Fate of Listeria monocytogenes During Freezing and Frozen Storage

A cell suspension of Listeria monocytogenes was frozen 30 min at ?18°C, or 10 min in liquid nitrogen (LN) at ?198°C. Solidification required 15 min at ?18°C and ca. 1 min at ?198°C. Freezing and storage 1 month in phosphate buffer (PB) at ?18°C caused 87% death and 79% injury. These were 54 and 45%, respectively, for cells in Tryptose Broth (TB) at ?18°C. Freezing and storage 1 month in LN caused no death or injury of cells suspended in PB, whereas some injury and death occurred in TB. Freezing at ?198°C followed by storage 1 month at ?18°C resulted in 60% death and 36% injury in PB, and 61 and 44.2%, in TB. Repeated freezing and thawing caused more death/injury than did a single freeze-thaw cycle.     

Inactivation of Listeria monocytogenes by Ultraviolet Energy

Short-wave UV-energy (100 μW/cm²) decreased the number of Listeria monocytogenes on Tryptose Agar (TA) ca. seven orders of magnitude in 4 min. Age of culture (48 vs 24 hr) did not alter the sensitivity of Listeria to this UV treatment. Increasing the intensity of irradiation (550 vs 100 μW/cm²) increased the rate at which L. monocytogenes was inactivated. Dry rather than moist cells of L. monocytogenes were most resistant to irradiation. In general, inactivation of Listeria with short-wave UV-energy yielded sigmoidal survivor curves. The population of Listeria on TA plates was not affected with long-wave UV-irradiation.     

Biogenic amines in Iranian white brine cheese: modelling and optimisation of processing factors

The simultaneous effects of processing factors such as ripening time (25–75 days), ripening temperature (4–14 °C) and brine concentration (10–13%) on biogenic amines content, proteolysis and sensory score of Iranian white brine cheese were studied, in 12 cheeses. Response surface methodology (RSM) was used to minimise biogenic amines content. At low level of ripening time, biogenic amines content decreased with increasing levels of brine concentration but at high level of ripening time, brine concentration had inverse effect. Ripening time showed quadratic effect on biogenic amines content. Based on biogenic amines content and sensory score, the optimum conditions were 13% brine and ripening at 9–14 °C for 43–65 days.     

Kinetics of Growth and Inhibition of Listeria monocytogenes in the Presence of Antioxidant Food Additives

Listeria monocytogenes strain Scott A in Tryptose Broth was treated with 100-300 ppm butylated hydroxyanisole (BHA), 300-700 ppm butylated hydroxytoluene (BHT) and 10-30 ppm tertiary butylhydroquinone (TBHQ). Resulting growth curves were fitted using the logistic model, and growth parameters [lag period (LP), generation time (GT), and maximum growth (MG)] were calculated. BHA and BHT inhibited Listeria monocytogenes by increasing LP and GT and decreasing MG. Extent of inhibition was concentration-dependent for cultures with BHA, but not with BHT. TBHQ at 10-30 ppm increased LP but did not affect other parameters. LP increased exponentially with increased BHA or TBHQ in Listeria culture. Concentrations of additive required to increase LP by one order of magnitude were 240 ppm for BHA and 26 ppm for TBHQ.