Effect of thermization and storage on the nutrition properties and shelf-stability of yoghurt from blends of cowmilk and coconut milk.

Thesis Abstract

Yoghurts were produced by blending reconstituted powdered cow milk (100 g of powdered
cow milk (CM) 0.5 litres of water) with 25 % and 50 % coconut milk (CCM) and a control
(100 % cow milk) before fermentation. After fermentation for 16 h at room temperature, the
yoghurts were subjected to two heat treatments (thermization) namely, 75 0C thermization
and 80 0C thermization and a control (unthermized). The proximate, physicochemical,
vitamin and microbiological quality of yoghurt samples were investigated during storage at
room temperature for 5 weeks. The results obtained, revealed that in each of the three
formulations of yoghurt, proximate composition, titratable acidity (TTA), total solids, free
fatty acids (FFA), viscosity, microbial load and vitamins decreased as the temperature of
thermization increased.
The pH and moisture increased from 4.30 and 82.36 %, respectively
with increase in thermization temperature to 4.59 and 83.39 %, respectively. Yoghurts
samples from 50 % cow milk (CM) + 50 % coconut milk (CCM) had the highest moisture
content (83.39 %) and lowest total solid content (17.84 %) compared to 82.98 % and 18.16
%, respectively from 75 % cow milk (CM) + 25 % coconut milk (CCM) and 82.92 % and
18.28 %, respectively from 100 % cow milk (CM). Titratable acidity increased significantly
(p < 0.05) with increase in percentage coconut milk ( 0.77 for 100 % cow milk (CM), 0.82 for
75 % cow milk (CM) + 25 % coconut milk (CCM) and 0.87 for 50 % cow milk (CM) + 50 %
coconut milk (CCM)). There was significant (p < 0.05) decrease in pH as the percent coconut
milk increased (4.48 for 100 % cow milk (CM), 4.40 for 75 % cow milk (CM) + 25 %
coconut milk (CCM) and 4.30 for 50 % cow milk (CM) + 50 % coconut milk (CCM)). The
highest viscosity of 282.59 ± 2.46 cPs was obtained for unthermized 100 % cow milk which
also had the highest protein content of 3.84 ± 0.11 %. Free fatty acid increased significantly
(p < 0.05) with increase in percent coconut milk and during storage. B-complex vitamins (B1,
B2 and B3) increased significantly (p<0.05) as the storage time increased while the
antioxidant vitamins (vitamin C and E) decreased significantly (p < 0.05). Vitamin A was not
detected in any of the samples. Total bacterial count increased significantly (p < 0.05) from
2.27 x 102 cfu/ml ± 0.14 to 6.77 x 102 cfu/ml ± 0.10 on storage within 3 weeks. Thereafter,
their numbers reduced. Fungi were detected in all the yoghurt samples but they reduced after
one week of storage. Sensory results indicated that yoghurts with coconut milk (CCM) and
those thermized at 80 0C had significant (p < 0.05) lower consumer preferences.