Hii, Law - Quality Evolution During Drying of FVFs
6.2.2. Chemical
6.2.2.1. Flavour
Flavour is the primary concern to consumers irrespective of the texture, shape and
colour of a dried product. Flavour of food consists of various food aroma compunds that
constitute the taste and odour of the food. Some flavour compounds are volatile and
these are carried away during moisture removal process. The change in shape and tex-
ture influence the microstructure of the food product and controls the release of flavour
during processing and consumption. Flavour properties of a food product can be ana-
lysed via chemical analyses (e.g. by chromatography method) or sensory evaluation.
Chemical evaluation is able to provide the quantitative details of the aroma compounds
but no indicative on the acceptance in terms of taste as perceived by human beings.
Therefore, sensory evaluation plays an ultimate role in deciding the final acceptability of
the food product. This is done by comparing the test product to a reference sample and
rating is given during evaluation.
Undesirable flavour can also be produced due to product spoilage. Dried food with
high fat content can easily become rancid due to fat oxidation. Oxygen level below 1% is
effective in delaying rancidity, staleness and other deteriorative activities in dried food
product (Perera, 2005). High fat content dried food could also pick up foreign odour eas-
ily such as from smoke exhaust, drying with other strong odour materials, contaminated
packaging bags and etc.
6.2.2.2. Water activity
The removal of moisture content reduces the water activity of the product during
drying. In general, the water activity (aw) in most dried food is relatively low, typically
less than 0.6, which is the recommended level for safe storage. Low water activity inhi-
bits the growth of various microorganisms and prevents oxidation and enzymatic reac-
tions. Table 6.4 shows typical water activity values to prevent growth of some common
microorganisms (Rahman, 2005).
Table 6.4. Critical water activity to inhibit microorganisms
Microorganisms aw (Critical range)Bacteria < 0.85 – 0.86Yeasts and moulds < 0.626.2.2.3. Shelf life
Storage and packaging play an important role in affecting the shelf life of a dried
product. Typically the optimum relative humidity for dried product storage is 55 – 70%
at moisture content ranging from 2 to 20% (Perera, 2005). Packaging should be mois-
ture proof and able to prevent the transfer of oxygen into the product and cause off fla-
vour formation. Alternatively, modified atmosphere packaging technique can be used to
extend the shelf life of the dried products (García-Esteban et al., 2004).
In general, under ideal condition, the shelf life of a food product can be predicted
based on microbial growth (Equations 6. 12 -6. 13).