AUTHOR=Zhu Meijun , Song Xia , Shen Xiaoye , Tang Juming 

TITLE=Listeria monocytogenes in Almond Meal: Desiccation Stability and Isothermal Inactivation

JOURNAL=Frontiers in Microbiology

VOLUME=11

YEAR=2020

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2020.01689

DOI=10.3389/fmicb.2020.01689

ISSN=1664-302X

ABSTRACT=<p>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 <italic>Listeria monocytogenes</italic> highlight the need to control <italic>L. monocytogenes</italic> in almond products. The objectives of this study were to examine the stability of <italic>L. monocytogenes</italic> in almond meal during extended storage and analyze thermal resistance of <italic>L. monocytogenes</italic> in almond meal of controlled moisture contents or water activity (a<sub>w</sub>) using thermal death time (TDT) cells and thermal water activity (TWA) cells, respectively. <italic>L. monocytogenes</italic> maintained a stable population in almond meal for 44–48 weeks at 4°C regardless of a<sub>w</sub>; however, we observed about 1.69 and 2.14 log<sub>10</sub> colony-forming units (CFU)/g reduction of <italic>L. monocytogenes</italic> in a<sub>w</sub> 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 <italic>L. monocytogenes</italic> in almond meal fitted the log-linear model well; thermal resistance of <italic>L. monocytogenes</italic> in almond meal was inversely related to the a<sub>w</sub> of samples. <italic>D</italic><sub>75</sub>-/<italic>D</italic><sub>80</sub>-values of <italic>L. monocytogenes</italic> in a<sub>w</sub> 0.25 and 0.45 almond meal obtained using TDT cells were 47.6/22.0 versus 17.2/11.0 min, respectively. <italic>D</italic><sub>80</sub>-, <italic>D</italic><sub>85</sub>-, and <italic>D</italic><sub>90</sub>-values of <italic>L. monocytogenes</italic> in a<sub>w</sub> 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 <italic>z</italic>-value of <italic>L. monocytogenes</italic> in a<sub>w</sub> 0.25 almond meal was not affected by TWA and TDT cell type (15.4–15.5°C), whereas <italic>z</italic>-value of <italic>L. monocytogenes</italic> in a<sub>w</sub> 0.45 almond meal was 10°C higher than that in a<sub>w</sub> 0.25 almond meal. This study contributes to our understanding of <italic>L. monocytogenes</italic> in nuts and impacts of a<sub>w</sub> on the development of thermal resistance in low-moisture foods.</p>