Using ion mobility spectrometry for screening the autoxidation of peanuts

Highlights

• An IMS configuration using the technique of constant RIP was established.

• It was possible to identify oxidised peanuts by IMS without upstream equipment.

• Hexanal showed two distinct peaks, correlated with autoxidation progress of peanuts.

• A rapid screening of peanuts with respect to quality seems possible.

Abstract

Autoxidation is a critical process in many fat containing foods that leads to reduced palatability because of the formation of off-flavour compounds. The aim of the study was to evaluate the applicability of ion mobility spectrometry (IMS) for detecting volatile off-flavour indicators from roasted peanuts which were subjected to storage at elevated temperature. IMS measurements were carried out using a sample inlet system that allowed to keep the reactant ion peak constant. It is evident from the ion mobility spectra that the level of autoxidation significantly affects the signal intensities in particular drift time regions. Supported by the IMS measurement of pure hexanal, and by qualitative analysis for volatile aldehydes and organic acids, the IMS peaks at relative drift times of approx. 1.7 have a large potential for being used as rancidity indicators.

Introduction

Lipid oxidation is an important group of reactions that, once in progress, lead to the formation of compounds which significantly affect the sensory quality of foods by causing off-flavour, and deviations in appearance and texture. Primary oxidation products that are further decomposed have also been suggested as having negative health implications, and reducing the nutritional value of fat-containing foods (Decker et al., 2010, Guéraud et al., 2010).

The fatty acid composition of peanuts largely depends on the cultivar. For example, the contents of oleic and linoleic acid, being the most dominant in peanuts, varied for US-grown peanuts of the type Runner from 49 to 57% and 22–31%, respectively (Shin, Pegg, Phillips, & Eitenmiller, 2010). Similar cultivars from Argentina significantly differed with respect to these fatty acids (40–49% oleic acid, and 33–41% linoleic acid; Grosso, Lamarque, Maestri, Zygadlo, & Guzmán, 1994). In regular oleic cultivars, hexanal, octanal, nonanal, and decanal can be considered as the major reaction products of autoxidation (Nawar, 1985). Hexanal is, for example, a frequently used marker to follow the progress of lipid oxidation in meat products and butter (Brunton et al., 2000, Panseri et al., 2011), infant formula (Guadalupe García-Llatas, 2007), potato crisps (Sanches-Silva, Rodríguez-Bernaldo de Quirós, López-Hernández, & Paseiro-Losada, 2004), nuts (Pastorelli et al., 2006, Williams et al., 2006), and baked foods (Purcaro, Moret, & Conte, 2008). Different extraction methods have been applied for monitoring volatiles from lipid oxidation, for example solid phase micro-extraction (SPME) (Brunton et al., 2000, Guadalupe García-Llatas, 2007, Panseri et al., 2011, Pastorelli et al., 2006, Purcaro et al., 2008), or automated dynamic headspace gas chromatography (Ha & Wang, 2013). Electronic noses were also used for analysing volatiles generated by autoxidation of nuts (Pastorelli et al., 2007, Williams et al., 2006). Although there is a wide range of research applications for e-noses, they are still far away from being applied in industrial processes because of shortcomings such as sensor poisoning, sensor drift and a lack of sensitivity (Loutfi, Coradeschi, Mani, Shankar, & Rayappan, 2015).

Ion mobility spectrometry (IMS) is an analytical technique for characterising chemical substances on the basis of the velocity of gaseous ions in an electrical field at ambient pressure (Eiceman, Karpas, & Hill, 2013). The analytical use stems from the correlation between the chemical composition of a sample and the gaseous ions that are released from it (Eiceman, 2002). IMS was initially used for real-time detection of warfare agents and drugs in small amounts, but further investigations discovered it as promising technique for domains such as food characterisation (Karpas, 2013). One of the earliest studies on the use of stand-alone IMS in the context with foods is from Karpas, Tilman, Gdalevsky & Lorber (2002) who successfully determined volatile biogenic amines in muscle foods. The same analyte was chosen by Karpas, Chaim, Gdalevsky, Tilman & Lorber (2002) for evaluating the diagnostic potential of microbial infections. In combination with a simple sample inlet system IMS was used to detect volatile compounds in fish or, in combination with a thermo reactor, it was applied to investigate the feeding regime of pigs by analysing the volatiles in their fat (Alonso et al., 2008, Menéndez et al., 2008). Márquez-Sillero, Cárdenas, and Valcárcel (2011) determined 2,4,6-trichloroanisole in wine with a single-drop ionic liquid extraction coupled to a multicapillary column separation and subsequent IMS detection and, in a follow-up study, they used a headspace multicapillary column ion mobility spectrometer for that purpose Márquez-Sillero, Cárdenas, and Valcárcel (2012). IMS coupled to a multicapillary column has also been tested for controlling and predicting the quality and shelf life of olive oil (Garrido-Delgado et al., 2015). Shuai et al. (2014) investigated a rapid screening method to detect the adulteration of flaxseed oil by using a pulsed glow discharge ionization source in negative mode, followed by multivariate analysis. The quantitation of hexanal in linseed oil and milk was possible using a coupled headspace – multicapillary column – IMS system (Márquez-Sillero, Cárdenas, Sielemann, & Valcárcel, 2014). This system allowed the separation of volatile compounds, but shows the disadvantage of the loss of the possibility for in-line measurements. Banach, Tiebe, and Hübert (2012) investigated IMS applicability for detecting spice adulteration, and concluded that conventional chemical analysis and olfactometric investigations are not obsolete, but e-nose and IMS may be useful tools for cost efficient quality monitoring by pre-identifying suspect samples for further analysis.

The aim of this study was to investigate the potential of using an IMS without any additional upstream equipment for tracing the autoxidation progress of blanched, roasted peanuts as affected by storage time. Advantages of this experimental set-up are a fast response, low purchase and maintenance costs, and a robust, transportable hardware for screening a high number of samples.