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 textural measurements and by bright-field as well as by Fourier transform infrared microscopical studies and sensory evaluation. Calcium chloride, pectin methylesterase (PME) or crystallized sucrose were used as pretreatment agents before freezing. Calcium chloride and PME treatments were used either at normal air pressure or in a vacuum. In addition, strawberries were dipped in calcium chloride solution after which they were sprinkled with crystallized sucrose. Strawberries were also just sprinkled with crystallized sucrose. Jams made from strawberries treated with CaCl₂and PME in a vacuum or with CaCl₂and crystallized sucrose, respectively, had the highest firmness values (about twice as great as the reference sample). Firmness of jam berries correlated negatively with firmness of jam media, i.e. jams with firmer strawberries had less firm medium. According to microscopical studies, both CaCl₂and PME in a vacuum and CaCl₂and sucrose pretreatments, respectively, affected the microstructure of strawberry tissues. These pretreatments seemed to stabilize the vascular tissue and to affect pectin, protein and structural carbohydrate. The use of a vacuum seemed to affect the pretreatment solutions, affording more effective absorption to the cortex and pith and providing stabilization there, especially for pectin and structural carbohydrate. According to sensory evaluation of the jams, different prefreezing treatments were shown to have a significant influence on the sensory attributes evaluated. The textural attributes in particular were statistically significantly different among the strawberry jams: wholeness of the berries in the jam (P<0.001), firmness (P<0.001) and clarity (P=0.001) of the jam medium as well as redness of the jam colour (P<0.05) were different among the strawberry jams analysed.