Formation of Volatiles in the Lipoxygenase Pathway as Affected by Fruit Type and Temperature Yafei Han and Sheryl Barringer * Department of Food Science and Technology, The Ohio State University, 2015 Fyffe Road, Columbus, OH 43210, USA. * Correspondence author: Sheryl Barringer, Department of Food Science and Technology, The Ohio State University, 2015 Fyffe Road, Columbus, OH 43210, USA. Tel: 614-688-3642; E-mail: barringer.11@osu.edu Received date: Sep 16, 2015; Accepted date: Nov 2, 2015; Published date: Nov 9, 2015 Copyright: © 2015 Han Y, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Abstract Objective: The objective of this study is to study the effect of temperature, lipoxygenase activity, substrate and fruit type on formation of volatiles in the LOX pathway. Methods: Volatiles in the headspace of tomato, bell pepper, tomatillo, strawberry, apple and pear were measured during and after fruit tissue disruption in real time using selected ion flow tube mass spectrometry (SIFT-MS) at 4, 25, 37 and 45°C for 60 min, and were compared to lipoxygenase (LOX) activity and fatty acid concentration. Results: In tomatoes, bell peppers and tomatillos, (Z)-3-hexenal, (E)-2-hexenal and hexanal increased rapidly after blending, peaked and declined, while in strawberry, apple and pear they sometimes peaked and sometimes remained constant over the 60 min. In tomato and bell pepper, the peak levels of (Z)-3-hexenal, (E)-2-hexenal and hexanal in the LOX pathway had good correlations to LOX activity, which was greatest at 25°C. In contrast, tomatillo, strawberry, apple and pear had lower LOX activity, their 6C aldehydes did not show strong correlation with LOX activity, and the volatiles increased with increasing temperature to 45°C. The concentrations of hexen-1-ol and 1- hexanol did not correlate to LOX activity for any of the fruits. In tomato, bell pepper and tomatillo, there was no significant correlation between the ratio of linoleic acid:linolenic and the ratio of hexanal:hexenal. In strawberry, apple and pear, there were good correlations between linolenic acid content and concentrations of (E)-2-hexenal, hexenal and hexen-1-ol. Conclusion: Thus the effect of enzyme activity and substrate composition on formation of volatiles in the lipoxygenase pathway remains complicated and difficult to predict. Keywords: Lipoxygenase; Temperature; Tomato; Bell pepper; Tomatillo; Strawberry; Apple; Pear; Selected ion fow tube mass spectrometry Introduction As aroma is one of the most important characteristics for fruit quality, volatile compounds are likely to have a huge efect on the perception and liking of fruit products by consumers. In fruits, important aroma compounds are derived from fatty acids, amino acids, phenols and terpenoids [1]. Tus the efect of enzyme activity and substrate composition on formation of volatiles in the lipoxygenase pathway remains complicated and difcult to predict. Among these compounds, volatiles generated from fatty acids via the lipoxygenase (LOX) pathway are responsible for the “green” fresh note, which is desirable in many fruit products [2]. Te LOX pathway is an enzymatic pathway for lipid oxidation. Te main products of the LOX pathway are C6 aldehydes and alcohols generated through sequential enzymatic reactions. Te major enzymes in this pathway include lipoxygenase, hydroperoxide lyase (HPL), alcohol dehydrogenase (ADH) and (Z)-3/(E)-2 isomerase [3]. Linoleic and linolenic acids are the substrates that are converted to volatiles such as hexanal, (Z)-3- hexenal, (E)-2-hexenal, 1-hexanol, hexen-1-ol, (E)-2-octenal, (E)-2- heptenal, (E)-2-pentenal 1-penten-3-one, hexanoic acid, (Z)-3-hexenyl acetate and hexyl acetate. Temperature afects the volatile concentrations in the headspace, due to two main factors: diferent partition coefcients at diferent temperatures, and diferent enzymatic activities at diferent temperatures. Te partition coefcient is the ratio between the concentrations of the volatile compound in the gas and liquid phase at equilibrium [4]. Te partition coefcient is temperature dependent; the log value of the partition coefcient is linearly related to the temperature [5]. Temperature also has an efect on activity of enzymes including those involved in the LOX pathway. Tomato LOX activity increases from 0°C to 20°C, reaches its maximum activity at 20°C to 30°C and decreases at higher temperatures [6,7]. In tomatoes, HPL activity increases from 5°C to 25°C and decreases from 25°C to 40°C [8,9]. Tomato ADH activity increases from 10 to 20°C and decreases from 20°C to 52°C [8,10]. Diferent fruits have diferent linoleic acid content, linolenic acid content and LOX activity, which may explain diferent LOX volatile profles among fruits. When extra linoleic acid was added to tomato homogenates, the hexanal level increased [11]. Similarly, the hexenal level was increased by addition of linolenic acid [11]. Te increase of hexanal and 1-hexanol upon addition of linoleic acid and the increase of (Z)-3-hexenal and (E)-2-hexenal upon addition of linolenic acid have been reported in other studies in tomato [12,3], bell pepper [13] and cucumber [14]. When LOX activity in tomato was decreased by reducing expression of LOX mRNA, the levels of hexenal and hexanal Han and Barringer, J Exp Food Chem 2016, 1:1 DOI: 10.4172/2472-0542.1000102 Research Article open access J Exp Food Chem ISSN:2472-0542 JEFC, an open access journal Volume 1 • Issue 1 • 1000102 J o u r n a l o f E x p e r i m e n t a l F o o d C h e m i s t r y ISSN: 2472-0542 Journal of Experimental Food Chemistry