for particles of 10 /im than for particles of 100 //m. Dry food powders with average sizes of 80 to 100 /zm are usually free flowing, whereas powders having sizes below 20 to 30 ¡im become cohesive, form secondary particles (clusters) of larger size, and form lumps when rewetted.

Adhesion without formation of bridges between the adjacent particles occurs as a result of either van der Waals or electrostatic forces and causes the formation of comparatively weak agglomerates. Adhesion associated with the formation of bridges produces much stronger agglomerates. Free flowing and cohesive food powders may undergo segregation during their storage, transportation, and handling. Primarily because of the differences in particle size and also in density, shape, and resilience, fine particles migrate to the bottom while large particles find themselves at the top of the vessel. As a result, some minor components of beverage blends (colors, flavors, vitamins) may become unevenly distributed between packages.

The purpose of particle size enlargement by agglomeration is to improve powder properties like bulk density, flowability, meterability, dusting, powder mix homogeneity, storage stability, and optical appearance. Powdered foods, which are in most cases intended to be dispersed in liquid, should also have good wettability, sinkability, dis-persibility, and (for soluble materials) solubility, that is good "instant properties." A powder layer spread on a liquid surface should imbibe the liquid, submerge, disperse, and dissolve within a few seconds with little mechanical aid and without forming lumps. A powder treated by a technical process to have such properties is called "in-stantized." Agglomeration is the predominant method for instantizing powdered foods, and an example of the dependency of the wetting time of a powder layer on the average agglomerate size is shown in Figure 1.

Another major quality factor for instant foods is the preservation of flavor components. Instant beverages containing, for example, coffee extract are particularly susceptible to flavor loss caused by high temperature or excessive contact with air, as, for example, in a fluidized bed.

Obviously, simultaneous improvement of all powder properties is impossible. Increasing agglomerate stability, for example, results in most cases in a decline of the in-

Figure 1. Wetting time (including standard deviations) of a powder layer of height ho = 5 mm for three commercial instantized food powders. CS, cocoa-sugar mix (21°C); SM, skim milk powder (21°C); WM, whole milk powder (50°C).

stant properties. The way the agglomerates are formed in the production process determines their properties, and comprehension of the basic physical principles of particle adhesion and the mechanisms likely to predominate in a given agglomeration process is helpful.

UNDESIRED AGGLOMERATION—CAKING

Agglomeration via caking may occur unintentionally since blends of particles are always exposed for some time to the ambient environmental conditions (temperature and/or humidity). For example, food powders that include lipids (soups, sauces, baking mixes) may undergo caking if the temperature exceeds the melting point of the lipids. As a result, sticky liquid bridges are formed. Once cooled, the lipids recrystallize, liquid bridges between particles become solid, and caking is reinforced. Although starchy and proteinaceous components are relatively insensitive to the environmental conditions, the soluble components of food powders (sugars, salts) absorb moisture and eventually change their state from solid to liquid.

The ability of sugars to soften depends on the conditions under which they were produced and stored. These conditions are responsible for the formation of areas of crystalline or amorphous structure. Amorphous sugars absorb much more moisture at a given water activity (relative humidity) and have lower glass-transition temperatures than crystalline sugars (2). Whereas a stable, crystalline structure is formed at equilibrium conditions, the amorphous one is created at nonequilibrium conditions. Relatively slow moisture withdrawal during carefully controlled crystallization (nuclei formation and crystal growth) leads to the development of a crystalline structure. Fast moisture withdrawal from a solution of carbohydrate via spray drying, roller drying, or freeze drying helps to produce mainly the amorphous form; even the mechanical impact of milling of sugar crystals produces an amorphous surface capable of recrystallization after absorbing water (3). Upon recrystallization, amorphous sucrose releases water, which facilitates formation of bridges between particles and initiates caking. Adding high molecular weight components (eg, maltodextrin) to a blend containing sugars may reduce caking (4).

Caking may be effectively suppressed by adding anti-caking agents like tricalcium phosphate, magnesium oxide, calcium silicate, and so on, which absorb a portion of moisture from the blend and thus reduce the amount of available moisture. Although total moisture content of the blend with or without anticaking agent stays virtually unchanged, it is relative humidity generated by the blend in a sealed chamber that reflects the amount of available moisture: blend with added anticaking agent generates lower RH than blend without anticaking agent. The effectiveness of the anticaking agents depends largely on their water-holding capacity, so that with an unlimited source of humidity (open storage), their impact is lessened.

Even packaged food powders may undergo caking influenced by the environment inside their packages. Being relatively isolated, the headspace inside the package is affected not only by the surface moisture of the particles and temperature in the warehouse, but by the permeability and heat conductivity of the package film. Variations in the temperature and humidity outside of the packaged material often accelerate an exchange in surface moisture between the ingredients and initiate caking.

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