## 1 Introduction  
Fatty acids play an important role in food safety and food quality. Dietary polyunsaturated fatty acids (PUFAs) has been found to have a number of health benefits including a positive effect on cardiovascular health [1]. n-3 PUFAs, abundant in oily fish, and n-6 PUFAs, found in grains are, of particular interest as they are associated with positive effects on a wider range of human health, including anti-inflammatory effect and neurological health [2]. PUFAs are mostly found as triglycerides. Triglycerides are prone to hydrolysis, producing free fatty acids (FFAs). The PUFA FFAs are unstable and can degrade further into hydroperoxides and short chain free fatty acids, causing rancidity, unpleasant taste and smell [3]. As well as affecting the quality, the presence of high levels of FFAs have implications for food safety; high levels of FFAs in different food products can indicate issues with storage time, temperature, moisture content, mould or infestation [4; 5; 6; 7; 8; 9]. Some examples are described below. According to [PERSON] et al., a rancid, bitter or 'off' flavour in dairy is attributed to short-chain FFAs [5]. The concentration of medium to long chain FFAs (which are most abundant) is correlated with the concentration of short-chain FFAs, so measuring these is an indirect measurement of rancid flavour. [PERSON] et al. found that the sensory threshold of rancid flavour was 1.95 mmol of FFA per 100 g of fat [5]. [PERSON] et al. [6] estimated a sensory threshold between 1.58 and 1.76 mmol of FFA per 100 g of fat in 2% fat milk, and [PERSON] et al. correlated FFAs levels of farm milk to sensory perception and only one sample with a FFA content above 2 mmol per 100 g of fat was considered rancid [7]. Altogether, these studies suggest the acceptable value of FFAs to be below 2 mmol per 100 g of fat. Extra virgin olive oil, which is a valuable gourmet product, may not have an FFA content of more than 1%, according to the European Union (EEC No 796/2002) [8]. Other olive oils for consumption have a limit of 3.3%, and oils with a higher FFA content are referred to as lampante and must be refined before consumption [8]. A high FFA content in olive oil is attributed to anomalies during the process of biosynthesis, microbial activities and environmental conditions. Infestation by the olive fly (_Bactrocera oleae_) is a major cause of high FFA content in olives, bruised olives that have fallen from the trees coupled with prolonged storage causes lipolysis and FFAs, and finally, a badly conducted extraction process also decreases oil quality [8]. In the European Union, the official method for determining the FFA content of olive oil is by dissolving the oil in ethyl ether and ethanol and titrating it against an alkali using a phenolphthalein indicator [8]. The level of FFAs in the fat of cocoa beans is a measure of the rancidity of the cocoa. High levels of FFA in cocoa are not acceptable, and in Europe, manufacturers may not legally sell cocoa butter containing over 1.75% FFA [9]. The presence of large amounts of FFAs in the fat of cocoa beans tends to indicate other problems with the cocoa; for example, either it has been wet for too long and/or it has a high percentage of mould, and thus is unsafe for human consumption. Developing effective technologies for the detection and quantification of FFAs is one of the challenges of the international food industry. Fatty acids may be analysed by traditional chromatographic methods; however, these are expensive, time consuming and must be performed in a lab by skilled personnel. In contrast, the novel biosensor approach based on screen-printing carbon electrodes has many benefits--they can be manufactured in a wide-range of geometries at low cost as carbon is an inexpensive material, therefore they can be considered as disposable; these characteristics lead to rapid, portable and user-friendly devices [10].