Baroprotection of vegetative bacteria by milk constituents: A study of Listeria innocua

The baroprotective effect of milk constituents on Listeria innocua 4202 treated at 350 or 500 MPa for 5 min was examined. High-pressure (HP) treatment of L. innocua 4202 (1×10⁹ cfu mL^-1) resulted in complete inactivation in simulated milk ultra-filtrate (SMUF), a ∼5.0 log reduction in phosphate-buffered saline and a ∼2.9 log reduction in milk. The addition of micellar casein to SMUF increased survival of the bacterium by 3 logs, compared with SMUF alone, but the protective effect was negated if the minerals associated with the casein micelles were removed. The colloidal minerals calcium (30 mm), magnesium (5 mm), citrate (10 mm) and phosphate (20 mm), suspended in SMUF increased survival by ∼3.3, ∼1.7, ∼3.3 and ∼3.5 logs, respectively. The buffering capacity of the suspending medium was found to be a key factor in microbial baroresistance. Buffering by phosphate and citrate in milk may protect microorganisms against changes in pH during HP treatment, whereas the divalent cations calcium and magnesium may protect cell membranes against HP.