Effect of instant controlled pressure drop drying on water loss and quality in apple slices

Abstract: Apple is one of the most widely cultivated fruits where its dried products rapidly occupied in the puffed products market. In recent year, instant controlled pressure drop drying (ICPDD) has been widely researched and generalized with its advantages of short time, no oxygen reduced oxidation in the processing under vacuum environment, the product is natural and nutritional. Instant controlled pressure drop drying is effective in giving apple slices crispy taste and prolonging the shelf-life of food. Thus, ICPDD has a great prospect in fruits and vegetables drying field, in which it's necessary to study the effect of instant flashing on water loss and product quality. In this paper, ICPDD drying technology, low-field nuclear magnetic resonance, color and texture detecting technology were mainly adopted to analyze the water loss and quality during flashing. High temperature, pressure differences and water removal can all influence the structure and quality of apple slices. Fresh Fuji apple was used in the experiment which was peeled, pitted and cut into sector slice with 5mm thickness and then was evenly put on the tray of hot air drying under 70℃ until the water content was decreased to about 70%. Then focus on the effect of instant controlled pressure drop drying on water diffusion and product quality. Experiment results showed that, different flashing temperatures had a little influence on water diffusion while higher temperature could accelerate water diffusion. Under the condition of 85, 95 and 105℃, the flashing times used to decrease the water content below 7% were nine, ten and eleven, separately. But compared with 95 ℃, the effect of 105 ℃ was no obvious. The drying rate increased first when it reached the maximum at the first flashing time and then decreased. But 105 ℃ was too high to keep a good color, and the crisp was too high to transport easily. Lower temperature 85 ℃ may be not enough to provide power for puffing. So that 95℃ was chosen as the flashing temperature in this research. Instant flashing really had an effect on water loss from the changes of water content where it firstly increased, and then decreased. Water state also changed from moisture with high degree of freedom to moisture with low degree of freedom that affected the water removal. After a few times of flashing, free water and immobilized water mainly existed in apple slices which were easily to be removed with lower bonding effect. Along with the increasing of flashing times, immobilized water and bound water with low liquidity play a dominant role in apple slices because they are difficult to remove. In order to obtain high quality apple chips, choosing in flashing times is important. How much the amount of moisture loss in flashing moment does directly decided the flashing times during drying, so that it could shorten vacuum time as much as possible to improve the sensor quality. Both water activity and color decreased with flashing times increasing, and long time in high temperature might facilitate the Maillard action. Appropriate flashing times could make water instantly vaporized to give rise to volume swelling and form spongy porous structure which gave apple chips crispy taste. Rapid water loss also caused case hardening to make hardness up which is in favor of keep a good shape. However, higher hardness would affect the chips taste. Therefore, 5 times was chosen as the suitable condition to gain great quality.