ORIGINAL PAPER

Effects of Added Phenolics on the Lipid Deteriorationand Antioxidant Content of Deep-Fried Potato Fritters

Dongxiao Sun-Waterhouse & Dongni Xue & Sandhya Wadhwa

Received: 12 August 2012 /Accepted: 4 November 2012# Springer Science+Business Media New York 2012

Abstract The health benefits of phenolic antioxidants justifytheir inclusion in foods like deep-fried potato fritters that areexposed to high heat during processing. In this study, phenolicantioxidants [quercetin, rutin or an apple phenolic extract(APE)] were incorporated into batters that were subsequentlyused in the preparation of deep-fried potato fritters. The fritterswere deep fried at 180 °C for 2.5 min or 165 °C for 3 minusing fresh or used canola oil. The study aimed to investigatethe effect of added phenolic antioxidants on the lipid deterio-ration during deep frying potato fritters. The effectiveness ofthese phenolics against lipid deterioration was also examinedfor the potato fritters that have been left in the air at roomtemperature for 0.5 or 1 h after deep frying. The total oxidation(Totox) value (based on the peroxide value and p-anisidinevalue) and free fatty acid content of the oil extracted from thedeep-fried potato fritters, as well as the total extractable phe-nolic content, of the fritters were evaluated. Results showedthat APE, quercetin and rutin suppressed oil deterioration todifferent extents, and their effectiveness was influenced bydeep-frying conditions, the number of times the oil hadbeen used and the time period that the fritter was exposed tothe air after deep frying. For the fresh or singly used oil, the

recommended deep-frying parameters are 180 °C for 2.5 min(“high heat short time” approach). For oil used more thanonce, deep frying of potato fritters at 165 °C for 3 min (“lowheat long time” approach) is generally recommended. Addingphenolics to the batter used for making potato fritters beforedeep frying increases product nutritional value and reduces oiloxidation, which indicates the feasibility of producing health-ier potato fritters.

Keywords Deep-fried potato fritters . Hydrolytic rancidity .

Nutritional value . Overall oxidation status . Total phenoliccontent

Introduction

Deep-fried foods such as potato fritters, potato chips, Frenchfries and fish sticks are popular fast foods due to theirunique flavour, appealing aroma, crispy texture and pleasanteating characteristics (Tsaknis et al. 1999; Schlosser 2001;Samra 2010; Soorgi et al. 2011). Consumers are increasing-ly looking for fast foods with reduced health hazards/risksand increased nutritional values, without compromisinggood taste (Jayarajah and Sevugan 2009). Numerous studieshave been reported examining the impacts of deep-friedfoods on human health, as well as how to prevent nutrientloss and lipid oxidation during deep frying of foods.

Typically, when foods are deep fried, water is lost, andcholesterol, saturated fats and energy density are increased(Ghidurus et al. 2010). Because the frying time is generallyshort, minimal negative impacts were found on the digestibilityof macro- and micro-nutrients (including proteins and carbo-hydrates), the stability of water-soluble vitamins and the bio-availability of magnesium and calcium in the fried foodsprepared using plant oils (Moreiras-Varela et al. 1988; Bognar1998; Perez-Granados et al. 2000a, b).

D. Sun-Waterhouse :D. Xue : S. WadhwaThe New Zealand Institute for Plant and Food Research Limited,Mt. Albert Research Centre, Private Bag 92169,Auckland 1020, New Zealand

D. XueFood Science, School of Chemical Sciences,University of Auckland, Private Bag 92019,Auckland, New Zealand

D. Sun-Waterhouse (*)Food Innovation, The New Zealand Institute for Plant and FoodResearch Limited, Mt. Albert Research Centre, Private Bag 92169,Auckland 1020, New Zealande-mail: Dongxiao.Sun-Waterhouse@plantandfood.co.nz

Food Bioprocess TechnolDOI 10.1007/s11947-012-1001-8

Fats and oils play important roles in the processing andsensory properties of these deep-fried foods (Stevenson et al.1984; Drewnowski and Almiron-Roig 2010). The character-istic fried flavour is due primarily to lipid degradation prod-ucts generated from frying oils (Pokorny 1999). Deep fryingcauses deterioration of frying oils via oxidation and hydroge-nation processes, which damage health-promoting unsaturat-ed fats and introduce health hazards such as oxidised productsand trans fats (Sanchez-Muniz et al. 1994; Fillion and Henry1998; Pokorn et al. 2003; Pedreschi 2009). Hydrolytic rancid-ity may also occur when food containing moisture is fried inheated oil (Chung et al. 2004): Water molecules can react withthe ester linkage of triacylglycerols producing diacylglycerolsand free fatty acids (i.e. FFA) which further accelerate thehydrolysis process (Frega et al. 1999) and impart undesirablearoma (O’Brien 1993). Such effects become more severewhen reused oils are used for frying (a common practice forfried foods prepared away from home) (Romero et al. 1998;Ng 2007). Health problems such as increased risk of cardio-vascular disease, obesity and hypertension are associated withlipid deterioration in deep-fried foods (Soriguer et al. 2003;Guallar-Castillón et al. 2007; Iqbal et al. 2008; Astrup et al.2011; Sayon-Orea et al. 2011).

