Starch water activity

Foods high ia sucrose, proteia, or starch (qv) tend to biad water less firmly and must be dried to a low moisture content to obtain microbial StabiHty. For example, grain and wheat flour can support mold growth at moisture contents above 15% (wet basis) and thus are stored at moisture contents below 14%. Stored grains and oil seeds must be kept at a water activity below 0.65 because certain molds can release aflatoxias as they grow. Aflatoxins are potent carciaogens (see Food toxicants, naturally occurring). 

Drying out can be prevented in one of two ways, either by packaging or by lowering the water activity of the product. Starch retrograda-tion can be inhibited by using a starch complexing emulsifier with or without the addition of fat. Mould growth is inhibited by a low water activity. 

These solve the shelf life problem by having a very low water activity aided by the quantity of dried fruit that they contain. The fat content and the low water activity inhibit starch retrogradation. 

LIG. 38 Graph of a modified state diagram for instant dent starch, which includes both DSC rg midpoint data plotted as a function of moisture content (% wb) and sorption isotherm data obtained at 20 °C. DSC Tg midpoint data were fit to the Gordon-Talyor equation, and sorption isotherm data were fit to the GAB equation. The moisture content associated with the Tg at 20 °C is called the critical me and is equal to 21.8% (wb), and the corresponding critical water activity value is 0.92. 

Figure 7.7 Clausius-Clapeyron relationship between water activity and temperature for native potato starch. Numbers on curves indicate water content, in g per g dry starch (from Fennema,...

Mannuronic acid, determination of, 735 Manometer, measuring vapor pressure to determine water activity, 61 -66 Martin process, starch isolation, 675 Mass spectrometry (MS), see also specific MS methods... 

In addition to lowering the water activity and forming sugar bridges between starch chains restricting flexibility, there is also an antiplasticizing effect of saccharide solutions relative to water.78... 

Zimeri and Kokini (2003) have shown that water acts as a plasticizer for gelatinized waxy maize starch (WMS) identified by the decrease in Tg with increasing moisture content (Figure 7.9). At room temperature, the glass transition occurs at a moisture content around 28% corresponding to a water activity (o ) value less than... 

Figure 7.31. State diagram for the extruded meat-stareh matrix. (The arrow indicates the water activity above which the extrudates become rubbery at room temperature) (Reproduced with permission from Moraru, Lee, Karwe, and Kokini, 2002, Phase behavior of a meat-starch extrudate illustrated on a state diagram, J. Food Sci. 67, pp. 3026-3032, Institute of Food Technologists.)...

The decrease in expansion seems to occur at moisture levels above 30°/o moisture for both cereal (starch-based) polymer systems, and protein (soy grits). This corresponds to a point on their adsorption curves where water activity rises rapidly with added moisture that is, at a level where the water added to a mix has little effect on primary hydration of polymers, but behaves as a diluent. In mechanical terms, this may be explained by proposing that at above levels of 25°/o-30% water plasticisation of the polymers is complete, and further added water acts as a lubricant, reducing the shear-induced temperature rise and particle damage necessary for the formation of homogeneous melts. 

Sucrose and other sugars increase the gelatinization temperature of starches the increase depends on the water activity of the sugar solution and the sugar the gelatinization temperature of starch increased with decrease in water activity of the sugar... 

Several studies have shown that anthocyanin stability increases with decreased water content, i.e., decreasing water activity (aw) (Wrolstad, 2000). Dry anthocyanin powders (aw < 0.3) are stable for several years when stored in hermetically sealed containers (Jackman and Smith, 1996). This property is advantageous when anthocyanins are produced into dry products and powders. Zajac et al. (1992) reported anthocyanin losses of 14% after 15 months of storage of a dry powder from black currant extracts at 20°C. When starch syrup or maltodextrins were included as carriers during the drying of the extracts, the anthocyanin losses were reduced to 2% and 3%, respectively. 

Dependence of the maximum force at breakage (MFB) with water activity for a dry starch model. D65 and D105 air dried at 65 and 105°C, respectively FD freeze-dried. Dotted line represents the glass transition temperature. 

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