Proceedings: Characteristics of freeze-dried cells

Microorganisms have been found to be more sensitive to selective media after freeze-drying. This increased sensitivity can be measured and thus the degree of sublethal injury can be determined as a function of various processing variables. In light of this, the use of selective media for the enrichment and detection of pathogens in freeze-dried products has to be reevaluated; indeed, the literature is now becoming abundant with such evaluations. In addition, the response of freeze-dried microorganisms has been found to be dependent on the medium in which they were grown; the phenomena of “metabolic injury” and “minimal medium recovery” are observed when microorganisms are grown in a complete and minimal medium, respectively. The expression of these two phenomena also can be used to assay for injury.Observations on the effects of freeze-drying on cell viability lead to the conclusion that freeze-drying is a complex stress. Damage to the cellular membrane structure and function, RNA integrity, and, possibly, DNA have been cited. The extrapolation of these macromolecular changes to specific viability responses for the purpose of elucidating the principal site of damage is still difficult. It is our opinion that the pre- and post-freeze-drying conditions to which the microorganisms are exposed can lead to a situation in which a particular macromolecular damage can become dominant over others, depending on the physiology of the cell.This knowledge can not only be applied for the purpose of improving detection of undesirable microbes but also for the preservation of desirable cultures, such as starter cultures in the dairy industry. Finally, the finding that microorganisms leak or release nucleic acids after freeze-drying, as they do after freezing and heating, can be applied to the problem of elimination of undesirable cytoplasmic components of organisms to be used as protein sources (4, 8).