The effect of different prefreezing treatments on the structure of strawberries before and after jam making

The structural changes in strawberry tissues during prefreezing treatments, freezing, thawing and jam making were studied by means of instrumental tex tural measurements and by bright-field as well as by Fourier transform infr ared microscopical studies and sensory evaluation. Calcium chloride, pectin methylesterase (PME) or crystallized sucrose were used as pretreatment age nts before freezing. Calcium chloride and PME treatments were used either a t normal air pressure or in a vacuum. In addition, strawberries rr cre dipp ed in calcium chloride solution after which they were sprinkled with crysta llized sucrose. Strawberries were also just sprinkled with crystallized suc rose. Jams made from strawberries treated with CaCl2 and PME in a vacuum or with CaCl2 and crystallized sucrose, respectively: had the highest firmnes s values (about twice as great as the reference sample). Firmness of jam be rries correlated negatively with firmness of jam media, i.e. jams with finn er strawberries had less firm medium. According to microscopical studies, b oth CaCl2 and PME in a vacuum and CaCl2 and sucrose pretreatments, respecti vely: affected the microstructure of strawberry tissues. These pretreatment s seemed to stabilize the vascular tissue and to affect pectin, protein and structural carbohydrate. The use of a vacuum seemed to affect the pretreat ment solutions, affording more effective absorption to the cortex and pith and providing stabilization there, especially for pectin and structural car bohydrate. According to sensory evaluation of the jams, different prefreezi ng treatments were shown to have a significant influence on the sensory att ributes evaluated. The textural attributes in particular were statistically significantly different among the strawberry jams: wholeness of the berrie s in the jam (P < 0.001), firmness (P < 0.001) and clarity (P = 0.001) of t he jam medium as well as redness of the jam colour (P < 0.05) were differen t among the strawberry jams analysed.