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Introduction Essential oils (EOs) produced from plants possess antimicrobial and antioxidant properties useful for natural food preservation. Specifically, Origanum commonly known as oregano EO (OEO) consists of carvacrol and thymol and possesses broad spectrum antimicrobial activity. The hypothesis was that OEO addition, as a preservative, would extend the shelf life of milk. Therefore, the study objective was to determine an optimal OEO inclusion rate for the preservation of milk quality and shelf life. Methods The OEO was added to batches of pasteurized milk at concentrations of 0.0, 0.5, 1.0, 1.5, and 2.0 mg/L. Milk samples were stored at 4 °C for 8 days, and subsamples were collected at 0 h and every 48 h. Milk components, titratable acidities, pH, color, superoxide dismutase (SOD), malondialdehyde (MDA), and total plate count (TPC) were measured. Results Addition of 2.0 mg/L OEO reduced TPC during the first 2 days compared with 0.0, 0.5, 1.0, and 1.5 mg/L OEO. By day 4, the TPC in the 0.0 mg/L OEO group reached 4.80 ± 0.16 log₁₀ CFU/mL, close to the maximum allowable count specified by the National Standard for pasteurized milk (5.0 log₁₀ CFU/mL). In contrast, the TPC in the 2.0 mg/L OEO group on day 8 remained well below the maximum allowable limit. Titratable acidity was higher in the 0.0 mg/L OEO group compared with all OEO-treated groups on day 6. By day 8, titratable acidity exceeded 18°T in all treatments. Fat and protein concentrations both demonstrated quadratic changes over time. Overall, fat concentrations were higher in the 0.5, 1.0, 1.5, and 2.0 mg/L OEO groups compared with the 0.0 mg/L OEO group ( p = 0.0456, p < 0.0001, p = 0.0061, and p = 0.0080, respectively). Similarly, protein concentrations were higher in the 0.5, 1.0, and 1.5 mg/L OEO groups compared with 0.0 mg/L OEO ( p = 0.0456, p < 0.0001, p = 0.0061, and p = 0.0080, respectively). Discussion In conclusion, 2.0 mg/L OEO was identified as the optimal dosage, effectively inhibiting bacterial growth and extending the refrigerated shelf life of pasteurized milk, thereby confirming OEO’s practical value as a natural preservative.