Evaluation of Process Parameters Used During the Fermentation and Drying of Italian-Style Salami

Abstract

Raw ground pork was inoculated with ca. 8 log CFU/mL of a five-strain cocktail of Salmonella spp. The inoculated meat was mixed with non-meat ingredients and the batter stuffed into 100 mm casings to produce an Italian style salami. Following fermentation, product was dried at either low, medium or high temperatures. Samples were taken for microbiological and chemical analyses from sticks of salami immediately after fermentation and at predetermined points throughout drying. The total reduction in levels of Salmonella was the result of the combination of both the fermentation and drying processes. Total log reductions of 5.08, 6.07 and 6.29 were observed for the low, medium, and high drying temperatures, respectively. The fermentation process resulted in log reductions of Salmonella ranging from 2.73 to 3.36 log CFU/g, with an additional 1.46- to 3.0-log reduction observed during drying.

Introduction

Fermented meat products such as Italian dry salami continue to be associated with outbreaks of foodborne illness. These ready-to-eat (TRE) products and their specialized production processes rely on factors such as low water activity, reduced pH, salt and/or combinations of these factors to proved microbial stability and to ensure their safety. In 2001 the USDA-FSIS proposed lethality performance standards fro RTE meat products, including a 6.5 log CU/g reduction in populations of Salmonella (1). The purpose of this study was to evaluate the impact of fermentation and three different commercial drying processes on the survival of Salmonella during the production of Italian dry salami.

Methods

Five strains of Salmonella spp. were used in combination to inoculate raw ground port. All strains were of known food origin from low water activity foods and include Salmonella Senftenberg, S. Enteritidis, S Montevideo, S. Tennessee and S. Typhimurium. Strains were revived from frozen cell cultures maintained in the Institute for Environmental Health (IEH) Culture Collection. Cultures were plated onto tryptic soy agar with 5% blood agar (TSA II 5% SB) and incubated overnight at 35±2°C. Isolated colonies were then transferred to tryptic soy broth (TSB) and incubated overnight at 35±2°C. Pure culture suspensions were then subjected to Cold-shock by storage at 4°C for 24 hours. Levels of Salmonella in the final suspension were determined by using serial dilutions and enumeration plating on XLT4 selective media. Once initial concentrations for each of the five strains were ascertained (108-109 CFU/mL), equal portions of the suspensions were combined into a cocktail for a final inoculum level of approximately  108-10CFU/mL.

The Salmonella cocktail was added to approximately 10% (15 lbs) of the total ground pork which was subsequently held at 1±2°C for 72 hrs. This 3-day storage at refrigeration temperatures allowed the Salmonella to adapt to the environmental conditions of the pork. The inoculated meat was then thoroughly mixed (~5-10 min) with the remaining (135 lbs) uninoculated meat and the non-meat ingredients, including nitrite, nitrate, spices, salt dextrose, and lactive acid starter culture, to generate a total of 150 lbs of salami batter. The lactic acid starter cultures were a mix of Pediococcus acidilactici and Staphylococcus carnosus. The mixture was held for an additional 24 hours at refrigeration temperature (1±2°C) prior to stuffing. The salami batter formulation, as well as the processing, fermentation and drying parameters for the process used in the study were obtained from the commercial U.S. processor.

Following the addition of non-meat ingredients, the batter was stuffed into 100 mm diameter collagen casings (n=30) of approximately 1 foot in length weighing ca. 2268 g (5 lbs). Ten sticks of salami were designated for each drying treatment and placed into one of the three environmental chambers (Figure 1). Salami sticks were fermented in the chambers for 24 hrs at a temperature of 40°C and a relative humidity (RH) of >95%. Following fermentation, each chamber was set to one of three different temperatures (low, medium, high) for drying. Three different drying schedules were maintained and monitored throughout the process. The salamis were dried until they reached final water activity of ≤0.92.

Samples were collected from salami sticks at schedules intervals throughout fermentation and drying, and analyzed for levels of lactic acid starter culture, Salmonella, pH, and water activity. Samples, for bot water activity and microbiological analyses, were collected aseptically (Figure 1) from each environmental chamber, from three random sticks of salami during fermentation and drying according to schedule. Approximately 25 g, representing the entire cross section of the individual salami, was obtained using a flame sanitized coring device which was passed through the entire salami to ensure that a random representation of the entire salami was samples. Triplicate samples were analyzed from each chamber (drying temperature treatment) for Salmonella. Levels of Salmonella in samples taken during the process were determined using serial dilutions and enumeration plating on XLT4 selective media. Samples were also analyzed for lactic acid bacteria using Petrifilm™ (3M, St. Paul, MN). Duplicate samples were taken and analyzed for water activity using an Aqualab Series 3 water activity meter (Decagon, Pullman, WA). Measurements for pH were taken throughout the process using a handheld stick pH meter (Figure 1).

Results

The values for pH and water activity (aw) for the three treatment conditions (low, medium and high temperature drying) are shown in Table 1 and Figure 2. For all three drying conditions the pH dropped to ≤4.8 at different times during the fermentation process however increased to pH 5.17, 5.18 and 5.17 by day 30 of drying for low, medium, and high drying treatments, respectively. All treatments reached a water activity of ≤0.92 by day 30 with average water activities of 0.908, 0.921 and 0.916 for the low, medium and high temperature drying treatment conditions (data not shown). Levels (log values) of lactic acid bacteria) (LAB) are shown in Table 2. The overall reduction in Salmonella for the low medium and high drying treatments are shown in Table 3 and Figure 3. In summary, the fermentation process resulted in approximately a 2.73- to 3.36-log reduction in levels of Salmonella. An additional 1.46- to 3.0-log reduction is reported during drying for a total log reduction of 5.08, 6.07 and 6.29 for the low, medium and high drying temperature treatments (Figure 3).

table1

Table 1. pH Values of samples taken from salami at the beginning of fermentation, at the end of fermentation, and at the end of drying for low, medium and high treatments

Figure1

Figure 1. Processing and sampling Italian dry salami sticks: (A) Salami sticks hanging in environmental chamber; (B) Taking pH of salami stick using hand held pH stick type probe; (C) Sampling using cork borer; (D) Core samples.

table2

Table 2. Levels (log CFU/g) of lactic acid bacteria in samples taken from salami at stuffing, at the end of fermentation and at the end of drying for low, medium and high treatments.

table3

Table 3. Reduction in levels of Salmonella from drying and fermentation.

Figure2

Figure 2. Water activity results for salami samples taken during drying.

Figure3

Figure 3. Salmonella results for samples of salami taken throughout fermentation and drying (XLT4).