Osmotic Dehydration Pretreatment Research Articles

Influence of osmotic pre-treatment and microwave application on properties of air dried strawberry related to structural changes

Strawberry halves, with and without osmotic pre-treatment, were dried at 40 °C air temperature, with and without microwave application (0.2 W/g). Changes in volume, pectic fractions, mechanical behaviour and glass transition temperature due to dehydration treatments were evaluated. Microwave application resulted in an increased water soluble pectic fraction as a consequence of both protopectin and oxalate soluble pectic fraction reductions. This solubilisation leads us to obtain dried samples with a more rigid, firmer structure which, indeed, showed greater glass transition temperature values. Nevertheless, these samples are softer when rehydrated, thus indicating a greater structural damage. The sugar presence induced by an osmotic dehydration pre-treatment seems to reinforce the solid cellular matrix, which is evident from the mechanical behaviour of rehydrated samples. So, the greatest structural damage caused by microwaves may be reduced by an osmotic dehydration pre-treatment.

Osmotic Dehydration Pretreatment in Drying of Fruits and Vegetables

Several vegetables and fruits, apple, ginger, carrot, and pumpkin were dehydrated under various osmotic conditions using sucrose and salt as the permeating agents. The dehydrated materials were then dried. The influence of solute concentration, process temperature and the type of solute on osmotic dehydration and further thermal drying were investigated. The nutrition loss during the osmotic process was measured using carotene as the nutrition index. The effect of calcium chloride present in osmotic solution on the product quality was also studied. A first order kinetic model was chosen to describe the mass transfer phenomena of the osmotic process. The equilibrium value of water loss, solute gained, kinetic constants K WL and K SG under various conditions are successfully predicted by the model. The relationship between the equilibrium value and four major factors that influence osmotic process of carrot was obtained based on the experimental data. The relations between the loss constant of carotene and the solute concentration in carrot and pumpkin were obtained based on the experimental data. The qualities of dried products are better for the osmotic dehydration pretreated samples than those dried directly.

DEEP FAT FRYING OF POTATO STRIPS—QUALITY ISSUES

Moisture loss and oil adsorption kinetics, structural properties (apparent density, true density, specific volume and internal porosity), color changes and viscoelastic behavior (compression tests, crispness) were investigated during deep fat frying of french fries. The effect of frying conditions (oil temperature, sample thickness and oil type), drying pretreatment and osmotic dehydration pretreatment on the above properties was also examined. The results showed that oil temperature and thickness of potato strips have a significant effect on oil uptake, moisture loss and color parameters of french fries, while the use of hydrogenated oil in the frying medium does not affect these properties. The porosity of french fries increases with oil temperature increases and sample thickness and it is higher for products fried with hydrogenated oil. Maximum stress and maximum strain increase during frying, while crispness of potato strips is higher for hydrogenated oil, and lower for refined oil. Air drying and osmotic pretreatment increase porosity of fried potatoes but decrease their oil and moisture content. A negative effect on color development with drying time was also observed. Pre-fry drying as well as osmotic pre-treatment increases the maximum stress and maximum strain of french fries during frying. Air drying pre-treatment increases the crispness of potato strips while osmotic pre-treatment does not affect it, with the exception of sugar solutions.

Effect of osmotic dedydration pretreatment on quality of french fries

Osmotic dehydration as a pretreatment before frying in order to produce low-fat french fries was examined. A relationship between frying kinetics and the type of solution used for osmotic dehydration before frying of french fries has been developed. The effect of osmotic dehydration on the structural properties (apparent density, true density, specific volume and internal porosity) and color parameters (lightness, a, b) of french fries was also investigated. Four types of solutions (sugar, NaCl, maltodextrine 12 and maltodextrine 21) were used. The results showed that osmotic pretreatment has a significant effect on oil uptake and moisture loss of french fries, as well as on the structural properties and color parameters of french fries. Osmotic dehydration pretreatment decreases oil and moisture content of french fries, while the porosity and intermit of color increased.

Glass transitions and shrinkage during drying and storage of osmosed apple pieces

The relationship between shrinkage during drying and ‘glass’–‘rubber’ transitions of apple tissue was studied. Our results did not indicate that structural collapse of apple pieces can be reduced by diminishing the difference Δ T between drying temperature and glass transition temperature ( T g) of the fruit matrix. Apple shrinkage was not consistently depressed by reducing drying temperature, infusing larger amounts of T g-depressing agents (sucrose, maltose, lactose) or infusing similar amounts of solutes with higher anhydrous T gs. However, sugars added during osmotic dehydration pretreatments help decreasing structural collapse during subsequent air drying, which was reflected by a 20–65% increase in final volume of samples treated with 50% sucrose and maltose solutions as compared to an air-dried, control sample. Dried samples remained in the “rubbery” state after drying and shrunk during subsequent storage.