Effects of high-pressure and heat treatments on physical and biochemical characteristics of oysters (Crassostrea gigas) M. Cruz-Romero, A.L. Kelly, J.P. Kerry ⁎ Department of Food and Nutritional Sciences, University College Cork, Cork, Ireland Received 13 February 2006; accepted 5 May 2006 Abstract Physical and biochemical changes in oysters following high-pressure (HP) treatment at 260 MPa for 3 min or heat treatment (cool pasteurisation (CP) at 50 °C for 10 min or traditional pasteurisation (TP) at 75 °C for 8 min) were investigated and compared to changes in untreated oysters. HP or TP oysters had higher (P < 0.05) pH values (6.49–6.58) than untreated or CP oysters (6.45–6.46). HP and heat treatment both modified the gross composition of oyster tissue. The protein content of HP-treated oysters (6.9%) was significantly (P < 0.05) lower compared to control or heat-treated oysters (7.9–9.1%). The moisture content of HP-treated whole oyster tissue (86.5%) was higher than that of heat-treated or untreated oysters (83.5–84.7%), but HP or CP treatments did not affect the salt content or water activity of oysters. However, all treatments increased Hunter L- (66.3–68.9) while decreasing a-(- 1.6 to - 2.4) and b- (15.8–14.5) values of oyster tissue; overall, HP treatment had less negative effects on tissue colour of oysters than thermal treatments. HP-treated, CP and TP oysters had higher shucking yields (15.5%, 12.5% and 2.6%, respectively) than untreated oysters. One significant advantage of HP treatment over heat treatment of oysters was that the former process opened the oyster and separated the muscle of the oyster from the shell. © 2006 Elsevier Ltd. All rights reserved. Keywords: High pressure; Oyster; Colour; Proximal Composition; pH; Water activity Industrial relevance: While the effectiveness of HP processing in microbial control in oysters is well accepted, little information exist on the effects on physical and biochemical characteristics of HP-treated oysters, compared to other processing technologies. Compared to heat-treatment, HP-treated oysters had better quality attributes (e.g., colour) and significantly higher yield than heat-treated or untreated oysters, and thus HP processing of oysters is potentially more suitable post- harvest treatment than heat treatment. 1. Introduction Oysters are non-motile marine organisms, dependant on filtering water through the gills to obtain food. As a consequence of filter-feeding, microorganisms and chemicals, including toxic metals, can be accumulated from the marine environment (Klontz & Rippey, 1991). Therefore, the quality of oysters is influenced strongly by the environment in which they grow and feed. Raw and undercooked bivalve molluscs (e.g., clams, oysters, and mussels) represent an important vector of infectious agents and marine biotoxins, largely due to this ability to concentrate pathogens and toxins during filter-feeding (Rippey, 1994). In recent years, much attention has focused on the develop- ment of processing methods to reduce microbial load in post- harvest oysters and, more specifically, to reduce the levels of Vibrio vulnificus. Included in these methods are relaying (Richards, 1988), depuration (Cliver, 1995; Richards, 1988), freezing (Parker, Maurer, Childers, & Lewis, 1994), use of additives (Birkenhauer & Oliver, 2003; Sun & Oliver, 1994a,b), irradiation (Andrews, Jahncke, & Mallikarjunan, 2003; Hu, Mallikarjunan, Koo, Andrews, & Jahnke, 2005; Mallet, Beghian, & Metcalf, 1991; Novak, Liuzzo, Grodner, & Lovell, 1966), thermoradiation (Ama, Hamby, & Toledo, 1994), high- pressure (HP) treatment (Andrews, 2004; Cook, 2003; Hu et al., 2005) and mild heat treatment (Andrews, Park, & Chen, 2000; Cook & Rupple, 1992). The success of the latter method led to the development of a patented “cold pasteurisation” (CP) technique, which involves dipping banded shell oysters in hot water (50 °C for 10 min), followed by the initiation of a rapid chill-down phase (Andrews, 2004; Andrews et al., 2000; Chen, Andrews, Grodner, & Park, 1996; Voisin, 2000). Each of these Innovative Food Science and Emerging Technologies 8 (2007) 30 – 38 www.elsevier.com/locate/ifset ⁎ Corresponding author. Tel.: +353 21 4903798; fax: +353 21 4270213. E-mail address: joe.kerry@ucc.ie (J.P. Kerry). 1466-8564/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ifset.2006.05.002