Extended shelf life milk
ESL milk bridges the gap between high temperature short time (HTST) pasteurised milk, with a shelf life of around 10 days under refrigeration, and ultra-heat treated (UHT) milk with a shelf life of at least 3 months at ambient temperature storage (Fitzgerald 2012). Although a generally accepted definition of ESL does not exist, the term has been used to refer to fresh milk with an extended shelf life regardless of the process used(Buckenhüskes 2014). The objective of the product is to combine longer shelf life and better organoleptic characteristics, a combination which lacks in both pasteurised and UHT milk (Rysstad and Kolstad 2006). The milk undergoes treatment in a manner that reduces the microbial count beyond normal pasteurisation, is packaged under extreme hygiene conditions, and has a defined prolonged shelf life under refrigeration conditions (Rysstad and Kolstad 2006; Lorenzen et al. 2011). Since the same conditions may be achieved by different temperature/time profiles, ‘ESL’ is an umbrella term for many different types of milk which also vary with regard to composition and flavor (Grabowski et al. 2013). Various processing schemes of ESL milk have been described by several authors ranging from high heat treatment for a few seconds to coupling pasteurisation with a non-thermal process. Buckenhüskes (2014), listed five available methods of ESL milk processing, while other researchers have only classified them as two methods (Rysstad and Kolstad 2006; Lorenzen et al. 2011; Grabowski et al. 2013). Apart from bacterial count reduction due to various techniques applied in ESL milk, the longer shelf life of ESL milk is also a result of reduced post process contamination due to the use of aseptic packaging as illustrated in Figure 2.2.
Literature has indicated that the high heat treatment method of ESL milk is based on 123-127 oC for 1-5 s or 135 oC for 0.5 s (Mayr et al. 2004a; Britz and Robinson 2008; Lorenzen et al. 2011). In South Africa, milk is subjected to 135 oC for about 0.5 s and packaged in the conventional manner. Although nothing has been published on the South African ESL milk produced using this method, it has been reported that generally this method causes sensory characteristics problems in the final product (Shmidt et al. 2012). Apart from high heat treatment another commonly used method is a combination of HTST pasteurisation and a non-thermal process such as microfiltration (Hoffman 2006) and bactofugation (Fox and McSweeney 1998; Fox et al. 2015) coupled with aseptic packaging. The main steps in ESL processing using pasteurisation and a non-thermal step for bacteria removal start with the chilling of raw milk, followed by heat treatment, homogenisation, bactofugation or microfiltration and lastly aseptic packaging. In South Africa the dairy industry use the bactofugation based process.
CHAPTER ONE GENERAL INTRODUCTION
CHAPTER TWO . LITERATURE REVIEW
2.1 Milk production and consumption trends
2.2 Extended shelf life milk
2.3 Bacteria associated with milk
2.4 Contamination routes in milk processing
2.5 Adaptation mechanisms of bacteria during milk processing
2.6 Spore-forming bacteria associated with ESL milk
2.7 Advances in detection and identification of microorganisms in the food industry
CHAPTER THREE HYPOTHESES AND OBJECTIVES .
3.1 Hypothesis 1
3.2 Hypothesis 2 .
3.2.1 Objective .
CHAPTER FOUR Bacillus and Paenibacillus spp. associated with extended shelf life milk during processing and storage .
4.1.1 Abstract .
4.1.3 Materials and methods
CHAPTER FIVE Diversity of Bacillus cereus strains in extended shelf life milk
5.1.3 Materials and methods
188.8.131.52 B. cereus 16S rRNA Sequencing
5.1.4 Results ..
CHAPTER SIX Source tracking Bacillus cereus in an extended shelf life milk processing plant using partial sequencing of rpoB and multilocus sequence typing
CHAPTER SEVEN GENERAL DISCUSSION
CHAPTER EIGHT CONCLUSIONS AND RECOMMENDATIONS
CHAPTER NINE REFERENCES
CHAPTER TE PUBLICATIONS AND AWARDS