Slide 1

(やまだぃちぅ) #1
Azarpazhooh, Ramaswamy - Osmotic Dehydration

bic acids in treated samples with sulfur dioxide followed by osmotic dip and vacuum
drying than in non-treated samples. Riva et al. (2005) observed that vitamin C was re-
tained higher by osmo-air dried apricot samples than by non-treated air dried samples.
They reported this phenomena as a lower phenolase activity and the protective effect of
the sugar specially sorbitol. Several authors have reported that the texture, flavor, and
color stability in dried fruit and vegetables are improved. This is especially important
since color may be a decisive factor in the consumer’s acceptance of a food (Krokida et
al., 2000).


4.6.1. Impact of osmotic dehydration on quality properties

Osmotic treatment of fruits and vegetables preceding convective drying may strong-
ly affect properties of the final product (Lewicki and Lukaszuk, 2000; Lewicki and Paw-
lak, 2003). During osmotic dehydration, many aspects of cell structures are affected such
as alteration of cell walls, splitting of the middle lamella, lysis of membranes (plasma-
lemma and tonoplast), tissue shrinkage (Alvarez et al., 1995) which could strongly influ-
ence the transport properties of the product during processing. All these phenomena
cause changes in the macroscopic properties of the sample, such as optical and mechani-
cal properties, which are related to the product appearance and texture, respectively. All
these changes greatly affect organoloptic properties of the osmo-dehydrated plant due
to solute uptake and leaching of natural acids, color, and flavor compounds out of osmo-
dehydrated plant tissue; as a result, natural composition of the product is modified
(Lazarides et al., 1995). Although compositional changes may have a positive and nega-
tive effect on the final product, rehydration of osmotically dried fruit is lower than in the
untreated fruit due to the rapid impregnation of a subsurface tissue layer with sugar
(Nsonzi and Ramaswamy, 1998a); moreover, if the osmosis takes more time, the rehy-
dration rate would be lower.


4.6.1.1. Impact of osmotic dehydration on color

Many investigators demonstrated that the quality (color, texture and rehydration
capacity) of air, freeze or vacuum- dried fruits and vegetables could be improved by a
prior osmotic step (Flink, 1975; Hawkes and Flink, 1978; Lerici et al., 1985; Nsonzi and
Ramaswamy, 1998a). There have been numerous research studies on the application of
color change during osmotic dehydration. The color of the products is measured by
lightness (L value), redness or greenness (a value) and yellowness or blueness (b
value), during or after drying. Falade et al. (2007) reported transparency and that the
color of the fruit may alter favorably due to physical and chemical changes during os-
motic dehydration. They evaluated L
, a, b values of osmosed and osmo-oven dried
watermelon, and reported that color parameters increase with an increase in osmotic
solution concentration. Osmotic dehydration improves fruit quality by stabilizing color
parameters and allows less color loss of fruit by enzymatic oxidative browning due to
infusion of extensive sugars. In addition, reducing the water activity of samples also de-
creases the non-enzymatic browning reaction (Krokida et al., 2000).


Osmotic dehydration limits or reduces the use of preservatives such as sulfur dio-
xide in fruits. In addition, substantial amount of air from the tissue is removed; therefore
blanching prior to osmotic dehydration also can be omitted (Torreggiani, 1993; Lenart,

Free download pdf