Slide 1

Hii, Law - Quality Evolution During Drying of FVFs

N = N 0 exp[k(t-tL)] (6.12)

ts = ln(Ns/N 0 )/k +tL (6.13)

where N 0 is the initial microbial load, N is the microbial level at time t, Ns the micro-
bial level at the end of shelf life, ts is the shelf life at constant temperature, k and tL are
the specific growth rate and lag time at constant storage temperature (Fu and Labuza,
1993 ).

6.2.3. Nutritional aspects

6.2.3.1. Food nutrients

Food nutrients degrade during drying and the magnitude of change depends on the
foodstuff and the drying conditions. Losses of nutrients can be minimized by applying
suitable pretreatments, selection of appropriate dying methods and optimization of dry-
ing conditions (Sablani, 2006). Generally, nutritional loss increases with the severity in
the process conditions during processing (Rahman, 2005). Table 6.5 shows the typical
nutritional changes that could occur during drying (Perera, 2005).
Table 6.5. Nutritional changes during drying of food

Type Possible changes

Colorie content Does not change, but is concentrated into a smaller mass as

moisture is removed

Fiber No change

Vitamin A Fairly well retained under controlled heat methods

Vitamin C Mostly destroyed during blanching and drying of vegetables

Thiamin, riboflavin

and niacin

Some losses during blanching but fairly good retention if the wa-

ter used to rehydrate is also consumed

Minerals Some maybe lost during rehydration if soaking water is not used.

Iron is not destroyed during drying.

Protein Can undergo heat denaturation, susceptible to light oxidation

and may undergo enzymatic degradation

Lipids May undergo enzymatic hydrolysis in the initial phase of drying.

At low water activity auto-oxidation of unsaturated fatty acids

causes rancidity

Carbohydrates Maillard browning and changes in flavour under high heat. Sugar

can be caramelized and give darker colour to dried product.

The degradation of a nutrient during drying can be described using a first order
reaction model (Benali, 2004). The degradation model (Equation 6. 14) can be expressed
by:

Degradation rate = k([N] [N])

dt

d([N] [N])

dt

d[N]

0 x =− 0 − x

= − (6. 14)