Oil stability depends on chemical and physical factors suchas light, temperature, pH, transition metal ion concentration,fatty acid (FA) composition and availability of oxygen (Frankelet al. 2002; Sørensen et al. 2008). Preventing or minimisinglipid rancidity can be achieved by adding antioxidants such asthe synthetic antioxidant butylated hydroxytoluene or naturalantioxidant vitamin E into edible oils. More recently, phenolicantioxidants have been investigated as an alternative antioxi-dant to combat oil deterioration, because of their dual benefits(i.e. health-promoting properties and ability to intercept freeradicals) (Bravo 1998; Arts and Hollman 2005; Anwar et al.2010; Sun-Waterhouse et al. 2011a, b, c). Adding an antioxi-dant to the oil for deep frying has been investigated (Allam andEl-Sayed 2004). In this study, we examine an alternative ap-proach in which antioxidants for fortification are added directlyto foods (battered potato fritters), to suppress lipid deteriorationduring deep frying: phenolic antioxidants, apple phenolicextract (APE), quercetin and rutin were fortified directly intothe batter of potato fritters (to improve oxidative stability of theoils absorbed by fritters). Canola oil is selected as the fryingmedium in this study, because it is good oil containing about6.3 % saturated FAs, 62.4 % monounsaturated FAs and 31.3 %polyunsaturated FAs, and commonly used for deep-fryingapplications (Ackman 1990). The deep-frying temperaturesgenerally range from 170 to 200 °C. Based on our preliminaryinformal sensory evaluation results, deep frying at 180 °C for2.5 min or 165 °C for 3 min was selected to produce perfectlycooked potato fritters. The feasibility of reusing deep frying oilonce and twice under different processing and post-fryinghandling conditions is also evaluated. Lipid deterioration was

evaluated based on overall oxidation status (Totox value) andhydrolytic rancidity (FFA content). Total phenolics retained inthe deep-fried fritters were measured to indicate the potentialnutritional advantage of adding phenolics directly into fritterbatter.

Materials and Methods

Chemicals and Materials

Pams® canola oil is marketed by Pams Products Ltd.,Auckland, New Zealand (light colour, naturally no cholesterol,high smoke point, containing saturated fat 15.0 g, trans fat<0.1 g, polyunsaturated fat 8.3 g, omega 3 fat 2.5 g andmonounsaturated fat 13.3 g per 25 mL serving). Washed paleyellow potatoes (Agria), Pams® self-raising flour and beer(Monteith’s Brewery Company, New Zealand) were purchasedfrom a local supermarket in New Zealand. APE was purchasedfrom Penglai Marine Bio-tech Co., Ltd, Shandong, China,containing 80 g phenolics per 100 g ingredient (manufacturerinformation). Active carbon (activated charcoal), catechin,rutin, quercetin, Folin–Ciocalteu’s phenol reagent (2 N), p-anisidine and tridecanoic acid were purchased from Sigma-Aldrich Chemie, Steinheim, Germany. Absolute ethanol andsodium carbonate were purchased from Merck, Darmstadt,Germany. Glacial acetic acid, potassium iodide, chloroform,concentrated HCl (36 %), isooctane, methanol, n-hexane(95 %), potassium dichromate, sodium thiosulphate and sul-phuric acid were purchased from Ajax Finechem Pty. Ltd.,Sydney, Australia. Ammonium chloride was purchased fromMay & Baker, Dagenham, England. Starch and sodium sul-phite were purchased from AnalaR, BDH Laboratory Chem-icals Ltd, Poole, England. Phenolphthalein indicator andsodium hydroxide were purchased from Scharlau, ScharlauChemie, Barcelona, Spain. All reagents are of analytical grade.

