The Biodiversity of the Microbiota Producing Heat-Resistant Enzymes Responsible for Spoilage in Processed Bovine Milk and Dairy Products

Abstract

Raw bovine milk is highly nutritious as well as pH-neutral, providing the ideal conditions for microbial growth. The microbiota of raw milk is diverse and originates from several sources of contamination including the external udder surface, milking equipment, air, water, feed, grass, feces, and soil. Many bacterial and fungal species can be found in raw milk. The autochthonous microbiota of raw milk immediately after milking generally comprises lactic acid bacteria such as Lactococcus, Lactobacillus, Streptococcus, and Leuconostoc species, which are technologically important for the dairy industry, although they do occasionally cause spoilage of dairy products. Differences in milking practices and storage conditions on each continent, country and region result in variable microbial population structures in raw milk. Raw milk is usually stored at cold temperatures, e.g., about 4°C before processing to reduce the growth of most bacteria. However, psychrotrophic bacteria can proliferate and contribute to spoilage of ultra-high temperature (UHT) treated and sterilized milk and other dairy products with a long shelf life due to their ability to produce extracellular heat resistant enzymes such as peptidases and lipases. Worldwide, species of Pseudomonas, with the ability to produce these spoilage enzymes, are the most common contaminants isolated from cold raw milk although other genera such as Serratia are also reported as important milk spoilers, while for others more research is needed on the heat resistance of the spoilage enzymes produced. The residual activity of extracellular enzymes after high heat treatment may lead to technological problems (off flavors, physico-chemical instability) during the shelf life of milk and dairy products. This review covers the contamination patterns of cold raw milk in several parts of the world, the growth potential of psychrotrophic bacteria, their ability to produce extracellular heat-resistant enzymes and the consequences for dairy products with a long shelf life. This problem is of increasing importance because of the large worldwide trade in fluid milk and milk powder.

Effects of microwave processing conditions on microbial safety and antimicrobial proteins in bovine milk

“…Raw milk was shown to contain 1 × 10 3 CFU/ml, as per standard agar plate analysis, with no growth observed on violet red bile agar . The microbial quality of the raw milk is in agreement with values reported in the literature for dairy practices in developed countries (Machado et al, ) and is unlikely to affect heat resistance of tested surrogates (Condron et al, ; Duffy, Whiting, & Sheridan, ).…”

Age Gelation, Sedimentation, and Creaming in UHT Milk: A Review

“…These heat‐resistant proteases are important in age gelation and, therefore, in limiting the shelf life of UHT milk products (Dufour et al., ; Hantsis‐Zacharov & Halpern, ; McKellar, ; Mitchell et al., ; Stevenson, Rowe, Wisdom, & Kilpatrick, ; Stoeckel et al., ). Some of the proteases from psychrotrophic bacteria are extremely heat‐stable, and markedly more heat‐stable than the plasmin system (Baglinière et al., ; Machado et al., ; Stoeckel et al., ). These proteases are not inactivated by the heat treatments used in typical UHT processing, whether direct or indirect, and, potentially, they can partially withstand heat treatments as high as 130 °C for several minutes and even retort‐sterilization conditions (Law, ; Mitchell et al., ; Richardson & Newstead, ; Richardson & Te Whaiti, ; Stoeckel et al., ).…”

“…Of the psychrotrophic bacteria found in milk, species from the Pseudomonas genus are the most common and most widely studied in respect to UHT milk stability; however, other genera such as Bacillus (Janstova, Lukasova, Drackova, & Vorlova, ; Machado et al., ) and Serratia (Baglinière et al., ; Bagliniere, Jardin, Gaucheron, de Carvalho, & Vanetti, ; Machado, Baglinière et al., ) species can also produce heat‐stable enzymes. Several studies have attempted to isolate and characterize the specific heat‐stable extracellular proteases secreted by the psychrotrophic bacteria, especially those from the Pseudomonas genus (Bagliniere et al., ; Bagliniere et al., ; Baur et al., ; Dufour et al., ; Mateos et al., ; Mitchell et al., ).…”

“…However, microbiological stability is not a guarantee that the milk will be acceptable throughout its shelf life. Storage for long periods can result in many chemical and physical changes to the milk, and some of these can results in unacceptable characteristics, or, in severe cases, abruptly end shelf life (Datta & Deeth, ; Deeth & Lewis, , ; Machado et al., ; Nieuwenhuijse & van Boekel, ; Stoeckel et al., ; Stoeckel et al., ).…”

The Extracellular Protease AprX from Pseudomonas and its Spoilage Potential for UHT Milk: A Review

“…The spoilage potential of AprX in UHT milk was studied by Machado et al. (); Matéos et al. (); Stuknytė et al.…”