501. The effect of storage at different relative humidities on the survival of micro-organisms in milk powder and in pure cultures dried in milk

Abstract

With 7 Figures)The survival of the microflora during the storage of dried milks has been shown (l) to vary considerably from sample to sample even when the bacteria appeared to be of similar types and the milks were dried in the same plant. The variation may have been due to differences in the moisture content of the dried milk, but no data were available with which to test this hypothesis. Preliminary experiments with three different types of milk powder showed that over the limited range of humidity used, the lower the relative humidity in which the milk powders were stored, the greater the survival of the bacteria contained in the powder. The scope of the investigation was then extended to cover a wider range of relative humidity and longer periods of storage. It was found that although in general the higher the humidity (or moisture content) the lower the survival rate of the microflora in the milk powder, survival was greater at 5-20 % relative humidity than at humidities above and below this level. A brief account of part of this work has been published elsewhere (2).The fact that survival was greatest at 15-25 % E.H. had been observed by Watts (3) for dried cultures of Streptococcus agalactiae. To determine whether this was a characteristic common to several types of bacteria the investigation was continued to include bacteria typical of the microflora of milk powders. Pure cultures of these bacteria in skim milk were dried and stored at varying relative humidities. Maximum survival was found at 5-20% R.H.Dried suspensions of bacterial spores have been shown to be markedly sensitive to the complete absence of humidity, although their viability was unaffected by exposure to 5 % R.H. for two years. METHODS Milk powdersWeighed portions of approximately 1 g. milk powder were spread in a thin layer in small Petri dishes (2 in. diam.) and placed in desiccators over aqueous sulphuric acid solutions of the specific gravities necessary to give the required relative humidities (4). The lids of the Petri dishes were raised slightly with small pieces of rubber tubing to allow free circulation of the air in the desiccator. At intervals two dishes were removed and immediately examined for moisture content and plate count. The moisture content was calculated as a percentage of the dry weight after heating at 100° C. for 3 hr.The plate count, after reconstitution at room temperature, was made on Yeastrel milk agar at 37° C. and was calculated per g. of fresh powder. The relative numbers of J. Dairy Research 20 5

Spray‐drying for the production of dried cultures

“…Working with spray-dried Lactococcus and Micrococcus strains isolated from milk, and with a typical strain of Bac. subtilis, Higginbottom (1953) showed that during storage at RHs from 80% to 100% a rapid fall in the numbers of viable bacteria was followed by rapid growth of bacteria and by overgrowth by moulds. At RHs below 80%, the number of surviving bacteria increased with decreasing humidity to maximum survival at about 10% RH but then tended to fall again towards 0% RH.…”

Evaluation of the desiccation tolerance of blastospores ofPaecilomyces fumosoroseus(Deuteromycotina: Hyphomycetes) using a lab-scale, air-drying chamber with controlled relative humidity

“…This led to our development of an air-drying chamber where we could control humidity while moving air over our blastospore preparations to remove excess water. Although controlled humidity environments can be developed with salt solutions of various concentrations, the drying process is slower and may result in unwanted microbial contamination particularly when many samples or samples of large mass are to be dried (Higginbottom 1953). A reasonably rapid method of drying living cells for use as biocontrol agents was needed to reduce unwanted microbial contamination and to lower drying costs.…”

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

“…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).…”

“…Overall, a decrease in the total microbial content occurs after prolonged storage of dried milk powders, with endosporeforming bacilli being able to survive both the heattreatments during powder production and the resulting desiccation, and are thus, the most likely to be isolated after prolonged storage (Keogh, 1966). Indeed, in a study using pure cultures of B. subtilis spores added to milk before drying no change in the viable count was observed up to 110 weeks at relative humidities between 5 and 50 % (Higginbottom, 1969). Long-term studies dealing with the numbers of thermophiles with time of storage are relatively few in number.…”

“…Studies on the effect of storage conditions on the longevity of microorganisms in milk powder, or their spores in the case of bacilli, have shown that the storage temperature, total solids content and the relative humidity are important factors (McDonough & Hargrove, 1968;Higginbottom, 1969;Reddy et al 1975;Mercurio & Tadjalli, 1979). With regard to possible freeze-thaw cycles affecting the long-term survival, Bacillus endospores are known to possess both extreme resistance to long-term desiccation and to multiple cycles of freezing and thawing (Nicholson et al 2000).…”