The Effect of processing factors and ultrasound on Mass Transfer of Botton Mushroom During Osmotic Dehydration

Button mushroom has short shelf life because of its high sensitivity to physical damage. There are a large number of researches on different methods of drying and combined methods of osmotic dehydration and air drying. There is less information about using salt solution as osmotic solution and the effect of ultrasound on dehydration rate of Button mushroom. Aiming to achieve the optimum method of osmotic dehydration, the effect of osmotic processing conditions on mass transfers were investigated in the present study.

In the first step, osmotic dehydration process was carried out by immersing samples in different sodium chloride concentration 6, 9, 12 and 15%, for 20,40,60,90 and 120 minutes at 25°C and 45°C and water loss, solid gain and weight reduction was studied. In the second step, in the constant conditions of salt concentration (12%), immersion time (60 min) and temperature (45°C), the mushroom samples were subjected to ultrasonic wave in two levels (pulse duration time to pulse rest time of 1:1 and 1:4) at frequency of 20 KHz and constant power 400 w, to determine their effect on water loss, solid gain and water reduction.

The results showed that, WL, SG and WR increased with osmotic of solution concentration, temperature and immersion time. Higher osmotic solution concentration used in osmotic dehydration resulted in greater mass transfer, which is already expected because of the increase in the gradient between the sample and the osmotic solution. When immersion time was increased to 60 min, despite the osmotic solution become diluted because of water entering the solution, solid gain and moisture diffusion increased. After this time, water loss was less increased and solid gain was more increased. Further solute uptake creates a resistant layer against permeability and solute movements on both sides of tissue. The effect of increasing the solution temperature of a highly concentrated solution on mass transfer was attributed to a decrease in the viscosity of osmotic solution, resulting in high diffusion rates for both water and solids. But in this process, at temperature of 45 °C, greater the mass transfer rate achieved, mainly due to the increase in cell permeability and permeability coefficient. It was found that salt concentration of 12%, immersion time of 60 min and temperature of 45°C gave the minimum solid gain (2.83%) and maximum water loss (29.36%) and weight reduction (24.53%). Also, the use of ultrasound with pulse ratio of 1:1 in constant osmotic dehydration condition was conducted better results. At 40 min processing time, the solid gain, water loss and weight reduction were 2.93%, 41.23% and 40.30%, respectively. Moisture content of mushrooms for osmotic dehydration and osmotic dehydration accompanied with ultrasound treatment reduced to 85 and 82 %, respectively.

Osmotic process can be used for dehydration of vegetables by placing them in a salt solution. Osmotic dehydration can be used as an effective method of preservation to produce high quality products with lower thermal degradation and to reduce the energy requirements. Also, the use of ultrasound with pulse ratio of 1:1 during osmotic dehydration could accelerate and facilitate transport of water and reduce processing time of osmotic dehydration.