Abstract. Qualitative changes of berries during freezing have been investigated with instrumental methods. In order to economize energy it is proposed to determine the freezing temperature of products. The objective characterisation of physical properties of products enabled to evaluate the quality, to incorporate it into the technological regimes of the treatment. Numerical results of process modelling are shown. The freezing regime is defined by several thermo-physical parameters of the production. Water stage turning into ice essentially affects the quality of the frozen product. In products ice crystal formation takes place at a relatively low temperature. This susceptibility of sugar containing products to temperature fluctuations possibly is due to their high concentrations of unfrozen water and lower melting temperature. It is possible to calculate the mass fraction of ice depending on the temperature and moisture content in product what is very important for predicting the product quality. Experimental investigation was carried out at the Institute of Horticulture, Latvia University of Agriculture.

The great significance of berries in human's diet is determined by the biologically active substances in them. Good quality is one of the main issues in every stage of freezing. It is necessary to research suitability of different berries cultivars to freezing, as well as influence of freezing on quality of the final product. Food quality and safety issues depend on the quality of raw stuffs and halffinished products. Research was carried out at the University of Agriculture, strawberries and blackcurrants grown in Latvia were used in the research. The freezing dynamics of fruits is characterised by temperature measurements in a layer and on its surface. Experience shows that only few products need very quick freezing, such us berries, whose quality is largely influenced by freezing rate. The aim of the research is the quality evaluation of processed products made of fresh and frozen berries. Desserts prepared from berries are products, quality of which depends on the storage conditions of half-finished products. The nutritive value of biologically active substances is retained, while the organoleptic indices worsen only a bit. The objective characterization of physical properties of products allowed to evaluate quality and to incorporate it into technological regimes of the treatment. Also new information was gained on the freezing processes that are mutually interconnected. It provides scientifically practical evidence for some technological developments of qualitative improvement in berry freezing methods and their usage range.

The aim of the study was to evaluate the effect of freezing, thickening and storage on the physical properties of the strawberry desserts made with addition of various hydrocolloids. The strawberry cultivars ‘Polka’, ‘Honeoye’, and ‘Senga Sengana’ harvested in Latvia were used in the study. The strawberries were processed in a blender for obtaining a homogenous mass. The strawberry mass was analyzed fresh and after storage at -18 °C. Both fresh and frozen strawberry mass samples with added gelatine, xanthan gum, and sodium carboxymethyl cellulose were whisked until obtaining dessert. For determination of the optimum hydrocolloid amount, sensory evaluation was done using a five-point hedonic scale. Moisture, soluble solids content, density, and pH of the product were determined. Apparent viscosity was measured using a rotational viscometer DV–III Ultra. Consistency of the dessert during its storage was evaluated using a texture analyser TA.XT.plus. The results of sensory evaluation allow concluding that in order to provide the most acceptable consistency, colour, and taste of strawberry dessert, the optimum quantity of carboxymethyl cellulose and xanthan gum to be added is 0.6% and that of gelatine is 6%. During freezing, the soluble solids content and pH level decreased in the strawberry mass, whereas the addition of hydrocolloids increased these values. The moisture content in strawberry puree, in its turn, as a result of freezing increased under the influence of syneresis, but added hydrocolloids, on the contrary, decreased this influence. Also the viscosity and consistency of the strawberry puree prepared from the researched cultivars decreased under the influence of freezing, whereas the addition of hydrocolloids increased both the values, making the strawberry dessert firmer. The research suggests that strawberry dessert with gelatine had the most pronounced changes during storage, but strawberry dessert with added xanthan gum and carboxymethyl cellulose changed the least.

Contributed by Dalija SegliòaThe

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