Impact of post fermentation cooling patterns on fatty acid profile, lipid oxidation and antioxidant features of cow and buffalo milk set yoghurt
Authors: Khan Imran Taj , Nadeem Muhammad , Imran Muhammad , Khalique Anjum 
Published: April 2020

Abstract
Background
In the manufacturing of set yoghurt, after reaching 4.6 pH, post fermentation cooling is applied to stop the bacterial activity. Depending upon the required textural and flavor attributes, one phase and two phase cooling patterns are accordingly selected. In one phase cooling, temperature of the yoghurt is rapidly decreased below 10 °C using blast freezing and then it is gradually dropped to 4-5 °C. In two phase cooling, temperature of yogurt is rapidly decreased to less than 20 °C and then it is gradually decreased to 4-5 °C. These cooling phases have a significant impact on textural and flavor perspectives of yoghurt. It is necessary to study the impact of industrially adopted cooling patterns on fatty acid profile, antioxidant characteristics, lipid oxidation and sensory characteristics of cow and buffalo milk set yoghurt.

Methods
This experiment was organized in a completely randomized design and every treatment was replicated five times to minimize the variation. Whole cow and buffalo milk without any standardization were converted to set yoghurt (400 g cups) using Strepotococcus thermophillus and Lactobacillus bulgaricus as starter bacteria. After reaching 4.6 pH, cow and buffalo yoghurt samples were exposed to three different cooling patterns. In first trial, samples of cow and buffalo yoghurt were cooled from 43 °C to 25 °C in 1 h and finally cooled to 4-5 °C in another hr. (T1). In second trial, samples were cooled from 43 °C to 18 °C in 1 hr. and finally cooled down to 4-5 °C in another 1 h. (T2). In third trial, samples were cooled from 43 °C to 4-5 °C in 2 h (T1). Alteration in fatty acid profile, total antioxidant capacity, reducing power, free fatty acids, peroxide value, conjugated dienes, vitamin A, E, color and flavor of cow and buffalo yoghurt samples were assessed for 20 days at the frequency of 10 days.

Results
All the three cooling patterns had a non-significant effect on compositional attributes of yoghurt. Buffalo milk yogurt had higher percentage of fat, protein and total solids than yoghurt prepared from cow milk (p < 0.05). At zero day, DPPH free radical scavenging activity of T2 and T3 was significantly higher than T1. This may be due to the longer exposure of T1 at relatively higher temperature than T2 and T3. Effect of storage period up to 10 days was non-significant in T2 and T3. Reducing power of cow and buffalo milk yoghurt was also significantly affected by the cooling patterns applied. Reducing power of T2 and T3 was considerably higher than T1 (p < 0.05). At zero-day, total antioxidant capacity of cow and buffalo milk yoghurt in T3 was 42.6 and 61.4%, respectively. At zero day, total antioxidant capacity of T2 and T3 was significantly higher than T1. Effect of storage on total antioxidant capacity of T2 and T3 remained non-significant till 10 days of storage. At zero day, the impact of cooling patterns on fatty acid profile of T1, T2 and T3 was non-significant, whereas, storage period had a marked impact on fatty acid profile. After 10 days, T1 was considerably different in fatty acids from T2 and T3. After 10 days of storage of cow milk yoghurt in T1, concentration of C4:0, C6:0, C8:0, C10:0, C12:0, C14:0, C16:0, C18:0, C18:1 and C18:2 decreased by 0.1, 0.11, 0.09, 0.07, 0.21, 0.38, 0.28, 0.27, 0.44 and 0.06%, respectively. Cow milk yoghurt in T1 after 10 days of storage, concentration of C4:0, C6:0, C8:0, C10:0, C12:0, C14:0, C16:0, C18:0, C18:1 and C18:2 decreased by 0.07, 0.15, 0.04, 0.17, 0.20, 0.34, 0.27, 0.36 and 0.04%, respectively. After 10 days of storage in T2 and T3, loss of fatty acids was 1.2 and 3.61% from C4:0 to C10:0, respectively. Milk type had no effect on peroxide value of yoghurt. Cooling of cow and buffalo yoghurt from 43 °C to 25 °C had a pronounced effect on peroxide value. At zero day, peroxide values of cow and buffalo yoghurt in T1 were 0.32 and 0.33 (MeqO2/kg). At zero day, peroxide value of cow and buffalo yoghurt in T2 were 0.24 and 0.26 (MeqO2/kg). At zero day, peroxide value cow and buffalo yoghurt in T3 were 0.23 and 0.25 (MeqO2/kg). Cooling patterns i.e. from 43 °C to 25, 18 and 5 °C (T1, T2 and T3) had a significant effect on the amount of vitamin A and E. Concentration of vitamin A and E in T1 were significantly less than T2 and T3. Cooling patterns had a significant effect on texture, T1 had a thick texture with higher viscosity as compared to T2 and T3. Thickness of yoghurt was in the order of T1 > T2 > T3 with no difference in color and flavor score till 10 days of storage.

Conclusion
Results of current investigation indicated that milk type and post fermentation cooling patterns had a pronounced effect on antioxidant characteristics, fatty acid profile, lipid oxidation and textural characteristics of yoghurt. Buffalo milk based yoghurt had more fat, protein, higher antioxidant capacity and vitamin content. Antioxidant and sensory characteristics of T1 were optimum till 10 days of storage.