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Abstract
Freeze drying, as relatively new process of drying in vacuum at very low temperatures,
ensures the preservation of all thermo labile compounds in the initial raw material, and
final low content of moisture provides microbiological stability and permanent preservation
of products. Red raspberry, because of relatively high content of water (90%), specific
structure of aggregate fruit, characteristic bright colour and flavour, was not preserved up
to now by conventional drying.
This paper gives an overview on the two different methods of drying fruits: freeze
drying and convective drying.
Raspberry sorts Willamette and Meeker were dried by freeze drying, as well as by
conventional drying with warm air of low relative humidity. Freeze drying was performed
in the device Christ Alpha I/5 under desublimers’ temperature of -55°C and processing
temperature of raw material of -35°C. Fruits of the sort Willamette were dried to a final
moisture content of 18.86%, and Meeker up to 16.15%. Also, some changes in chemical
composition, overall aroma, sensory characteristics, density and volume, water activity,
loss of vitamin C, the degree of rehydration of the dried fruit in comparison to fresh one
were tested. Gained results show that freeze drying can effectively preserve the chemical
composition, volume, colour and aroma of raspberry.In the paper of authors from foreign countries the influence of different technologies for
the products preservation was tested as freezing, freeze drying (or lyophilisation) and
conventional drying. Tests were done with the main goal to determine the qualitative
changes on strawberry, raspberry, gooseberry, elder, apricot, sour cherry, apple and
cornelian cherry. All preservation technologies have reduced the amount of biologically
active compounds, such as vitamin C and phenolic compounds. Related to the chemical
composition of fresh fruit, decrease of tested parameters was affected by freezing for 15%,
freeze drying for 28-32% and conventional drying for 45-48%.
The process of freeze drying is among one of the methods for preservation of raw
materials of plant origin (fruits, vegetables, spices and herbs). Final product has a
high nutritional value (as fresh fruits and vegetables). Freeze dried products do require
special storage conditions (e.g. absence of light, packaging materials with low gas
permeability, inert atmosphere, etc.). In this way dried products represent the basis for
instant soups, bakery, dairy and confectionery products.