Water activity food processing

Figure 0.3b shows that the desorption isotherm, indicating the course of a drying process, lies slightly above the adsorption isotherm pertaining to the storage of moisture-sensitive food. As a rule, the position of the hysteresis loop changes when adsorption and desorption are repeated with the same sample. The effect of water activity on processes that can influence food quality is presented in Fig. 0.4. Decreased water activity retards the growth of microorganisms, slows enzyme catalyzed reactions (particularly involving hydrolases cf. 2.2.2.1) and, lastly.

Dehydration Processing. Dehydration is one of the oldest means of preserving food. Microbes generally do not grow below a minimum water activity, of 0.65 defined as the equiHbrium relative humidity surrounding food ia a sealed container at a given temperature, ie, no microbes can... 

Adsorption of (bio)polymers occurs ubiquitously, and among the biopolymers, proteins are most surface active. Wherever and whenever a protein-containing (aqueous) solution is exposed to a (solid) surface, it results in the spontaneous accumulation of protein molecules at the solid-water interface, thereby altering the characteristics of the sorbent surface and, in most cases, of the protein molecules as well (Malmsten 2003). Therefore, the interaction between proteins and interfaces attracts attention from a wide variety of disciplines, ranging from environmental sciences to food processing and medical sciences. 

Now we turn our attention to the water and the solids that compose the myriad of fresh and processed foods we consume. When a component is added to water (or coexists with water, as in a fresh food), the overall mobility of the water decreases, compared to that of pure water. The magnitude of the decrease depends on the number, amount, and nature of the component(s) added, as well as the effect of any processing methods used. In the past, researchers focused their attention on the relationship between water (activity, availability, mobility) and food stability. Based on the introduction of the polymer science approach to food stability by Slade and Levine (1985, 1988, 1991), the focus has shifted to the relationship... 

WATER ACTIVITY, MOISTURE CONTENT, RELATIVE STABILITY LEVEL, EXAMPLE PROCESSING, PRESERVATION AND PACKAGING TECHNOLOGIES, TEXTURAL ATTRIBUTES, AND EXAMPLE FOOD PRODUCTS CORRESPONDING TO EACH OF REGIONS I, II, AND III IN FIGURE 15... 

Throughout this review, the concept of mobility has been highlighted as a key parameter for understanding and predicting the processability and stability of food systems. Mobility is the common denominator of the three methods examined in this review—water activity, nuclear magnetic resonance, and glass transition. An emerging aspect of the picture for food... 

Niacin is relatively stable to most food-processing operations. It is stable to exposure to air and resistant to autoclaving (and is therefore stable to pasteurization and UHT treatments). The amide linkage of nicotinamide can be hydrolysed to the free carboxylic acid (nicotinic acid) by treament with acid but the vitamin activity is unaffected. Like other water-soluble vitamins, niacin can be lost by leaching. 

Figure A2.2.1 Water activity stability map (adapted form Labuza, 1970). A representation of a typical sorption isotherm for food materials and of the effects of water activity on the relative reaction rates of several chemical processes, as well as the growth of microorganisms, in foods are shown.

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