An Investigation about effects of mixture of carbon dioxide and nitrogen gas properties of raw bovine milk using response surface methodology

The purpose of this study was to investigate the effects of flushing of CO2 / N2 gases at different ratios and gas treatment duration on physicochemical and microbiological properties of raw milk using a combined design. In this research acidity, pH, freezing point, total count, total yeast and mold count and total coliform count were performed on 15 raw milk samples between 9 and 15 minutes after injections. Statistical analysis showed that the combined treatment of nitrogen and carbon dioxide had significant effect on the acidity and pH of raw milk (p˂0.05), so that with increasing the amount of carbon dioxide to 100%, the pH decreased and acidity increased. The combined effect of both gases on the freezing point of raw milk was significant (p˂0.05). Increasing the treatment time of raw milk from 9 to 15 minutes had significant effect (p˂0.05) on reducing the microbial load, but no significant reduction was observed due to the combination of gases. According to ANOVA results, in 100% nitrogen and duration of 15 minutes, the best results in terms of microbiology and chemistry were obtained. The effect of milk treated with a mixture of CO2 and N2 gases on yogurt properties, as a food model, has also been investigated. Yogurt samples were investigated for chemical, microbial, and organoleptic properties during 22 days storage time. The results showed that the raw milk treated with gas mixture did not have an adverse significant effect on the sensory and physicochemical properties of yogurt (p> 0.05). However, yogurt acidity, pH and synthesis were significantly affected during storage time (p˂0.05). Therefore, the application of raw milk treatment with 100% nitrogen gas for 15 minutes can be considered as a bacteriostatic agent in the preservation of raw milk in industrial scale.