Strawberries are perennially popular fruit due to their sweet taste and nutritional properties – even though their availability may be limited to a relatively short season. Moreover, they are notoriously susceptible to microbial spoilage.
Food manufacturers do use freeze dried strawberries in dry formulations, such as breakfast cereals, with good colour, flavour and taste results; but the researchers involved in the new study, accepted for publication in the Journal of Food Engineering, point out that the texture tends to collapse after rehydration.
This, they say, limits the application of freeze-dried strawberry pieces in liquid formulations.
In a bid to extend the application possibilities, Professor Min Zhang and colleagues from Jiangnan University’s School of Food Science and Technology investigated the effects of coating the freeze-dried strawberry pieces, and the best way to dry them after coating.
They hypothesized that whey protein isolates (WPI) could slow down the rehydration, since it has low water vapour permeability (WVP). By contrast, glycerol increases WVP sby reducing internal hydrogen bonding in films, they said.
Coating and drying methods
The team spray dried strawberries that had been chopped into 1cm3 pieces, with a vacuum level varying from 60-80 Pa to 10-20 Pa. The fruit pieces had a final moisture content of 6.7 per cent.
Aqueous solutions were prepared by swelling WPI in distilled water, adding lactose and glycerine, and stirred for 20 minutes until totally dissolved. The solutions were put into a dipping bath at 70ºc, left for 20 minutes to denature the proteins, then cooled to room temperature.
The strawberry pieces were then dipped either in the WPI solutions or in water, as a control, placed on plastic plates, and air-dried at selected temperatures.
To test rehydration, coated and dried strawberry pieces were weighed, then put into milk, then weighed again. The dissolution method was used to take colour measurements.
In addition, the team looked into coating and spouted bed drying of the pieces of fruit – a system that allowed for the pieces to be coated and dried simultaneously.
Slower rehyration, lower costs
The rehyrdration ratio of the coated and dried strawberry pieces was seen to be significantly less than normal freeze dried strawberry pieces in milk.
The researchers determined that the optimal formula for coating, in order to reduce the rehydration ration, was 10 per cent whey protein, 3 per cent glycerol, and 10 per cent lactose.
When the strawberry pieces were dried in the spouted bed, they were seen to have “a light surface colour, vivid red, and the shape almost the same as the freeze-dried strawberry pieces,” wrote the researchers.
They also noted that slight shrinkage can lead to a reduction in the rehydration ratio, but with no great effect on appearance.
On the other hand, coating and drying time and costs were reduced.
“In general, spouted bed drying is an excellent drying method when it is applied to coat freeze-dried strawberry pieces.”
Journal of Food Engineering (published online ahead of print)
“Effect of coating on post-drying of freeze dried strawberry pieces.”
Authors: Lue-lue Huang, Min Zhang, Wei-quiang Yan, Arun Mujumdar, Dong-feng Sun.