Scale-up unit of a unique moderately high pressure unit to enhance microbial inactivation Kidane Shibeshi, Mohammed M. Farid ⇑ Department of Chemical and Materials Engineering, The University of Auckland, Private Bag 992019, Auckland, New Zealand article info Article history: Received 12 February 2010 Received in revised form 18 January 2011 Accepted 14 March 2011 Available online 21 March 2011 Keywords: PATS HPP Geobacillus stearothermophilus D-value Pumpkin soup L-Ascorbic acid Shelf life abstract The efficacy of a scale-up of a moderately high pressure unit built in this work was investigated with regards to inactivation of Geobacillus stearothermophilus spores suspended in pumpkin soup, and effect of the process on L-ascorbic acid. In this design saturated steam was used as a heating medium. The treat- ment unit is a double pipe heat exchanger in which the food is pumped in its inner tube, while steam is passed in the annular region to heat the sample. This technology comprises a unique approach of gener- ating a mild pressure (80–100 MPa) utilizing thermal expansion of the liquid being treated. The results show that this unique application decreased the D-values of Geobacillus stearothermophilus ATCC 7953 spores suspended in soup samples in comparison to thermal treatment alone. The improvement was more significant at lower treatment temperatures. The D-values obtained were in a good agreement with that of the small unit built earlier in which oil was used as a heating medium. The effect of treatment on L-ascorbic acid was similar to that of thermal treatment. The treated samples were subjected to shelf life study by storing them at two different temperatures. No evidence of spore recovery was noted during the post-treatment storage period. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Currently, commercial sterile shelf stable (pH > 4.6) foods are produced by thermal processes. Although, this method ensures safety and extends food shelf life, studies have shown that the quality and nutritional values of the products are often signifi- cantly affected (San Martín et al., 2002; Muñoz et al., 2007). Now- adays, there is increasing consumer’s demand for high-quality, fresh like, and microbiologically safe foods. This demand has initi- ated the development of alternative food preservation technologies such as ultra-high pressure processing (UHP). Ultra-high pressure is capable of destroying most vegetative microorganisms (Robertson et al., 2008) and inactivating some of the enzymes (Deliza et al., 2005), while maintaining the quality of the product (Moerman, 2005; Robertson et al., 2008). However, pressure treatment alone is not sufficient to inactivate bacterial spores (Nakayama et al., 1996; Stewart et al., 2000; Ananta et al., 2001; Rajan et al., 2006a), unless extreme pressure of more than 1000 MPa is used, which is difficult to achieve in industrial UHP units (Canovas and Juliano, 2008). An approach that could enhance spore inactivation is to combine high pressure processing with other preservation methods/techniques such as thermal. According to Canovas and Juliano’s (2008) review, a number of approaches were patented for commercial food sterilisation of se- lected low-acid foods; however, these approaches are not practical from either microbiological or economical perspective. Neverthe- less, one emerging technology which has received considerable interest is pressure assisted thermal sterilisation (PATS) (Balasubr- amaniam, 2009). This technology has the benefits of lowering ster- ilization temperature and hence minimizing damage to nutrients in the food, which makes it suitable for processing of low-acid foods. The unique advantage of this process is the rapid tempera- ture increase of the content upon compression and rapid cooling upon decompression. In recent years, several workers have investigated the efficacy of pressure combined with thermal treatment on spore inactivation at temperatures up to 121 °C(Ananta et al., 2001; Koutchma et al., 2005; Okazaki and Suzuki, 2006; Patazca et al., 2006; Rajan et al., 2006a,b; Ahn et al., 2007a; Wimalaratne et al., 2008; Mathys et al., 2009; Ratphitagsanti et al., 2009). Of those studies, Ananta et al. (2001) investigated the inactiva- tion of Geobacillus stearothrmophilus ATCC 7953 spores in mashed broccoli and cocoa mass at temperature ranges from 60 to 120 °C and pressure from 50 to 600 MPa. They showed that spore inacti- vation was enhanced by increasing treatment temperature. Furthermore, inactivation of Geobacillus stearothermophilus spores suspended in egg patties by combined treatment (700 MPa, 105 °C for 5 min) was sufficient to achieve 4 log cfu/ml reduction 0260-8774/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2011.03.016 ⇑ Corresponding author. Tel./fax: +64 9 3737599. E-mail address: m.farid@auckland.ac.nz (M.M. Farid). Journal of Food Engineering 105 (2011) 522–529 Contents lists available at ScienceDirect Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng