Effects of Acid Type and Alta™2341 on Listeria monocytogenes in a Queso Blanco Type of Cheese

Abstract

The behavior of Listeria monocytogenes was evaluated during storage of a queso blanco type of cheese produced with acidulants (citric, malic, or acetic acids) and a commercial lactic acid bacterium fermentation product, ALTA™2341 (ALTA). The cheese was prepared by direct acidification (final pH 5.2), with and without 0.6% ALTA, inoculated with 106 CFU/g of L. monocytogenes, and stored at 4 or 20°C for 42 and 7 days, respectively. Levels of L. monocytogenes increased in cheese coagulated with citric or malic acids and stored at 4°C, but decreased slightly in cheese coagulated with acetic acid. At 20°C, counts of L. monocytogenes increased in cheeses acidified with citric or malic acid, but counts did not increase appreciably in cheese acidified with acetic acid. When cheese was stored at 4°C, the presence of 0.6% ALTA resulted in lower counts of L. monocytogenes compared with counts in cheese that did not contain ALTA. However, at 20°C populations of L. monocytogenes increased in cheese containing ALTA regardless of acid type. Additional studies compared the effects of acetic acid, alone or in combination with 0.6 or 2.5% ALTA, against low (102 CFU/g) and high (106 CFU/g) inoculum levels. When inoculum levels were low, pathogen counts decreased by > 1.1 log10 CFU/g in all formulations at 4°C. After 7 days at 20°C, pathogen counts increased in the queso blanco type of cheese prepared with acetic acid alone. In contrast, in the presence of 0.6 or 2.5% ALTA, 7-day counts were less than the initial inoculum. With high inoculum levels at 4°C, counts of L. monocytogenes were less than the initial inoculum in the acetic acid-coagulated queso blanco type of cheese with or without ALTA. At 20°C, counts increased in the queso blanco type of cheese prepared with 0.6% ALTA, but decreased appreciably in cheese prepared with 2.5% ALTA. These results demonstrate that acetic acid is significantly more effective than malic or citric acids for controlling L. monocytogenes in queso blanco, and that inclusion of ALTA can provide added protection against the pathogen.

Validation of Extrusion as a Killing Step for Enterococcus faecium in a Balanced Carbohydrate-Protein Meal by Using a Response Surface Design

“…According to the National Advisory Committee on Microbiological Criteria for Foods (18), rapid and significant reductions in microbial populations are frequently observed after inoculation if the food includes bactericidal ingredients or others used to extend the shelf life of the product. Research has also shown that 0.5-to 2.5-log reductions in Listeria monocytogenes were observed after inoculation of fresh soft cheese, bologna, and ham containing lactic acid bacteria fermentate or nisin (7,8). In the prepared mix, potassium sorbate was added as an antimicrobial agent.…”

EFFECT OF STORAGE TEMPERATURE ON THE GROWTH OF LISTERIA MONOCYTOGENES ON QUESO BLANCO SLICES*

“…Although several studies have established that a variety of soft cheese supports the growth of L. monocytogenes , these data cannot be used to predict the probability of L. monocytogenes growth in Queso Blanco (Glass et al . 1995; Ramsaran et al .…”

“…However, the model developed for these growth parameters should be validated in other soft cheese. Formulations and manufacturing practices for Mexican‐style cheese present a significant risk for product contamination and subsequent growth by L. monocytogenes (Glass et al . 1995, 1998).…”

Stress Response of Escherichia coli

“…Adaptation to acid enhanced the survival of pathogenic E. coli in acidic foods and caused changes in other physiological characteristics (Leyer and Johnson 1992, 1993). E. coli O157:H7 survived but did not grow in fermented dry sausage (final pH 4.8) for 8 wk of storage at 4 °C (Glass and others 1992, 1995). In addition, acid‐adapted E. coli O157:H7 had an increased resistance to lactic acid, and showed enhanced survival compared to nonadapted cells during sausage fermentation, in shredded dry salami (pH 5.0), and in apple cider (pH 3.5) (Leyer and others 1995).…”