Impact of pH and Water Activity on Growth of Staphylococcus aureus in Shelf Stable Ready-to-Eat Snack Sausages

Introduction: Staphylococcus aureus is a pathogen of concern in shelf stable meats because it can grow in low pH and water activity (a_w) environments.  Historically, USDA has accepted shelf stable meat and poultry as having a a_w <0.91, pH <5.0, or a combination of a_w <0.95 and pH <5.2.  Recently, the validity of these critical parameters has come into question.

Purpose: Investigate the impact of pH and a_w on the inhibition of Staphylococcus aureus in shelf stable snack sausages acidified with citric acid.

Methods: Three batches of cured beef-pork snack sausages were prepared using encapsulated citric acid to achieve pH values of 5.6, 5.1, or 4.7, cooked to 71°C, and then dried to a_w 0.96, 0.92, or 0.88 to yield nine treatments.   Treatments were surface-inoculated with 3-log CFU/g S. aureus (three-strain mixture) and individual 25-g portions were vacuum-packaged and stored at 21°C for 4 weeks.  Triplicate samples were assayed for S. aureus growth at 0, 1, 2, and 4 weeks by plating on Baird-Parker agar with egg yolk tellurite enrichment; experiments were replicated twice.

Results: The treatment with a_w of 0.96 and pH of 5.6 supported 5.5 and 7.0 log S. aureus growth at 1 and 4 weeks, respectively.  In addition, S. aureus populations increased ~1-log within 1 week for treatments with pH of 5.6 and either a_w of 0.92 or 0.88 but no additional growth was observed for the rest of the study, thus not confirming shelf stability.  In contrast, none of the treatments with pH ≤5.1 and a_w <0.96 supported S. aureus growth throughout this storage period.  

Significance: This study provided new pH and a_w parameters, similar to those previously outlined by USDA, that effectively inhibit S. aureus growth in semi-dry, pH-reduced snack sausages.  Results also suggest that reduced pH has greater relative impact on S. aureus growth than a_w in shelf-stable snack sausages.