Dry inoculation methods for nonfat milk powder

Abstract

Carriers with inoculated microorganisms are often used to validate low-moisture food safety interventions. In this study, we evaluated dry inoculation methods using silicon dioxide (SiO 2) and a small portion of nonfat milk powder (NFMP) as dry carriers for NFMP. Silicon dioxide was characterized by vapor sorption analysis. One milliliter of inoculum of a 5-strain Salmonella cocktail (serovars Agona, Reading, Tennessee, Montevideo, and Mbandaka) or Enterococcus faecium NRRL B-2354 was inoculated onto 1 g of SiO 2 or 10 g of NFMP as carriers. Both inoculated carriers were air-dried for 72 h [22°C, relative humidity (RH) ~30%], equilibrated to water activity (a w) 0.25 ± 0.02 (24 h at 22°C, RH 25%), and mixed with preconditioned NFMP (a w = 0.25 ± 0.02) to reach an inoculation level of 8.2 ± 0.2 log cfu/g. Inoculated NFMP was stored at 22°C, RH 25%, and its bacterial populations were monitored for 30 d. Both sets in equilibrated NFMP were subjected to isothermal treatments in closed aluminum cells at 85, 90, and 95°C. Silicon dioxide maintained moisture content (0.29 ± 0.03%, dry basis) at different water activities. The NFMP inoculated with both carriers exhibited stable bacterial populations over 30 d at 22°C. Strains in NFMP inoculated with SiO 2 showed equal or higher D-values but equal z-values compared with those inoculated with a small portion of NFMP. Enterococcus faecium exhibited comparable thermal resistance to Salmonella under all tested conditions. This study supports E. faecium as a Salmonella surrogate in thermal processing of NFMP and the use of SiO 2 to inoculate NFMP.

Evaluation of Enterococcus faecium NRRL B-2354 as a surrogate for Salmonella enterica in milk powders at different storage times and temperatures

“…The z-values of Salmonella and E. faecium were determined to be 14.77 and 16.39°C for WMP and 12.82 and 16.58°C for NFDM, respectively. The z-values of Salmonella and E. faecium found in this study were very close to the z-values of Salmonella and E. faecium (16.4 and 16.5°C) reported by Liu et al (2019) and z-values of L. monocytogenes (14.6 to 16.0°C) in NFDM at different water activities as reported by Ballom et al (2020). The determined microbial inactivation kinetics of Salmonella and E. faecium could be a practical tool for the food industry to identify appropriate thermal processing conditions for pasteurization of milk powders.…”

“…Significantly higher (P < 0.05) D and δ values of E. faecium were also observed at 85, 90, and 95°C than Salmonella. Similarly, Liu et al (2019) conducted isothermal treatments for NFDM with storage time of 48 h after inoculation and found that the decline of Salmonella exceeded that of E. faecium during isothermal treatment of NFDM at all 3 temperatures (85, 90, and 95°C). These results suggest that E. faecium is a suitable nonpathogenic surrogate for Salmonella in the thermal processing of milk powder at different process temperatures with 30 d of storage.…”

Influence of low water activity on the thermal resistance of Salmonella Enteritidis PT30 and Enterococcus faecium as its surrogate in egg powders

“…As an example, the D 80 ℃ -values for egg white powder samples was 25.8 ± 0.3 min, 13.3 ± 0.3 min, and 6.1 ± 0.1 min at a w,20 ℃ values of 0.30 ± 0.02, 0.45 ± 0.02, and 0.60 ± 0.02, respectively. This increase in the thermal resistance of Salmonella with reducing a w has also been observed in wheat flour, almond flour, whey protein, milk powder, and cocoa powder conditioned at similar a w values at room temperature before thermal treatments (Liu et al., 2019; Tsai et al., 2019; Xu et al., 2019). Because bacteria cells adapt quickly to a low a w environment by losing water, their proteins and ribosomes become more stable, and water loses mobility inside the bacteria cell (Leuenberger et al., 2017).…”

“…The absence of S. Enteritidis PT30 and E. faecium in egg powders samples was corroborated by diluting 10 random 1 g samples in 9 ml 0.1% (w/v) buffer peptone water, and then plating on modified TSAYE [Tryptic soy agar supplemented with 0.6% (w/v) yeast extract (TSAYE) plus 0.05% ammonium iron (III) citrate, and 0.03% sodium thiosulfate pentahydrate] and e-TSAYE (TSAYE plus 0.05% ammonium iron (III) citrate, and 0.03% esculin thiosulfate) for E. faecium (Liu et al., 2019; McLaughlin and Balaa, 2006) .…”

“…The samples were then homogenized for 2 min at 260 r/min with a Seward Stomacher (Seward, London, UK) (Xu et al., 2019). Egg powder samples were appropriately tenfold serial diluted in 0.1% (w/v) buffer peptone water and then spread-plated in triplicate onto modified TSAYE for S. Enteritidis PT30 and e-TSAYE for E. faecium (Liu et al., 2019; McLaughlin and Balaa, 2006). The plates were incubated at 37 ℃ for 48 h; the colonies were counted and converted in log CFU/g.…”