Preparation of Potato Fritters

Batter was formulated in the absence (control sample) orpresence of a phenolic ingredient, APE, quercetin or rutin, ata concentration of 300 mg/kg batter. Batter was prepared bymixing Pams self-raising flour (25 g) and Monteith’s beer(37 mL) until a smooth and thick texture was achieved (notlumpy). Uncooked potatoes were washed, peeled and gratedusing a hand grater to produce coarse potato strands. Thepotato strands (25 g) were moulded using a glass Petri dish(diameter, 100 mm) to achieve a constant fritter shape andsize. Batter (20 g batter per fritter) was added to coat all thepotato strands (allowing 5 min soaking to achieve an evencoating). The coated potato fritters were placed in stainlesssteel mesh baskets and deep fried using a BDF450 SyncroDeep Fryer (Breville Pty. Ltd., Sydney, Australia) which has

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an electronic thermostat (digital temperature control). Thedeep fryer contained 1 l of canola oil [“fresh” oil (oil fromthe unopened container and before the expiry date), “reusedonce” or “reused twice” oil (oil has been used for deepfrying potato fritters once or twice, respectively, and sub-jected to filtration)]. Deep-frying conditions were set as“high heat short time” (180 °C for 2.5 min) or “low heatlonger time” (165 °C for 3 min). After deep frying, thepotato fritters in the stainless steel mesh baskets were takenfrom the fryer, and the basket was banged three times toremove excess oil. The deep-fried potato fritters were thenplaced on layers of kitchen paper towels for 2 min (to absorbexcess oil) and transferred onto plates. These potato fritterswere left for 0.5 or 1 h on the plates in the air (at roomtemperature, 20±1 °C; relative humidity, 51 %) to cool, anda portion of which was used for an informal sensory evalu-ation, with the remaining being immediately subjected to oilextraction for chemical analysis. Two batches of deep-friedpotato fritters were produced for each fritter formulation(control and APE, rutin, or quercetin fritter), each deep-frying condition (180 °C for 2.5 min or 165 °C for 3 min)and for each set of handling condition after deep frying (leftfor 0.5 or 1 h at room temperature). Informal sensory evalu-ation was conducted on the control and APE-fortified deep-fried potato fritters that were freshly prepared with fresh orreused oil at 180 °C for 2.5 min or 165 °C for 3 min.

Extraction of Oils from Deep-Fried Potato Fritters

Oil was extracted in duplicate from each type of potato frittersusing the following procedures. Deep-fried potato fritter wasground using a coffee grinder (CG 2B, Breville, NSW,Australia). Two successive extractions were conducted: 10 g ofthe ground fritter was placed in a beaker, and 50 mL ofchloroform and 50 mL of methanol were added. The mixturewas homogenised using a TP 18/10 Ultra-Turrax homogeniser(at 12,000 rpm for 2.5 min; IKA Werke, Janke and Kunkel,Staufen, Germany). The solvent layers were collected respec-tively. These steps were repeated, and the combined solventlayers were evaporated using a Labconco RapidVap® Concen-trator (40min at 40 °C and 10 kPa; model 79100-01, LabconcoCorp., Kansas City, MO, USA) under nitrogen gas and dried inthe Ultra-Low Cold Trap Centrivap® Centrifugal Concentrator(model 78100-01, Labconco Corp., Kansas City, MO) at40 °C for 4 h under vacuum. The extracted oils were tightlysealed, covered with aluminium foil and kept in a −80 °Cfreezer (Thermo Electron Corporation, Asheville, USA) untilchemical analysis.

Totox Value Determination

The Totox value indicates the overall oxidation status ofoil. Totox values were calculated using the equation of

Totox value02PV+p-AV, where PV0peroxide value andp-AV0p-anisidine value.

The PVs of the extracted oils were determined using theAmerican Oil Chemists’ Society (AOCS) (1998a) andexpressed as peroxide milliequivalent per kilogram oil. Analiquot (0.2 g) of oil was used for each assay. The p-AVs ofthe extracted oils were determined using AOCS (1998b). Analiquot (0.2 g) of oil was used for each assay. Absorbance oftesting samples was measured using a 96-well microplate(200 μL) and a spectrophotometer (SpectraMax Plus 384,MDS Analytical Technologies, Hawthorn, Australia).

Free Fatty Acid Determination

FFAs were determined using the direct titration method of theAOCS (2000). Percent FFA is calculated: FFA (in percent, asoleic acid)0[(VNaOH×NNaOH×282.46) / (1000×m)]×100.

Total Extractable Phenolic Content Determination

The extraction of phenolics from the extracted oils followedthe method of Sun-Waterhouse et al. (2011b). Extractionwas repeated three times for each oil sample. The driedphenolic extracts were reconstituted in methanol (1 mL,methanol/water 25:75 v/v) for the Folin–Ciocalteu assay(Singleton et al. 1997). The total extractable phenolic con-tent (TEPC) is expressed as milligram catechin equivalentsper kilogram potato fritter.

