Solar Drying: Fundamentals,Applications and Innovations - National ...

Aware and Thorat - Solar Drying of Fruits and Vegetables
sensitive to oxidative degradation. Sulphuring can destroy thiamine and riboflavin while
pre-treatment such as dipping in sulphite solutions reduce the loss of vitamins during
drying. As much as 80% decrease in the carotene content of some vegetables may occur
if they are dried without enzyme inactivation. However, if the product is adequately
blanched then carotene loss can be reduced to 5%. Steam blanching retains higher
amounts of vitamin C in spinach compared with hot-water blanching (Ramesh et al.,
2001). Blanching in sulphite solution can retain more ascorbic acid in okra (Inyang and
Ike, 1998).
Pre-drying treatments such as addition of sugars are needed for vegetable drying
in order to avoid damages to tissue structures. Previous work has shown that nonreducing
disaccharides such as sucrose and trehalose can protect biological systems
from the adverse effects of freezing and drying. Especially trehalose is known to have
many advantages. For example, sweetness of a 10% trehalose solution is 45% as that of
a 10% sucrose solution. Trehalose is a non-reducing sugar and therefore does not react
with amino acids or proteins to cause Maillard browning. Pretreatment with trehalose
solution has been claimed to be very effective for producing high quality dried vegetable
chips Aktas et al., (2004) found that pretreatment with trehalose solution improved the
reconstitutional properties of dried sliced carrot and potato samples compared with the
dried products pre-treated with sucrose solution, which is generally used for osmotic
dehydration as shown in Figure 2.1. Osmotic treatment is a simultaneous water and solute
diffusion process (Rastogi et al., 2004). Many studies showed that sucrose treatment
increases the water loss compared with the other osmotic solutions (Reppa et al.,
1998; Khiabani et al., 2002).
Various pretreatment methods may be used in conjunction with the drying
process to maintain or even improve the quality of a dried product. Among many methods
of pretreatment blanching is one of the most common. Blanching is usually performed
prior to drying to inactivate enzymes responsible for various undesirable enzymatic
reactions. Blanching also helps with color retention and modification of product
texture (Mate et al., 1999; Ahmed et al., 2001). Moreover, blanching can help increase
the drying rate, hence reducing the drying time (Severini et al., 2005). Dipping or soaking
a product (especially vegetable) in organic acids such as citric acid, lactic acid or
acetic acid (Karapinar and Gonul, 1992; Yu et al., 2001) is an alternative to blanching as
these pretreatment methods can help reduce the number of normal flora and pathogenic
organisms. Some organic acids such as acetic acid have been noted to reduce the activity
of enzymes responsible for browning (Naphaporn et al., 2010). These are the various
pretreatments that can be employed prior to solar drying of fruits and vegetable.
60 Drying of Foods, Vegetables and Fruits