Listeria monocytogenes in Almond Meal: Desiccation Stability and Isothermal Inactivation

Abstract

Almond are among the most consumed tree nuts and used in a variety of food products. Recent almond butter recalls due to potential contamination of Listeria monocytogenes highlight the need to control L. monocytogenes in almond products. The objectives of this study were to examine the stability of L. monocytogenes in almond meal during extended storage and analyze thermal resistance of L. monocytogenes in almond meal of controlled moisture contents or water activity (a w) using thermal death time (TDT) cells and thermal water activity (TWA) cells, respectively. L. monocytogenes maintained a stable population in almond meal for 44-48 weeks at 4 • C regardless of a w ; however, we observed about 1.69 and 2.14 log 10 colony-forming units (CFU)/g reduction of L. monocytogenes in a w 0.25 and 0.45 almond meal over 44 to 48 weeks of storage at 22 • C. Under all test conditions using either TDT or TWA cells, the inactivation kinetics of L. monocytogenes in almond meal fitted the log-linear model well; thermal resistance of L. monocytogenes in almond meal was inversely related to the a w of samples. D 75-/D 80-values of L. monocytogenes in a w 0.25 and 0.45 almond meal obtained using TDT cells were 47.6/22.0 versus 17.2/11.0 min, respectively. D 80-, D 85-, and D 90-values of L. monocytogenes in a w 0.25 almond meal obtained using TWA cells were 59.5 ± 2.1, 27.7 ± 0.7, and 13.2 ± 1.1 min, respectively, in contrast to 22.0 ± 1.1, 10.6 ± 0.2, and 4.6 ± 0.4 min obtained using TDT cells. The z-value of L. monocytogenes in a w 0.25 almond meal was not affected by TWA and TDT cell type (15.4-15.5 • C), whereas zvalue of L. monocytogenes in a w 0.45 almond meal was 10 • C higher than that in a w 0.25 almond meal. This study contributes to our understanding of L. monocytogenes in nuts and impacts of a w on the development of thermal resistance in low-moisture foods.

Heat Inactivation of Listeria monocytogenes on Pecans, Macadamia Nuts, and Sunflower Seeds

“…In whole nuts and seeds, extremely high survival rates were observed even at temperatures above 400°C. On MN and SS, the D values at 90°C were 3- and 4-fold greater than the largest value observed in almond meal by Zhu et al ( 16 ). These differences were probably due to the use of whole nuts with surface inoculation in contrast to ground almonds with uniform distribution of the inoculum.…”

“…Ballom et al ( 29 ) determined L. monocytogenes ’ D values at 80°C in nonfat dry milk to be 4.3 and 14.6 min at a w s of 0.45 and 0.25, respectively. In the most recent report on thermal inactivation of L. monocytogenes , Zhu et al ( 16 ) measured D values in almond meal at a w s of 0.25 of 60, 28, and 13 min at 80, 85, and 90°C, respectively, under experimental conditions that kept the a w constant. Comparing the different findings described above, multiple factors, such as the food matrix, experimental conditions, and water activity, seem to influence thermal-kinetic parameters.…”

“…These values were 5- to 6-fold smaller in high-moisture products, such as milk and chocolate milk. The heat inactivation kinetics of a mixture of 3 L. monocytogenes strains were determined in an almond meal model at temperatures that ranged from 75 to 90°C ( 16 ). In the latter study, D values of 13.2 min were measured at 90°C and at a constant a w of 0.25.…”