High-Performance Liquid Chromatography Profilingof Apple Phenolic Extract Ingredient

The APE ingredient was mixed with 25 % methanol to givea concentration of 30 mg/mL. The resultant mixture wasvortexed and centrifuged (Eppendorf Centrifuge 5702,Hamburg, Germany) at 3,000 rpm for 15 min. The superna-tant recovered was analysed by HPLC following the methodof Stevenson et al. (2006), using a Shimadzu LC10AvpHPLC equipped with a SPDM10Avp PDA detector(Shimadzu Co., Kyoto, Japan) and a Synergi Fusion–RP 80A column (4 μm, 250×4.6 mm, Phenomenex, Torrance, CA).Solvents were (A) acetonitrile containing 0.1 % formic acidand (B) 96:2.5:1.5 (water/acetonitrile/formic acid), and theflow rate was 0.5 mL/min. The initial mobile phase, 0 % A,was ramped linearly from 0 to 5 % A (0–5 min then held for5 min), from 5 to 15 % A (10–25 min), from 15 to 19 % A(25–30min), from 19 to 25%A (30–39min), from 25 to 45%A (39–43 min), and finally from 45 to 95 % A (43–48 minthen held for 5 min) before resetting to the original conditions.The wavelength of the UV–vis absorbance detection rangedfrom 200 to 600 nm. The sample injection volume was 10 μL.Individual phenolics were identified based on their retentiontime and absorbance maximum (λmax). External standards

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including catechin, caffeic acid, chlorogenic acid, epicatechin,phloretin, phloridzin, quercetin, quercetin glycosides, rutin,myricetin, myricetin glycosides, kaempferol glucoside, 2, 4-dihydroxybenzoic, p-hydroxybenzoic, protocatechuic, sali-cylic, syringic, ferulic and p-, m-and o-coumaric acids wereused for component identification.

Statistical Analysis

At least three replicate determinations were obtained foreach datum point. Data were analysed using the Dunnettmethod in one-way ANOVA.

Results and Discussion

Overall Sensory Characteristics of the Control and FortifiedPotato Fritters

All the control and fortified fritters had nearly the sameshape, diameter and thickness. When the same formulations,processing and handling conditions were applied, the potatofritters deep fried at 180 °C for 2.5 min appeared to beslightly darker in colour and crispier in texture than thosefried at 160 °C for 3 min. The fritters that have been left inthe air for 1 h were slightly softer, compared to those left for0.5 h. The fritters deep fried with fresh oil generally had aslightly lighter colour, compared to those prepared withreused oil. Undesired notes derived from reused oils becamedetectable for the fritters deep fried with “reused twice” oil,and these notes were not detected for all the APE-fritters.

Minimal flavours originated from the APE ingredient weredetected in the APE-fortified fritters, possibly due to thedominant deep-frying fritter flavour and the very low APEconcentration in each fritter. Under the same conditions, thebatter of the APE-fortified fritters had a slightly darkercolour than the control fritters (due to the brown colourintroduced by APE), although there was no significant dif-ference in the colour inside the potato strands between thecontrol and fortified fritters.

Overall Oxidative Status of Different Potato Fritters

The overall oxidation status of an oil can be evaluated basedon the Totox values (Table 1). The Totox values of deep-fried potato fritters were influenced by the presence and typeof antioxidant, the nature of oil for deep frying (fresh orreused), deep-frying temperature and time period, and thetime length of exposure in the air after deep frying. Most ofthe Totox values were below 30 (the maximum acceptablethreshold being 30) (O’Connor et al. 2007), except for thecontrol potato fritters that have been deep fried at 180 °C for2.5 min using the “reused twice” oil, or using the “reused oilonce” and left for 1 h afterwards. In general, deep fryingwith reused oil caused increased oxidation. The differencein the Totox values between “fresh” oil and “reused once”oil was bigger than between the “reused once” and “reusedtwice” oil. Leaving the deep-fried fritters in the air for alonger period resulted in elevated Totox values.

