This study was designed to produce the yogurt ice cream containing probiotic
microorganisms with the recommended levels (106-107) of live cells at the time
consumption. The mixture was supplemented before freezing with prebiotics (inulin
and oligofructose) and cryoprotectant (glycerol) to see their (prebiotics and glycerol)
effects on the survival of Streptococcus thermophilus, Lactobacillus bulgaricus,
Lactobacillus acidophilus and Bifidobacterium lactis during freezing process and in
storage period. Along with bacterial population, the effects of prebiotics and glycerol
supplementation on physicochemical properties like air holding capacity, fat
components, protein contents, total solids, hardness, stickiness, melting rate, glass
transition, air cell size and ice crystal size were also investigated. Glass transition
temperature was analyzed in each treatment mixture before freezing with differential
scanning calorimeter. The results from the data obtained at various stages of study
have shown different variation pattern for each property.
The initial count of each bacterium before freezing in all treatments with in
experiment was similar and during the freezing process there was non-significant
change in bacterial population. During the storage period at -20°C in the first three
weeks there was less loss in all the samples (P<0.05). With the passage of time the
death rate is increased in all the samples but this decrease was very less with
supplementation as compared to control samples (P<0.05). In the prebiotic yogurt ice
cream the greatest loss was observed in L. acidophilus (P<0.05), while the S.
thermophilus concentration was the maximum among all the bacteria (P<0.05). At the
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end of 12 weeks storage period all the bacteria maintained the minimum required
(106-107 CFU/g) concentration (P<0.05). The addition of prebiotics and glycerol has
increased the total solids of all the samples (P<0.05) that would ultimately led to
more overrun percentage. The supplementation of prebiotics and glycerol have shown
non-significant effect on the fat quantity while decreased the protein concentration
significantly (P<0.05). Fat and protein contents remained unchanged during the
storage period of 90 days (P<0.05). The hardness increased with prebiotics and
decreased with glycerol supplementation, while the stickiness increased with the
increasing prebiotics and glycerol concentration (P<0.05). The melting rate has
shown different behavior although the dry matter contents increased with prebiotics
and glycerol but it did not support the slow melting (P<0.05). Prebiotics show less
effect on glass transition temperature, the increase was very less almost to 1°C.
Glycerol has shown most of the effect and it decreased Tg to near about 10°C in 4%
supplemented samples (P<0.05). The overrun percentage show most of its effect on
probiotics as these bacteria are anaerobic and grow best in absence of oxygen, but the
addition of glycerol minimized its effect on survival rate of the bacteria. The overrun
have shown no effect on total solids, fat and protein level but it decreased the melting
rate at 22 °C. The air act as insulator and prevent the melting of yogurt ice cream
(P<0.05). The hardness and stickiness also decreased with increasing level of overrun
(P<0.05). The prebiotics and glycerol supplementation have shown non- significant
change in air cell size and ice crystal size while overrun percentage has significantly
decreased the air cell and ice crystal sizes (P<0.05).
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In conclusion, the addition of prebiotics and glycerol increased the survival
rate by decreasing the freeze damage caused by large ice crystal formation and also
improved the physicochemical properties of yogurt ice cream.