Acrylamide formation and reduction in fried potatoes

Abstract

In April 2002, Swedish researchers shocked the food safety world when they presented preliminary findings of acrylamide in some fried and baked foods, most notably potato chips and French fries, at levels of 30-2300 m/kg. Reports of the presence of acrylamide in a range of fried and oven-cooked foods have caused worldwide concern because this compound has been classified as probably carcinogenic in humans with significant toxicological effects namely neurotoxic and mutagenic (Rosen and Hellenäs, 2002; Tareke et al., 2002).

Before its discovery in foods, acrylamide was known as an industrial chemical and a component of cigarette smoke. Acrylamide is a known carcinogen substance in experimental animals that occurs in carbohydrate-rich foods as a result of cooking methods at high temperatures. As acrylamide has not been detected in unheated or boiled foods, it was considered to be formed during heating at high temperatures. They attributed this fact to the higher temperatures reached in Maillard nonenzymatic browning reactions required for desirable color, flavor and aroma production, specially in the surface of fried potatoes (Coughlin, 2003). For instance, Tareke et al. (2002) showed that acrylamide was formed by heating above 120°C certain starch-based foods, such as potato chips, French fries, bread and processed cereals. French fries and potato crisps exhibit relatively high values of acrylamide 424g/kg and 1739g/kg, respectively.

Recent epidemiological studies by the University of Maastricht supported by the Dutch Food Safety Agency indicate a positive association between dietary acrylamide and the risk of certain types of cancer (Hogervorst et al., 2007). These researchers observed increased risks of endometrial and ovarian cancer with increasing dietary acrylamide intake, particularly among never smokers. Risk of breast cancer was not associated with acrylamide intake. Choosing a balanced and varied diet, and avoiding overcooking of certain starchy foods, will contribute to reducing dietary intake of acrylamide.

Acrylamide concentration determination nowadays appears to be a necessity since very high concentrations of this potentially toxic molecule were detected in amylaceous fried foodstuffs and cause cancer in rats (Rosen and Hellenäs, 2002). The analytical methods for acrylamide determination relies on using (i) gas chromatography and mass spectrometry -GC-MS- (Tareke et al., 2000), (ii) liquid chromatography and tandem mass spectrometry -LC-MS-MS- (Rosen and Hellenäs, 2002). Granby and Fagt (2004) validated an analytical method for analyzing acrylamide in coffee. Recently, a liquid chromatography-tandem mass spectrometry analytical methodology for simultaneous analysis of acrylamide and their precursors such as asparagine and glucose was implemented with a detection limit for acrylamide of 20 µg/kg for French fry analysis (Nielsen et al., 2006).

Acrylamide formed in potatoes during frying is highly related to the colour of potato chips (Rosen and Hellenäs, 2002; Mottram et al., 2002; Stadler et al., 2002; Pedreschi et al., 2005). Some international research groups have separately confirmed a major Maillard reaction pathway for acrylamide formation (Coughlin, 2003). For instance, Mottram et al. (2002) showed how acrylamide could be formed from food components during heat treatment as a result of the Maillard reaction between amino acids and reducing sugars. On the other hand, Stadler et al. (2002) have shown also that acrylamide can be released by the thermal treatment of certain amino acids such as asparagine, particularly in combination with reducing sugars, and of early Maillard reaction products (N-glycosides).

Wide variations of acrylamide concentration in foods are, at least partially, caused by different levels of precursors of acrylamide in various batches of raw materials (levels of asparagine and sugars fluctuate widely in raw potato tubers). For example, Rydberg et al. (2003) showed that both addition of glucose and asparagine to the levels naturally occurring in potatoes would increase the acrylamide levels in fried potatoes. Both potato variety and field site (fertilization and storage conditions) had a noticeable influence upon acrylamide formation since they affect acrylamide precursor concentrations in the tubers. In addition to potato tuber composition, other factors involved in acrylamide formation are the processing conditions (pre-treatments, temperatures and times of frying, type of frying and post-frying treatments).

The potential capability of different potato varieties to form acrylamide during heat treatment correlated well with the concentration in the tubers of reducing sugars (especially glucose and fructose) and asparagine. The potato cultivars show large differences in their potential to form acrylamide which was primarily linked to their sugar contents (Amrein et al., 2003). Since potato tubers are especially high in the aminoacid asparagine, it is now thought that Maillard reaction is most likely responsible for the majority of the acrylamide found in potato chips and French fries (Friedman, 2003). Martin and Ames (2001) found that asparagine was the free amino acid present in the highest amount in potatoes (93.9 mg/100g). Davies (1977) and Hippe (1988) reported that asparagine is present in potatoes in varying, relative high amounts of 0.5-3% of dry matter. On the other hand, potential of acrylamide formation is also related to the sugar content such as glucose and fructose (Biedermann et al., 2002). Sugars accumulate in potato tubers, when there is an imbalance between starch degradation, starch synthesis, and respiration of carbohydrates. Storage temperature and physiologhical age of the tubers are the most important factors that affect this process of sweetening. Potatoes aimed for processing are stored at relatively high temperature (e.g. 8 °C). In practice a limit of 1.5-2.0 mg/g of fresh weight of reducing sugars in potato tubers is used as an indicator for suitability for processing (Burton, 1969). Beside diversity in storability between cultivars, large variation is often found between different potato lots/fields of the same cultivar within and between years and hence they should be managed accordingly (Olsson et al., 2004). The amount of sugars and aminoacids in potato tubers is influenced by different factors such as: potato cultivar, fertilizers, climate, storage conditions, etc.

Acrylamide is formed during frying mainly due to reaction of asparagine and reducing sugars and, as this reaction pathway is clearly correlated to Maillard reaction (Mottram et al., 2002; Stadler et al., 2002; Pedreschi at al., 2005). For this reason, it has been proposed by several authors to perform quick and easy measurement by way of image analysis of chips browning, rather than using painstaking chromatographic methods (Pedreschi et al., 2007). However, standard procedures for acrylamide determination involve slow and expensive methods of chromatography and mass spectroscopy and, thus cannot be used for routine analysis. It was therefore a logical step to develop alternative methods based on image analysis of chips browning to measure acrylamide concentration. Currently, a substantial body of research has been carried out world-wide to build greater understanding of acrylamide, how it is formed in foods, what the risks are for consumers and how to reduce occurrence levels. This book chapter shows not only the principal mechanism(s) of acrylamide formation but also several acrylamide mitigation procedures reported in the literature that could be very useful for fried potato manufacturers in order in reduce to reasonable levels the acrylamide generation in their processing lines.