Microbiological Quality of Pasteurized Milk in Hawai`i - University of ...

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Log cfu/ml . Hongfei he, Yong Li, Alfred L., et al. 23 10.0 M n=5 8.0 6.0 4.0 L n=9 a* a a a a a a a a a a b FDA regulatory limit (4.3) 2.0 0.0 -5 0 3 -5 0 3 Time (days ) *Means with no common superscripts are significantly different (P
24 Microbiological Quality of Pasteurized Milk in Hawai`i one way to determine the sites of contamination. On the other hand, organic (O) milk samples in this study demonstrated high quality performance in microbial testing. It may be possible that the different milk handling strategies designed for O milk production and handling may contribute to its quality; the chilled distribution system of O milk is the probably primary factor that contributed to product safety and extended shelf-life. Direct airfreight with shorter transportation time may be a major contributing factor for the superior microbiological quality of O milk. It must also be noted that in some rare occasions, O milk samples did exceed the legal limits set by the FDA. This is probably due to poor handling e.g. transportation from air cargo site to the retail stores is not performed by refrigerated trucks as has been observed in a few occasions when one walked passed the stores. Bulk tank milk imported into the state without refrigeration generally requires 5-7 days of transit. The volatile temperatures during transportation may have a direct effect on the shelf-life, quality and potential safety of the milk products. Increased temperatures have been shown to encourage growth of microorganisms and increase the activity of degradative enzymes (Cromie, 1991, Hantsis-Zacharov and Halpern, 2007, Sorhaug and Stepaniak, 1997). Excessive high bacterial level may challenge the efficacy of re-pasteurization, resulting in final products with higher bacterial counts thus affecting their subsequent shelf-life. The heat-stable enzymes produced by bacteria may cause serious quality defects of re-pasteurized milk (Sorhaug and Stepaniak, 1997). For example, proteins present in milk can be digested by proteases, resulting in production of bitter-tasting peptides and curdling of the milk; Lipases are able to degrade lipids and cause rancidity of milk (Hantsis- Zacharov and Halpern, 2007). Unlike Honolulu where the majority (>65%) of milk is imported, the majority of the milk supply on the Big Island was produced locally (>70%). This circumstance may be a possible explanation for the differences in the rate of spoilage in between the two counties (>80% vs 11%). It must be noted that the Pasteurized Milk Ordinance (PMO) allows the bulk shipping of pasteurized milk without refrigeration. The guideline recommends the temperature of 7.2ºC (45ºF) as the indicator for rejecting milk. Hence, as long as the temperature is met, the milk can be reprocessed for fluid milk use. The current standards do not set a legal microbial count. In addition, there is no data on the bacterial level after 7 days in transit without refrigeration. The state of California however prohibits the re-pasteurization of milk for fluid milk market but allows it for other utilization (CA FAC # 35832). Such regulation does not exist in Hawai'i and as such no law is violated or challenged. The PMO, when drafted and revised, probably, did not consider the situation where milk is being transported and held for more than five days without refrigeration. There is also a need for future studies to determine the microbial population of milk following such mode of transportation and handling. Sampling of the bulk tanks milk prior to repasteurization would be required if the current PMO standards is to be validated. Any such sampling would need the cooperation of the processor and the regulatory agency. In a study conducted 15 months later, results showed storage temperatures affected the final microbial numbers in milk (Table 1). When this was compared to the microbial level in the earlier studies, only 8% of the samples at 4ºC and 25% of the samples at 7ºC storage had bacterial counts higher than the regulatory limit at 5 days prior to expiration date. These results indicate that: a) the milk products from the same processing plant had different quality during the two different sampling periods and b) postpasteurization contamination was still the likely culprit. Previous report suggests the differences are probably due to multiple factors existing in the processing environment, such as pasteurization and storage conditions, packaging equipment, and personal hygiene (Gruetzmacher and Bradley, 1999). Nevertheless, 25% incidence of high bacteria counts should raise concerns given the fact that the major consumers of milk are infants, toddlers and young children. It must also be noted that shelf-life of milk in the retail space is not governed by any regulations in any state. The duration a product is on the shelf space is primarily a product quality control set by the respective creamery. In summary, these studies validate the increased complaints from the consumers on milk spoilage prior to the expiration date in Hawaii. We did not resolve where the contamination occurs or identify the source of the bacteria. These spoilage-related organisms can be present in raw milk, but they also may enter milk products during production and processing. Identifying the spoilage organisms and their reservoirs are critical for extending fluid milk shelf-life. In the present study, our findings suggest that psychrotrophic bacteria were the predominant organisms causing spoilage in Hawaii locally processed milk and this is confirmed by the subsequent study using molecular techniques to identify the spoilage-related bacteria in pasteurized milk (He et al. 2009). Since these bacteria are heat sensitive, the contaminants could be attributed to postpasteurization contamination. Our findings provide important information on the quality status of locally available pasteurized milk in retail stores and stress the need for further studies to determine the source of the spoilage organisms. Acknowledgement This study was supported via a grant provided by Hawaii Department of Agriculture and the Hawaii Farm Bureau Federation ORS# 2006 1665-4. Pac. Agric. Nat. Resour. Vol. 2: 20-25, 2010
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