Artículo de microbiologia

Food Control 22 (2011) 697e701

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Food Control
journal homepage: www.elsevier.com/locate/foodcont

Pseudomonas spp. and Hafnia alvei growth in UHT milk at cold storage
T.R. Chen a, *, Q.K. Wei b, Y.J. Chen a
a b

Department of Food Science, National Chia-Yi University, 300 University Road, Chia-Yi, 60004, Taiwan, ROC Department of Food NutritionScience, Chung-Hwa Institute of Technology, 89 Wunhua 1st street, Rende Township, Tainan 71703, Taiwan, ROC

a r t i c l e i n f o
Article history: Received 7 March 2010 Received in revised form 30 September 2010 Accepted 12 October 2010 Keywords: Pseudomonas Hafnia alvei Gram-negative psychrotrophic bacteria UHT milk

a b s t r a c t
Gram-negative psychrotrophic bacteria isolated fromsamples of spoiled commercial ultra-heat treated (UHT) milk from Taiwan were comprised of 67% Pseudomonas spp. and 22% Enterobacteriaceae. Whole milk, low-fat milk, and flavored milk samples were tested. The modified Gompertz equation was used to model the data and provided a good fit for the growth of Pseudomonas spp. and Hafnia alvei at 7  C for 14 days in both whole UHT milk and low-fat UHT milk.From this model, the estimated lag time for H. alvei was longer than that for Pseudomonas fluorescens in both whole UHT milk and low-fat UHT milk. The growth rate of H. alvei was higher than that of P. fluorescens. The pH value was significantly reduced by H. alvei than P. spp. after cultured in whole and low-fat UHT milk at 7  C for 14 days. Ó 2010 Elsevier Ltd. All rights reserved.

1. IntroductionMost people in Taiwan consume commercially pasteurized ultra-heat treated (UHT) milk, including whole UHT milk (WUM), low-fat UHT milk (LFUM) and flavored UHT milk (FUM). After being kept in refrigerated storage for one or two weeks, pasteurized milk can become spoiled due to Gram-positive spore-forming bacteria, such as Bacillus subtilis, and Gram-negative bacteria, such as Pseudomonas spp. andEnterobacteriaceae (Ternstrom, Lindberg, & Molin, 1993). Bacterial spores can enter milk at the farm, survive the pasteurization process, and grow slowly in refrigerated milk after germination (Ternstrom et al., 1993). Contamination with Gram-negative psychrotrophic (GNP) bacteria has been reported in pasteurized and refrigerated milk (Cousin, 1982; Ercolini, Russo, Ferrocino, & Villani, 2009;Frank, Christen, & Bullerman, 1993). The recontamination of GNP is undesirable since they can grow rapidly in chill-stored milk, thus decrease the keeping quality and shorten the shelf-life. This post-pasteurization recontamination most frequently takes place during the filling procedure (Eneroth, Christiansson, Brendehaug, & Molin, 1998; Relyea, Wiedemann, & Boor, 1998), which made the milk and milkaerosols to be exposed to the surrounding air. Besides, condensed water on the machinery may pollute its way into the milk (Eneroth, Ahrne, & Molin, 2000). Incompletely-cleaning surfaces in the filling

machine could easily cause contamination with the milk (Schroder, Cousins, & McKinnon, 2009). Mathematical models that predict the growth of bacteria are useful tools for the food industry.Predictive modeling has been successfully used to optimize production processes in order to improve the quality and safety of dairy products (Cassin, Paoli, & Lammerding, 1998; De Jong, Verschueren, Vissers, Straatsma, & Smit, 2002). Raw and pasteurized milk usually undergoes spoilage when held at refrigeration temperatures because of the effects of psychrotrophic contaminates (Elliott, Emmons, & Yates,1974). The change of populations of microorganisms was needed to be detected in milk at cold storage. In this study, we investigated the prevalence of Pseudomonas fluorescens and Hafnia alvei in spoiled UHT milk in Taiwan, and we utilized Gompertz model (Zwietering, Jongenburger, Rombouts, & Van’t Riet, 1990) to predict the growth of P. fluorescens and H. alvei in UHT milk at cold storage...