No Need to Boil or Bake: Non-Thermal Food Preservation

Cebrián et al. (2016) conducted laboratory studies in conjunction with a literature review in order to define foodborne bacterial pathogen sensitivity to non-thermal food preservation techniques¹. Their results show that for certain bacterial pathogens in specific food matrix environments, non-thermal processing might be a suitable substitute for pasteurization by heat.

Food producers use many different kinds of preservation processes to ensure microbiological food safety, including thermal techniques such as heat and pasteurization. Unfortunately, in addition to rendering foodstuffs microbiologically safe, heating a foodstuff can adversely change physical characteristics; not all foods are suitable for thermal processing. When factors such as taste, aroma, and texture are altered, it often leads to consumer dissatisfaction and rejection.

Non-thermal alternatives to heat treatments do exist but there is less available information on the sensitivity of bacterial pathogens to these processes. Cebrián et al. examined four non-thermal processes (see Table 1) under harmonized experimental conditions in order to classify the sensitivity of common foodborne bacterial contaminants (listed in Table 2).

Manosonication (MS)	Application of ultrasound waves >16-18Hz causes transient cavitation in bacterial cells, inducing high temperatures and shockwaves
Pulsed electric field (PEF)	Short duration (1-100µs) electric field pulses at 10-50kV/cm disrupt cell envelopes by electropermeabilization
High hydrostatic pressure (HHP)	100-15,000MPa pressures disrupt cell membranes and cause oxidative damage
Ultraviolet Irradiation (UV)	UV light induces genetic damage and affects cell replication

Table 1: Non-thermal processing methods

Staphylococcus aureus	Salmonella species	Listeria monocytogenes
Cronobacter sazakii	Escherichia coli	Campylobacter jejuni

Table 2: Foodborne bacterial contaminants

The researchers examined bacterial viability following the non-thermal preservation methods, applying the treatments under standardized experimental conditions where possible to cultures grown in McIlvaine buffer at pH7. The team also examined the impact of other variables such as growth stage, pH, and temperature in conjunction with non-thermal processing. They also considered data from other studies when summarizing results.

Cebrián et al. found that S. aureus showed resistance to MS and HHP treatment, whereas C. sazakii and Salmonella species including S. enterica showed sensitivity. Other studies found high levels of resistance to HHP from E. coli contamination, whereas Campylobacter were sensitive to this treatment. The researchers also found inter and intra-species differences in sensitivity to the non-thermal preservation treatments. Resistance varied with strain in Salmonella species, S. aureus and C. sazakii.

Noting that PEF varies according to pH, Cebrián et al. conducted resistance testing under acidic and neutral conditions. They found resistance in L. monocytogenes and E. coli at pH7, and C. sazakii and Salmonella species at pH4.

The researchers found that all treatments worked better in terms of reducing bacterial viability when combined with heat, suggesting that a combination of treatments might be optimal for food processing.

They also note that other factors during treatment, such as the physical properties of the food matrix (pH, water activity, composition) can also affect resistance. Food matrix can shield bacteria from UV for example. Pre-treatments factors can also affect resistance. For example, the bacterial growth phase influences sensitivity, with those in stationary phase being most stable and resistant to processing. Other influences on cellular physiological state, such as induction of stress also determine bacterial response to non-thermal processing.

Cebrián et al. conclude by suggesting that using non-thermal preservation treatments in combination with heat and optimal pH, for example, may maximize success. Furthermore, the results show that MS and UV are most suitable as alternatives to heat processing for heat-labile foods.

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Reference

1. Cebrián, G. et al. (2016) “Comparative Resistance of Bacterial Foodborne Pathogens to Non-thermal Technologies for Food Preservation“, Frontiers in Microbiology 7:734 doi:10.3389/fmicb.2016.00734