FREEZE DRYING CHARACTERISTICS OF MUSHROOMS
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Date
2018
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Publisher
Acharya N.G. Ranga Agricultural University
Abstract
Mushroom is a macro-fungus with a distinctive fruiting body, which can be
hypogeous or epigeous, large enough to be seen with the naked eye and to be picked by
hand. Mushroom has been valued throughout the world as both food and medicine for
thousands of years. Button mushroom (Agaricus bisporus L.) is the most widely
cultivated and consumed mushroom in the world. It contributes about 90 per cent of
total country’s production as against its global share of about 40 per cent. Button
mushrooms are highly perishable due to their high moisture content. Once the fruiting
body matures, degradation process starts and it becomes inconsumable after some time.
Due to its short shelf-life, mushroom is usually dehydrated for preservation. Drying of
mushrooms by conventional methods is generally practiced to reduce water activity, but,
results in nutritional losses, characteristic colour deterioration and structure
deformation. To overcome these problems, freeze-drying of mushroom slices was
investigated.
White button mushrooms after cleaning were vertically cut into 2, 4, 6 and 8 mm
thick slices. Sliced mushrooms were frozen at −20 °C and then subjected to the freeze
drying at various heating plate temperatures of 10, 20, 30 and 40 °C. Weight reduction
values were recorded during the drying to determine the drying characteristics such as
moisture content (% d.b.), moisture ratio and drying rate (% d.b. min-1). The
experimental moisture ratio data were fitted to the five thin layer drying models. Three
statistical parameters, namely coefficient of determination (R2), reduced-χ2and root
mean square error (RMSE), were used to test the mathematical models. Freeze dried
mushroom slices were rehydrated in water at 20, 40 and 60 °C. The effect of slice
thickness and heating plate temperature on physicochemical properties like bulk
density, porosity, firmness, shrinkage, water activity, colour, ascorbic acid, protein,
dietary fiber and microbial properties like total bacterial, yeast and mould were
evaluated immediately after freeze drying and during the storage.
Moisture content of the fresh slices was found to be in the range of 1547 to
1427% (d.b.) and it was reduced to less than 5.41% (d.b.) after the freeze drying at
various heating plate temperatures. Freeze drying of mushroom slices took place
entirely in falling rate period. Moisture content, moisture ratio and drying rate decreased
continuously with an increase in the heating plate temperature and decrease in the slice
thickness. All the models gave the best fitting results, however, the Page model showed
lower RMSE (0.0016), reduced- χ2 (3.52x10-4) and highest R2 (1.000) value. Samples
with lower thickness and dried at higher plate temperature exhibited higher rehydration
ratio (RR). All the samples rehydrated at 20 °C exhibited the highest RR values.
Increase in slice thickness resulted in higher bulk density (0.1524 g cm-3), firmness
(8.98 N), water activity (0.311), shrinkage (5.32%), ascorbic acid (25.95 mg/100 g) and
protein content (31.02%), but, lower porosity (79.85%) and rehydration ratio (2.89).
The increase in heating plate temperature resulted in increase in rehydration ratio
(6.3067), porosity (89.31%), firmness (8.98 N) and colour change, but, decrease in
shrinkage (1.8%), bulk density (0.07 g cm-3), ascorbic acid (16.18 mg/100 g), protein
(24.58%) and dietary fibre content (16.23%). No microbial growth was observed in
freeze dried mushroom slices. Firmness, rehydration ratio, porosity, ascorbic acid and
protein content of all the samples decreased during the storage, whereas, the water
activity increased with the storage period. This may be due to the absorption of moisture
during the storage. Microbial analysis indicated by yeast count, mould count and total
plate count was not detectable during the first three weeks of storage, whereas in the
fourth week negligible growth was observed. This may be due to the low water activity
of stored samples.
Keywords: Slice thickness; Plate temperature; Modeling; Rehydration; Physicochemical
properties
Description
D5554
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