Inactivation of Escherichia coli O157:H7 and spoilage yeasts in germicidal UV-C-irradiated and heat-treated clear apple juice
Highlights
► Yeasts were generally more UV-C resistant than E. coli O157:H7. ► E. coli O157:H7 strains were more heat resistant than the yeasts. ► In most challenge studies conducted, an individual E. coli or yeast strain was found more resistant than composited inocula.
Introduction
The aggressive campaigns of the industry and government health agencies to increase fruit and vegetable consumption and the desire of consumers to lead healthful lifestyles have significantly influenced the market of fruit juices. Fruit juice consumption is a convenient way of meeting the daily recommended fruit and vegetable intake since 1/3–1/2 cup of pure, undiluted juice is already equivalent to one serving of fruit or vegetable (Food and Nutrition Research Institute [FNRI], 1994, USDHHS & USDA, 2005). However as with other food products, the occurrence of pathogenic and spoilage microorganisms in fruit juices have constantly challenged the industry. The occurrences of outbreaks of infections of Escherichia coli O157:H7 and other pathogens due to unpasteurized fruit juices have prompted the United States Food Drug Administration (USFDA) to ratify the Federal Juice Hazard Analysis Critical Control Point (HACCP) that compels manufacturers to subject juice products to a processing step or combinations of processes capable of reducing a population of a target pathogen by 5 log cycles (Federal Register [FR], 2001, Goodrich et al., 2005). Spoilage microorganisms in juices include yeast species which produce metabolites that negatively affect sensory quality (Basak et al., 2002, Zook et al., 1999).
Heating is currently the most common method by which fruit juices are pasteurized (Basak et al., 2002). The USFDA recognizes the efficacy of heating in reducing both pathogenic and spoilage microorganisms for juice product safety and shelf life stability (Donahue, Canitez, & Bushway, 2004). Jay, Loessner, and Golden (2005) discussed that thermal processing is applied to foods based on its ability to alter microbial cell structures and inactivate enzymes necessary for metabolism and growth. Many essential cellular processes are dependent on the integrity of the cell membranes and cell walls. Inactivation of metabolic enzymes would also mean the cessation of many biological reactions within the cell that could eventually lead to cell death. On the other hand, since heating of juices may negatively affect the nutritional and sensory qualities of products, some processors elect to use non-thermal techniques to inactivate microorganisms in juices.
Several studies have been conducted to develop and evaluate the efficacies of alternative pasteurization processes for fruit juices. These include physical means such as ultrasound processing (Baumann et al., 2005, D’Amico et al., 2006, Patil et al., 2009); high pressure processing (Brinēz et al., 2006, Jordan et al., 2001) and pulsed electric field (PEF) processing of fruit juices (Ayhan et al., 2002). A number of studies have determined the efficacy of ultraviolet (UV) irradiation as a means of eliminating pathogenic and spoilage microorganisms in fruit juices (Basaran et al., 2004, Donahue et al., 2004, Franz et al., 2009, Wright et al., 2000). The Institute of Food Technologists [IFT] (2000) discussed that UV processing involves that use of radiation (100–400 nm) from the UV region of the electromagnetic spectrum. This UV range may be further divided and classified as UV-A (315–400 nm), UV-B (280–315 nm), UV-C (200–280 nm), and the vacuum UV range (100–200 nm). The UV-C range is germicidal and can effectively inactivate bacteria and viruses by breaking down the DNA of microorganisms, impairing DNA transcription, replication and compromising essential cellular functions (Unluturk, Atilgan, Handan Baysal, & Tari, 2008). Specifically, exposure to UV light results in the cross-linking of neighboring pyrimidine nucleotide bases in the same DNA strand, eventually causing cell death (Sizer & Balasubramaniam, 1999).
Since different microorganisms have different susceptibilities toward processing methods, the 5 log pathogen reduction standard applies to the most resistant pathogen under a specific processing condition (FR 2001). Similarly, such standard may be applied to the most resistant spoilage organism. Furthermore, the spoilage organism that exhibits the greatest growth rate in conditions similar to product storage may also be chosen as indicator for shelf life and quality evaluation or process establishment and validation. This study determined and compared the heat and ultraviolet inactivation rates in apple juice of individual and composited inocula of E. coli O157:H7 (strains CR-3, MN-28 and HCIPH 96055) and juice spoilage yeast strains namely, Debaryomyces hansenii, Clavispora lusitaniae, Torulaspora delbrueckii, Pichia fermentans and Saccharomyces cerevisiae. Furthermore, the fates of the test organisms in untreated apple juice suspending medium were also determined. The results established from this study can assist in selecting appropriate target organisms, whether singly or in a cocktail of inocula, which should be used in challenge studies and eventually in the establishment and validation of process lethalities for similar commodities.
Section snippets
Apple juice suspending medium
All inactivation and growth studies were determined in a commercially available clear apple juice suspending medium (Harvest 100% Apple Juice, Kirin-Tropicana, Tokyo, Japan). Several juices in 410-ml polyethylene bottles from the same production batch were aseptically pooled to make enough volume for the studies. Juice physicochemical properties such as pH and soluble solids (SS, °Brix) were determined. The SS and pH were respectively measured using a handheld refractometer (Master-A1T, AS ONE,
Inactivation patterns
The commercially available, clear apple juice used as microbial suspending medium had the following physicochemical characteristics: pH 3.68, 12.5 °Brix. The calculated D values of the E. coli O157:H7 strains and spoilage yeasts in the UV-C-irradiated and heated apple juice are presented in Table 1. Based on the D values of all test organisms, irradiating the test apple juice with UV-C will require longer processing time to achieve the same lethalities that resulted from heating the same
Conclusions
The resistances of both tested pathogens and spoilage microorganisms toward UV-C and heat treatment significantly varied within strains and species. For UV-C irradiation of the suspending juice medium, using P. fermentans as a target organism will ensure the inactivation of all tested yeast species and E. coli O157:H7 strains. S. cerevisiae inactivation in heat-treated (55 °C) juice will also ensure microbial safety and shelf life stability. However for a thermal process specifically aiming for
Acknowledgments
The author extends his gratitude to Hiroyuki Nakano of the Graduate School of Biosphere Science, Hiroshima University, for his guidance and support. Acknowledgment is also due to Yutaka Kikoku of the Research and Development Center of the Aohata Corp., Takehara City, Hiroshima, Japan for providing all the yeast species. The help extended by Ryousuke Hashiguchi, Lee Ming Mak, Rie Kai and Saki Takei in conducting the experiments are also being acknowledged.
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