Survival of Selected Organisms During Spray Drying of Skim Milk and Storage of Nonfat Dry Milk

Abstract

Pasteurized concentrated skim milk containing 35 to 40% total solids was inoculated with cultures of Bacillus subtilis, Micrococcus flavus, or Escherichia coli to contain 1 × 106 organisms per g and spray-dried to determine the effect of exit air temperatures of 93.3, 82.2, and 71.1 C on survival of the organisms and moisture content of the finished product. The numbers of survivors increased as the drying temperature decreased. The percent survivors varied from 27.57 in the product made from milk inoculated with M. flavus and dried at 71.1 C to 0.02 in the product made from milk inoculated with E. coli and dried at 93.3 C. The organism most resistant to drying and most persistent during storage was B. subtilis, followed by M. flavus and E. coli, with the latter showing low survival during drying and abrupt die-off during the first 4 weeks of storage. The moisture content of the dry milks varied from 2.75 to 4.80% with low moisture associated with high drying temperature.

Survival of thermophilic spore-forming bacteria in a 90+ year old milk powder from Ernest Shackelton's Cape Royds Hut in Antarctica

“…4 % after 36 weeks. Of those species that did survive B. subtilis was the most resistant to drying and recovery after storage (Thompson et al 1978). In another investigation performed by Celestino et al (1997) a medium-heat whole-milk powder (which was neither stored under an inert gas nor vacuum) showed no significant difference between the total plate counts of aerobic thermophiles and anaerobic thermophiles over an eight month period.…”

“…The three critical storage parameters enhancing the survival of microorganisms following milk powder production and long-term storage are the water activity, relative humidity and the temperature (Higginbottom, 1969;Thompson et al 1978;Stapelfeldt et al 1997). Extremely dry conditions can lead to morphological changes in cells and generally, a slow reduction of the microbial population (Troller, 1991).…”

“…Extremely dry conditions can lead to morphological changes in cells and generally, a slow reduction of the microbial population (Troller, 1991). Additional contributing factors include the preheat-treatment of the powder (Findlay et al 1946), the milk powder structure, storage conditions (particularly the lack of oxygen), the type of organisms present (Thompson et al 1978 ;Kieseker & Aitken, 1993;Celestino et al 1997) and type and concentration of milk solids, i.e. skim or whole milk (Mazas et al 1999).…”

“…Long-term studies dealing with the numbers of thermophiles with time of storage are relatively few in number. In a study using mesophilic recovery of B. subtilis at 32 8C by Thompson et al (1978), the authors studied the effect of varying temperatures (71 . 1 to 93 .…”

Milk and dairy products

“…During storage of dry milks, some vegetative microorganisms present may slowly die-off (Thompson et al, 1978) but others may survive over prolonged periods. Spore-formers, being the most resistant, retain viability indefinitely.…”