Extended storage of cold raw milk on yogurt manufacturing

Rafael Fagnani,Bruno Garcia Botaro,Josiane Schuck,Fernando Cesar Dos Santos

doi:10.1590/s0100-204x2017000200004

Abstract

Abstract: The objective of this work was to evaluate the effect of the extended cold storage of raw milk on the kinetics of fermentation on yogurt production, as well as on the product’s microbiological and physicochemical properties during shelf life. Three treatments were evaluated: yogurts made with raw milk stored for 4, 72, and 168 hours. Kinetics of fermentation was assessed through the lactic acid production rate, growth rate of Streptococcus thermophilus and Lactobacillus bulgaricus, and time to reach pH 5.0 and 4.5. The physicochemical and microbiological characteristics of raw milk and yogurts were also analyzed during cold storage at 4°C. The microbial quality of raw milk was affected by the storage time of 168 hours, with a decreasing tendency in the mesophiles:psychrotrophs ratio. Extended storage of raw milk beyond 72 hours negatively affects yogurt production, despite the low initial bacterial count, decreasing lactic acid production, S. thermophilus growth rate, pH, and protein content. Therefore, to optimize fermentation and yogurt shelf life, the maximum storage time for raw milk at 4oC should not exceed 72 hours.

Highlights

Milk cooling in dairy farms and the bulk hauling system are common practices within the dairy production chain of many countries to maintain the microbiological quality of raw milk until arrival at the industry
It has been reported that raw milk with high bacterial counts, especially of psychrotrophic microorganisms, may impair the production and yield of yogurts, reducing the products’ shelf life and acceptance
The cleavage of the fat globule membrane by phospholipases to glycerol and fatty acids leads to milk fat degradation (Lopez et al, 2010). That these studies are mainly focused on cheese yield, not on how raw milk storage can affect the quality of processed yogurts related to fermentation and shelf life (Leitner et al, 2008)

Summary

Introduction

Milk cooling in dairy farms and the bulk hauling system are common practices within the dairy production chain of many countries to maintain the microbiological quality of raw milk until arrival at the industry. It was observed that the cold storage of raw milk results in casein dissociation, in the release of inorganic calcium from the micelles, and in the increased activities of plasmins and associated enzymes, i.e., proteases and lipases, affecting shelf life and the quality of heat-processed dairy products (Malacarne et al, 2013). The cleavage of the fat globule membrane by phospholipases to glycerol and fatty acids leads to milk fat degradation (Lopez et al, 2010) It should be noted, that these studies are mainly focused on cheese yield, not on how raw milk storage can affect the quality of processed yogurts related to fermentation and shelf life (Leitner et al, 2008)

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Results