Microbiological Quality of Pasteurized Milk in Hawaiʻi
Pacific Agriculture & Natural Resources 2010, Volume 2, Number 1
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Abstract
Complaints of spoilage of pasteurized milk purchased from Honolulu groceries highlighted the need to address the emerging milk quality issues for the consumer’s consumption. The objective of this study was to evaluate the microbiological quality of pasteurized milk available in Hawaiʻi’s market. Bacterial counts of different types of milk samples (imported-M, locally produced-L and imported organic-O) were compared during refrigerated storage. Imported bulk milk from mainland are re-pasteurized in Hawaiʻi and packaged under different brands. Hawaiʻi locally produced milk are marketed with the “Island Fresh” label. The expiration or sell by date is based on the pasteurization. Imported organic milk from the mainland were prepackaged and shipped by air freight. At 5d before the expiration date, 70% of the mainland samples and 62% of the local samples had aerobic bacteria count exceeding the regulatory limit of 20,000 cfu/ml for grade ‘A’ pasteurized milk set by the United States Food and Drug Administration (FDA). Local raw milk samples from the farms supplying the processor were examined and results indicated good microbiological quality. High levels of psychrotrophic bacteria were identified as a major contributor to the pasteurized milk spoilage before the expiration date. The implications of this study were further discussed.
Introduction
Hawaii’s dairy industry faced numerous challenges in the recent decade. Aging operators faced with higher feed costs, fluctuations in raw milk prices and increased environmental regulations (both at the state and federal levels) found it easier to close operations versus investing in costly new technologies for waste management. Most of the dairy operations were located in small acreage without the opportunity to expand and were faced with increasing pressure of housing development within a stone throw from their farm sites. Displaced dairy operations resulted in the need for larger volumes of milk to be imported to meet local demands for fluid milk. Presently, there are three types of imported milk from the U.S. mainland: 1) pre- packaged organic milk with refrigerated shipping or air- freighted; 2) ultra pasteurized milk with long shelf-life, and 3) conventional high temperature short time (HTST) pasteurized milk is cooled and shipped in bulk tanks (>6,000 gallons) for the trans-ocean journey that normally takes 5-7 days. Particularly, the conventional pasteurized milk is held in insulated tankers without refrigeration during transportation. Initial shipment of bulk tankers had refrigeration but due to higher cost and limited space for electrical terminals in a ship, this method was changed. In Honolulu or Hilo, imported bulk tank milk was then re-pasteurized (HTST), packaged and placed for retail with a shelf-life of 17-19 days. The shelf-life dates back to the most recent pasteurization (local) date.
Consumer complaints of pasteurized milk spoilage in Honolulu prior to the printed expiration date began in late 2002 and increased in 2003. The increased complaints coincided with the increase of imported milk into the market. Numerous studies have indicated that spoilage of processed milk is primarily due to bacterial activity that results in loss of sensory quality (Cromie, 1991, Boor and Murphy, 2002, Hayes and Boor, 2001). Among various microorganisms, psychrotrophic bacteria are by far the most important factor influencing pasteurized milk quality (Sorhaug and Stepaniak, 1997). Psychrotrophic bacteria are a group of microorganisms capable of growing at 7oC (44.6oF) or less. These organisms are remarkable for their rapid growth and short generation multiplication under low temperatures; in raw milk or in pasteurized milk under refrigeration. The extracellular proteolytic and lipolytic enzymes produced by these bacteria can degrade nutritional components of milk and cause undesirable changes in sensory characteristics (Hantsis- Zacharov and Halpern, 2007). Although most of the psychrotrophic bacteria are destroyed by pasteurization, high level of these organisms in raw milk may produce sufficient enzymes that can tolerate pasteurization and reduce processed milk quality. Furthermore, post- pasteurization contamination by psychrotrophic bacteria present in the processing environment is currently a detrimental factor for extending the shelf- life of HTST pasteurized milk (Eneroth et al., 2000). This study was designed to determine the bacterial levels of retail fluid milk produced locally versus milk imported and re-pasteurized. The general goal was to validate the legitimacy of the consumer complaints. The hypothesis was that early spoilage of imported milk was due to higher psychrotrophic bacteria count even under proper handling.
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