Inhibition of Staphylococcus aureus in Whey Treated with Hydrogen Peroxide during Extended Non-refrigerated Storage

Introduction: During production of cheese, liquid whey is drained from warm cheese curd at temperatures ranging between 38 and 49°C and then transferred to large storage tanks before further processing. Depending on cooling capabilities, the whey temperature may exceed 7°C for >4 hours, particularly during summer months.  This environment provides conditions under which Staphylococcus aureus may produce heat-stable enterotoxin. Hydrogen peroxide can be added to the liquid whey to inhibit microbial growth.

Purpose: To assess the ability of whey treated with hydrogen peroxide to inhibit the growth of S. aureus when stored non-refrigerated for 24 hours.

Methods: Two whey types (with and without starter culture; four lots each) were inoculated with 3-log S. aureus per ml. Whey was then treated with 0, 10, or 100 ppm H₂O₂, and samples incubated at either 21 or 32°C.  Duplicate samples were assayed for S. aureus at 0, 4, 8, 12, and 24 hours by plating on Baird-Parker agar.

Results: When stored at 21°C, the no-starter culture whey without H₂O₂ supported >1 log increase of S. aureus at 8 hours, whereas none of the other treatments supported growth for the 24 hour holding period.  When stored at 32°C, the no-starter whey with 0 or 10 ppm H₂O₂ supported >1 log growth at 4 and 8 h, respectively.  In contrast, no growth was observed in the 100-ppm treatment for no-starter whey or in any of the whey with starter culture.  The pH of whey without H₂O₂ decreased from 6.6 to <5.0, regardless of inclusion of starter culture whereas the pH of treatments with 100 ppm H₂O₂ remained stable. 

Significance: Data suggest that whey produced with a starter culture will inhibit S. aureus if stored at <32°C for up to 24 hours. Alternatively, S. aureus can be inhibited in whey through temperature-time control or addition of 100-ppm hydrogen peroxide.