Thirty milk samples [high-temperature short-time (HTST) milk, extended shelf life (ESL) milk (directly heated, indirectly heated, microfiltered), ultra-high temperature milk] from 17 German dairies were analysed. Total viable counts of directly or indirectly heated ESL milk were significantly lower than those in microfiltered ESL and HTST milk. Evaluation of indigenous enzyme activity revealed sufficient heat treatments in all milk samples. The manufacturing processes were differentiated by estimating furosine and acid soluble whey proteins. Sensory examinations revealed a preference for HTST heated and microfiltered ESL milk. However, a significant discrimination of drinking milk types was not possible. Vitamin losses were not detected, and concentrations of vitamins in different types of milk were comparable.Keywords Drinking milk, Microbiological status, Head load evaluation, Sensory quality, Vitamin status.
I N T R O D U C T I O NExtended shelf life (ESL) milk has a shelf life of about 3 weeks under chill chain conditions and fills the gap between high-temperature short-time (HTST)-heated milk, which typically is assigned a shelf life of 10 days and ultra-high temperature (UHT)-heated milk, which can be stored for a few months without cooling (Hoffmann et al. 2006). In Germany, ESL milk has a market share of 20-25%. For comparison, UHT milk has a share of approximately 70%, and HTST milk has a market share of 5-10% (BLE 2008). The term ESL milk as well as the manufacturing process is not legally defined in the European Union. A thermal treatment or combination of heat treatment and membrane filtration are involved in the production of ESL milk. The thermal process requires direct or indirect heating at 123-127°C with a holding time of 1-5 s. Traditional HTST pasteurisation is carried out at 72-75°C for 15-30 s, and UHT milk is heated at a minimum of 135°C for a few seconds (sterilisation value F 0 ‡ 3 min) (Schwermann and Schwenzow 2008a,b;Kaufmann et al. 2009). The combined treatment process for ESL milk includes microfiltration of skim milk through ceramic membranes with an average pore diameter of 0.8-1.4 lm. As a result, a spore reduction of 3-5 log 10 steps is achieved and most other forms of microorganisms are also separated. The so enriched retentate and a specific amount of cream are heated at 123-127°C, homogenised, and mixed with the HTST heated skim milk permeate (Hoffmann et al. 2006;Schwermann and Schwenzow 2008c;Henke 2009).Microbial spoilage of heat treated drinking milk is caused by heat-resistant or recontaminating micro-organisms. The former group is represented by spore forming micro-organisms, mainly Bacillus spp. and Enterococci. The latter group consists of Gram-positive nonspore forming bacteria and Gram-negative bacteria. These organisms gain significance if they are psychrophilic or psychrotrophic and able to multiply at refrigeration temperatures (Blake et al. 1995;Mayr et al. 2004;Kress et al. 2005;Kaufmann and Kulozik 2008).Several parameters are suited for the evaluation of milk...
