Water activity packaging

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. 

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... 

Humectant properties of sucrose and other carbohydrates may be important for certain products, as water activity can be controlled by the level of humectants. Replacement of the carbohydrates by sweeteners may therefore influence the water activity of products and require other technologies, other packaging materials or additional steps to control microorganisms. 

A low water activity a ), which for C. botulinum ranges from 0.94 (type B) to 0.97 (type E), enables the outgrowth and toxin production in packaged... 

Foods dried to water activities in the range of 0.65 to 0.85 are often referred to as intermediate moisture roods. These partially dried foods tend to be soft and to rehydrale easily The remaining water acts as a plasticizer. Because molds and yeast may be able to grow in these partially dried products, they must be preserved by heaL vacuum, or modified atmosphere packaging, refrigeration, or chemical means. 

More impi manl than moisture content, per se. is the objective of creating a preserved food substance lhai is stable and can he eaten directly. An idealized IMF system will have ilia microbial stability at reduced water activity. 17) storage siabiliiy without special conditions. l3i reduction of weight and more compactness of product, and (41 can be consumed from the package without rehvtlrulion Some authorities believe that IMF systems represent excellent potential in the development of the snack market... 

The importance of temperature control is significant for laboratory comparisons, accelerated shelf-life studies, and packaging requirements. Also, temperature may be essential when measuring aw levels near critical values, especially for government regulations or critical control points. The precision required in water activity applications will determine the need for temperature control. Several reasons for sample temperature control are ... 

The maximum shelf-life of bulk packaged WMP containing 3% moisture is about 6 months at 30°C (Kjaergaard Jensen, 1988). The oxidation of WMP, as measured by peroxide value, is dependent on the moisture content of the powder, van Mil and Jans (1991) reported that under similar storage conditions, the peroxide value of WMP increases more rapidly for powder containing 3% moisture than in powder containing 2.4% moisture. The water activity (aw) range for WMP is usually 0.13 0.20, with a typical value from 0.16 to 0.18 (Wewala, 1990). Stapelfeldt et al. (1997) found that the quality of WMP is maintained best at aw between 0.11 and 0.23, whereas the quality of the powder decreases when stored at aw of 0.31 at 45°C. However, the critical aw for improved oxidative stability of WMP stored at 40°C for one year is 0.21 0.24 at a moisture level of 3.4% (Wewala, 1990). 

The storage condition of raw materials, particularly hygroscopic substances, is important, and as a minimum water activity (Aw) of 0.70 is required for osmophilic yeasts, 0.80 for most spoilage moulds and 0.91 for most spoilage bacteria, precautions should be taken to ensure that dry materials are held below these levels. Some packaging used for raw materials, such as unlined paper sacks, may absorb moisture and may itself be subject to microbial deterioration and so contaminate the contents. For this reason polythene-lined sacks are preferable. Some liquid or semi-solid raw materials contain preservatives, but others such as syrups depend upon osmotic pressure to prevent the growth of os-mophiles, which are often present. With this type of material it is important that they are held at a con-... 

RETORT CALCULATION During the retort process we assume that no oxygen is present so that only water transport is considered. At retort temperatures the diffusion coefficient of water in materials of interest is sufficiently high so that a pseudo-steady-state model, in which linear profiles are assumed across all structural layers, can be used. The EvOH layer is assumed to be at a uniform water activity at a given time which is good assumption based on water permeability measurements in EvOH at humidities found in package applications (13 ). 

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