Impact of vacuum frying on quality of potato crisps and frying oil
Introduction
Deep-fat frying, similar to other heat processing practices, may change sensorial properties and nutritional value of the respective product. Moreover, food safety concerns may arise due to formation of toxic ‘processing contaminants’ (Stadler, 2012, chap. 9). A lot of attention has been paid to acrylamide, classified by the International Agency for Research on Cancer (IARC) as a ‘probable human carcinogen’ (Group 2A) (IARC, 1994), since it was found in heat processed starch-rich foods in 2002. The potentially increased risk of developing cancer for consumers in all age groups due to dietary exposure has been confirmed in the recent EFSA ‘Scientific Opinion on Acrylamide in Food’ (EFSA, 2015).
Acrylamide and its formation pathways in various foodstuffs have been investigated in many studies (EFSA, 2015, Keramat et al., 2011, Nguyen et al., 2016, Nguyen et al., 2017). In general terms, acrylamide naturally originates via Maillard reaction in starchy rich foods with low moisture content when heated above 120 °C, especially in foods containing asparagine and reducing sugars (Biedermann et al., 2002, Mottram et al., 2002). According to EFSA Scientific Panel (EFSA, 2015), potato fried products, such as potato crisps, may significantly contribute to the total dietary exposure in some population groups.
Much effort has been spent on optimisation of processing conditions, enabling the mitigation of this processing contaminant in potato crisps and similar potato-based products fried at high temperatures. The FoodDrinkEurope acrylamide “Toolbox” (Acrylamide Toolbox, 2013) summarises methods to manage levels of acrylamide as a process contaminant. Firstly, agronomic factors are put in place to reduce the sugar content in potato tubers. Secondly several effective intervention steps, which may prevent and/or reduce acrylamide formation in potato crisps (‘French fries and other cut deep-fried potato products’ category), have been found, including: (i) pre-treatment such as blanching and washing (reduction of acrylamide precursors); (ii) use of asparaginase (asparagine deamination); and (iii) regulation of thermal input and moisture.
In the last case, vacuum frying is mentioned as an alternate thermal input control system that, due to lower temperatures, reduces the extent of Millard reactions, including acrylamide formation. The application of these preventive/mitigation measures resulted in a significant downward trend for mean levels of acrylamide in potato crisps, from 763 ± 91.1 μg kg−1 in 2002 to 358 ± 2.5 μg kg−1 in 2011 (EFSA, 2015). However, a small proportion of potato crisps with concentrations exceeding the ‘indicative’ level set to 1000 μg kg−1 by the EU Commission 2013/647/EU (Commission Recommendation, 2013) can still be found on the market. In this context, it is evident that strategies and measures allowing minimization of acrylamide content in this ‘risky’ commodity are needed. So far, the only currently known published study (Granda, Moreira, & Tichy, 2004) focused on acrylamide formation in vacuum fried potato crisps (prepared from varieties grown in the USA) without evaluating the quality of oil absorbed into crisps fried under low pressure.
Potato crisps absorb relatively high amounts of frying oil, typically around 30% (w/w) and some products even more (Moreira, Castell-Perez, & Barrufet, 1999). Both nutritional and sensorial quality of the oil is closely associated with changes that occur throughout the frying process. During conventional frying, where temperatures reach 170 °C, many chemical reactions, amongst the most significant being oxidation and polymerisation, take place. In addition to hydroperoxy, epoxy, hydroxy, and carbonyl groups, these large molecules contain -C-O-C- and -C-O-O-C- linkages (Choe & Min, 2007). The extent of linkaging depends on the composition of the frying oil, specifically on the degree of unsaturation in fatty acids. Key factors playing an important role in these chemical reactions are oxygen and moisture content. The indicators of oil bath quality are not only free fatty acids (released by triacylglycerol hydrolysis), but also various, both low and high molecular weight, oxidation products forming through auto-oxidation reactions. In addition to the deterioration of sensory properties, some of these products are anti-nutritional or even toxic (Zhang, Saleh, Chen, & Shen, 2012). In this context, at the 3rd International Symposium on Deep-Fat Frying in 2000, delegates from the German Society for Fat Science published recommendations for the classification of frying oil quality with limits for total polar compounds in oil (<24%) and for polymerised triacyglycerols (<12%) (Stier, 2013). Limited information regarding the quality of different frying oils during vacuum frying of potato crisps is presented (Basuny et al., 2012, Crosa et al., 2014).
In the past decade, a concern has emerged regarding the content of 3-monochloropropane-1,2-diol (3-MCPD) esters that occur especially in refined edible fats and oils. Since there is a suspicion, that free 3-MCPD (classified as possible human carcinogen) may be released from its bound form by action of gastrointestinal lipases (IARC, 2012), the changes of 3-MCPD esters during frying should be investigated in more detail as they are transferred into potato crisps together with the frying oil.
In the frame of this study, we have tested vacuum frying as a process that is carried out under pressures well below atmospheric levels. Due to lower frying temperatures, vacuum frying reduces undesirable reactions that may occur both in the respective matrix and oil bath. Most of the relevant studies were aimed at its potential to preserve ‘freshness/nutritional value’ of respective product through preventing degradation of natural components, e.g., vitamins, pigments, etc., but also preventing toxic compounds formation (Dueik and Bouchon, 2011, Moreira, 2014). The purpose of our study was not only to minimise acrylamide formation under optimised vacuum frying conditions and to assess the palatability of the final product, but also to evaluate adverse chemical changes in the frying oil, both in terms of the formation of breakdown products and the occurrence of 3-MCPD esters.
Section snippets
Chemicals
Acrylamide (purity 99.5%) was from Sigma-Aldrich (Buchs, Switzerland). 13C3-Acrylamide (isotopic purity ≥99%) was from Cambridge Isotope Laboratories (Tewksbury, MA). Magnesium sulfate (p.a. purity ≥98%) was from Fluka (Tokyo, Japan). Sodium chloride (purity ≥99%), sodium phosphate dibasic dodecahydrate, ethylenediaminetetraacetic acid disodium salt dehydrate (Na2EDTA), aqueous ammonia solution (25%, v/v) (all of purity ≥99%) and anhydrous sodium sulfate were from Penta (Chrudim, Czech
Considerations behind the study design
In the first part of our study, we focused on optimisation of vacuum frying conditions, considering the existing knowledge on the underlying processes. One of the key parameters we had to achieve was as low as possible moisture content (below 2.5% w/w) in the final fried crisps, which is a condition that is necessary in order to avoid microbiological spoilage. At the same time, we also took into account that beyond this point (low moisture level achieved), activation energy of acrylamide
Conclusions
Vacuum frying of potato slices at reduced oil bath temperature (125 °C in this particular case) has been demonstrated as a highly promising processing technology enabling production of safer crisps with improved nutritional value. The major benefits documented in our study are summarised as follows:
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Vacuum fried crisps (product with moisture content <2.5%) contained very low acrylamide levels, not exceeding 250 μg/kg even when prepared from potatoes with a relatively high sugar content. Under such
Acknowledgements
Financial support from specific university research (MSMT No 21/2012) is gratefully acknowledged. This work was supported by the “Operational Programme Prague – Competitiveness” (CZ.2.16/3.1.00/21537 and CZ.2.16/3.1.00/24503) and the “National Programme of Sustainability I” – NPU I (LO1601 – No.: MSMT-43760/2015). Financial support from specific university research (MSMT No 20-SVV/2017).
Special acknowledgment also goes to associate professor Zuzana Reblova and associate professor Zdenka
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