Effects of ultrasound waves on rehydration of carrot slices in osmotic drying pretreatment

Introduction                                   Material and Method

Osmotic dehydration is one the new drying methods of food stuffs (Tavakolipour, 2002) which can be used in combination of hot air as preprocess before dehydration or as dehydrating method (Zargari, 1992; Fito et al, 1992). In this method , food slices put in an osmotic solution more than product (salty or sweet solu-tions).Osmotic pressure cause to gradually removing of cell wall’s water, penetrating a few of salts or minerals from solution to food stuff texture. Finally it observed that moisture has been decreased and dry solid increased (Rahman et al , 2007; Simal et al , 2003).

Water content of Food stuff decreases after osmotic dehydration while dry matter increases . there are some parameters including rehydration process including osmotic solution (type, concentration) , thickness of food stuff , shaking etc(Soti khaibani, 2003).

Required carrot bout freshly of fruit vegetable market, required glucose BX 80 bout from Shadineh Co Isfahan, Iran .

Slice Preparing

Carrots with 2 cm diameter were selected to perform experiments .first of all their head and tail which didn’t have suitable thickness cut , the middle part cut into slices with 2 cm diameter and their size , shape uniformed.

Preparing of Osmotic Solution

To prepare this solution glucose solution 50 % w/w with BX 80 % applied, diluted with distilled in ration 10:6 to produce a 50 % w/w solution.

Ultrasound Wave Treatment

To apply this treatment, ultrasound bath produced by pars Nahand co equipped with 3 ultrasound generator with 50 w power (150w together) used, carrot slices with osmotic solution in with 1:4 ratio (200 g slices and

800 g osmotic solution) mixed, put in an 1lit beaker in ultrasound bath filled by 5 L water as a waves transferring medium and temperature moderating, samples treated for 1-3 hr. Samples were shook every 1 min equally by glass bar during this time. Water temperature controlled during process, fixed in range 38-40 ˚c by removing some hot water. In the end, slices let to be removed their water by a metal net and dried by straw paper.

Rehydration Experiment

Since rehydration considers as one of the quality indexes of dried food , 20 g of all dried samples soaked in 200 ml distilled water at 20 ˚c for 1 hr , then dried by straw paper after removing water . water intake measured by the following formula:

A=(W-D)/D ×100

In which:

W= sample weight after rehydration

D= Dried sample weight before rehydration

A= moisture percent base on dry matter

All the results were analyzed by Duncan and tests using SPSS software.

Results and DiscussionResults of Rehydration Experiment

Figure 1 indicated that samples with no ultra sound treatment reached to constant weight in the last of period, while ultra sound treated samples reached to their maximum weight in 3rd hours, then their weight decreased. This implies that ultra sound treated samples have more porosity and more intake of materials of osmotic solution, therefore causes to more race of rehydration, but slices weight decreases gradually by absorbing more water and entering more absorbed materials into this water. Final weight was significantly more than control after 5 hr , although ultra sound treated samples need less time to rehydrate.

Figure 1. Rehydration of dried samples

Figure 3. Comparison of averages of interaction effect

Figure 2. The effect of osmotic solution during pre osmotic process

Rehydration amount increased in 1-3 hr of pre osmotic treatment and the most rehydration observed in salty solution with no ultra sound treatment (Fig 3).

Conclusion

The most and the least rehydration observed in control (salty solution) after 3 hr and 1 hr respectively .the final weight of ultra sound treated samples were significantly more than control. Ultra sound treated samples also reached to the constant weight after 3 hr, then their weight subtract while samples with no ultra sound treatment reached to the constant weight to the end of period.