Chemical compositions and muddy flavour/odour of protein hydrolysate from Nile tilapia and broadhead catfish mince and protein isolate Suthasinee Yarnpakdee a , Soottawat Benjakul a,⇑ , Pimpimol Penjamras b , Hordur G. Kristinsson c,d a Department of Food Technology, Faculty of Agro-Industry, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand b Scientific Equipment Center, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand c Matis – Icelandic Food and Biotechnology R & D, Vinlandsleid 12, IS-113 Reykjavik, Iceland d Department of Food Science and Human Nutrition, University of Florida, Gainesville, FL 32611, USA article info Article history: Received 14 March 2013 Received in revised form 6 July 2013 Accepted 9 July 2013 Available online 17 July 2013 Keywords: Muddy compounds Protein hydrolysate Lipid Nile tilapia Broadhead catfish abstract Chemical compositions and muddy compounds in dorsal and ventral muscles of Nile tilapia and broad- head catfish were comparatively studied. On a dry weight basis, Nile tilapia was rich in protein (93.1– 93.8%), whilst broadhead catfish contained protein (55.2–59.5%) and lipid (36.6–42.4%) as the major con- stituents. Ventral portion had higher lipid or phospholipid contents with coincidentally higher geosmin and/or 2-methylisoborneol (2-MIB) contents. Geosmin was found in mince of Nile tilapia and broadhead catfish at levels of 1.5 and 3.2 lg/kg, respectively. Broadhead catfish mince had 2-MIB at level of 0.8 lg/ kg, but no 2-MIB was detected in Nile tilapia counterpart. When pre-washing and alkaline solubilisation were applied for preparing protein isolate (PI), lipid and phospholipid contents were lowered with con- comitant decrease in geosmin and 2-MIB contents. Protein hydrolysate produced from PI had a lighter colour and a lower amount of muddy compounds, compared with that prepared from mince. Therefore, PI from both Nile tilapia and broadhead catfish could serve as the promising proteinaceous material, yielding protein hydrolysate with the negligible muddy odour and flavour. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Nowadays, aquaculture provides approximately one-third of the world’s fishery products. Muddy flavour and odour are amongst the most severe problems encountered in aquaculture and cause unacceptability by consumer as well as the reduction of market value of product. Two chemical compounds associated with muddy taints are known as geosmin and 2-methylisoborneol (2-MIB). The former compound renders an earthy pond-bottom taste, whilst the latter one is associated with a musty taste (Robin, Cravedi, Hillenweck, Deshayes, & Vallod, 2006). These metabolites produced by cyanobacteria, actinomycetes and certain fungi, are excreted into the environment (Jensen et al., 1994; Lovell & Sackey, 1973). Typically, fish readily absorbs these compounds through the gills, then transfer through the digestive tract, and finally accumu- late in lipid rich tissue. Although both compounds are present at an extremely low level in fish tissue, they can cause the muddy fla- vour due to their low threshold values. Sensory threshold of geos- min in rainbow trout was estimated to be 0.9 lg/kg (Rohrlack, Christoffersen, & Friberg-Jensen, 2005). This is slightly higher than that of channel catfish (0.7 lg/kg) (Dionigi, Johnsen, & Vinyard, 2000; Johnsen & Kelly, 1990). Threshold value of 2-MIB in rainbow trout was 0.6 lg/kg (Persson, 1980). To reduce muddy flavour, several strategies have been pro- posed. Rohani and Yunus (1994) reported that soaking of gutted tilapia in salt solution (5% w/v) for 30 min prior to deboning was able to lower muddy off-flavour of surimi to some degree as re- flected by the higher hedonic score. Additionally, soaking, dipping or washing the farm-fish with tamarind pulp, lemon juice, lemon grass and banana leaf ash can also minimise muddy taint associ- ated with fish tissue (Bakar & Hamzah, 1997; Mohsin, Bakar, & Selamat, 1999). Recently, acid- and alkaline-aided solubilisation has shown significant potential to reduce lipids and phospholipids in fish muscle. DeWitt et al. (2007) reported that 2-MIB and gesos- min spiked in channel catfish (Ictalurus punctatus) were effectively removed by acid and alkaline solubilisation. Due to their lipophilic nature, these muddy compounds might be localised in the lipid rich portion in fish muscle. However, little information regarding the distribution of muddy compounds in Nile tilapia and broad- head catfish muscle has been reported. Additionally, the develop- ment of muddy flavour and odour associated with hydrolysate and its reduction has not been reported. The objective of this study was to investigate the effect of muscle position and pretreatment on muddy flavour and odour associated with the muscle and pro- tein hydrolysate produced from Nile tilapia and broadhead catfish. 0308-8146/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodchem.2013.07.043 ⇑ Corresponding author. Tel.: +66 7428 6334; fax: +66 7455 8866. E-mail address: soottawat.b@psu.ac.th (S. Benjakul). Food Chemistry 142 (2014) 210–216 Contents lists available at ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem