CHARACTERIZATION OF A HEAT-RESISTANT MESOPHILIC BACILLUS SPECIES AFFECTING QUALITY OF UHT-MILK - A PRELIMINARY-REPORT

Heat resistant mesophilic sporeformers (HRS) first occurred in Germany after the new Milk Ordinance (EC-Directive 85/397, Milchverordnung 23 .6.1989) came into effect. According to these regulations a maximum co lony count of 10 cfu/0.1 ml is required after 15 days incubation of th e packages at 30 degrees C. HRS reach colony counts up to 10(5)/ml wit hout changing milk composition or sensoric properties. The problem now adays is evident in a number of dairies in several countries. A biolog ical characterization of the HRS including morphological patterns as w ell as growth conditions and biochemical properties is presented. The taxonomy is not yet clear. Several working groups are involved using g enetical or phenotypical methods to establish a new taxon group. Produ cts involved are UHT-milk, UHT-cream, chocolate milk and milk powder. Contamination occurs in all types of UHT-equipment of several supplier s. For detection of the HRS only a conventional autoclaving method is available, faster methods as gene probes or PCR are under development. Regarding the time consuming and insensitive detection method the way of contamination as well as the contamination sources are not clear. Possibilities at the dairy and the farm level are discussed, however, problems regarding reprocessing of contaminated UHT-milk or processing of contaminated milk powder are under consideration too. The probably extreme heat resistance of the HRS was investigated by several labora tories. Results are differing, most probably because HRS-strains in na turally contaminated milk seem to be much more resistant than laborato ry cultured strains. With regard to the consumers safety investigation s concerning the pathogenicity of the HRS started as early as the prob lem was known. Pathogenicity tests in mice and embryonized chicken egg s and cytotoxicity tests on different cell lines are presented. At thi s moment there is no evidence for any pathogenic or toxic properties o f the HRS.