When “fresh” or “reused once” oil was used and thefritters were left for 0.5 h after deep frying, the potato frittersprepared at 180 °C for 2.5 min had lower Totox values than

Table 1 Totox values of potato fritters

Oil Potato fritter formulation 180 °C×2.5 min 165 °C×3 min

Left aside 1 h Left aside 0.5 h Left aside 1 h Left aside 0.5 h

Fresh Control 12.5±0.36 A, i 5.56±0.15 D, h 8.40±0.51 B, g 7.80±0.26 C, f

APE 7.8±0.56 A, l 4.00±0.30 C, i 6.43±0.29 B, i 6.03±0.20 B, g

Rutin 10.8±0.35 A, j 4.75±0.36 D, i 7.50±0.34 B, h 6.69±0.22 C, g

Quercetin 9.6±0.10 A, k 4.53±0.32 D, i 7.00±0.23 B, h 6.34±0.20 C, g

Reused once Control 31.0±0.00 A, b 19.4±0.36 D, e 27.5±0.34 B, b 21.9±0.47 C, b

APE 18.9±0.05 A, h 12.5±0.69 D, g 16.0±0.22 B, f 13.0±0.35 C, e

Rutin 24.0±0.31 A, f 13.3±0.52 D, f 18.8±0.27 B, e 14.0±0.57 C, e

Quercetin 22.8±0.35 A, g 13.1±0.35 D, f 18.2±0.40 B, e 13.6±0.64 C, e

Reused twice Control 34.9±0.54 A, a 31.6±0.43 B, a 29.2±0.24 C, a 26.3±0.45 D, a

APE 25.8±0.69 A, e 22.3±0.50 B, d 21.9±0.35 B, d 19.0±0.69 C, d

Rutin 28.7±0.26 A, c 27.6±0.48 B, b 25.8±0.23 C, c 20.3±0.52 D, c

Quercetin 27.2±0.24 A, d 25.4±0.60 B, c 25.6±0.35 B, c 20.1±0.35 C, c

Note: “APE”, “left aside 1 or 0.5 h”, “fresh” and “reused once or twice” refer to “apple phenolic extract”, “storage time period 1 or 0.5 h at roomtemperature after deep frying”, “oil from the unopened container after purchase and before the expiration date” and “oil has been used once or twicefor deep frying potato fritters”, respectively. Different uppercase letters (within the same row) indicate statistically significant differences at P<0.05.Different lowercase letters (within the same column) indicate statistically significant differences at P<0.05

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those at 165 °C for 3 min. For the fritters left for 1 h after beingdeep fried with “reused twice” oil, the trend was reversed.

Adding a phenolic antioxidant ingredient to batter de-creased the overall oxidation. For the same type of oil (freshor reused) and under the same deep-frying and post-fryinghandling conditions, the control fritters had higher Totoxvalues than those fortified with phenolics. APE appeared tobe the most effective antioxidant ingredient, and quercetingenerally exhibited a slightly better protective effect on thepotato fritter than rutin.

Unsaturated FAs like those in canola oil are highly sus-ceptible to rancidity and can decompose in a cascade ofmechanisms (secondary oxidation) generating off-flavours.The more double bonds and/or electron-donating groups,the easier it is for a FA to be oxidised (Zhang et al. 2007).Exposure to high temperatures such as those for deep fryingwould accelerate the autoxidation process via forming ther-mally oxidised compounds (Choe and Min 2006). Oil oxi-dation is not one simple reaction but a complex series ofreactions between unsaturated fatty alkyl groups and activeoxygen. The mechanisms of lipid oxidation depend on thedistinct food components (e.g. different batter formulationsand potato type) and specific environment (e.g. differentdeep-frying conditions). Oil oxidation can be assessed basedon production of primary and secondary breakdown prod-ucts (estimated by PV and p-AV, respectively).

Free Fatty Acid Contents of Different Potato Fritters

Under the same deep-frying and handling conditions, theFFA contents of potato fritters with or without an addedphenolic ingredient were very similar. This indicates that thephenolics may have minimal direct effect on hydrolyticreactions during deep frying and subsequent exposure inthe air, although these phenolics could significantly sup-press oxidation (Coppen 1989). Figure 1 only shows theFFA contents of the control potato fritters (produced in theabsence of added phenolics), in order to demonstrate moreclearly the dominant influencing factors on the FFA con-tent of potato fritters, i.e. the type of oil for deep frying,deep-frying conditions and post-frying fritter handling.The overall FFA content of all the control samples isbelow 0.6 %. The fritters that were deep fried using“fresh” or “reused once” oil had FFA contents under thetypical FFA limit (0.5 %) for potato chips (Moreira et al.1999), except for those deep fried at 165 °C for 3 min andleft in the air for 1 h.

In general, leaving the fritters openly in the air after deepfrying resulted in higher FFA contents. This phenomenonmay be due to increased water/moisture exposure and ab-sorption. The FFA contents of the potato fritters deep fried at165 °C for 3 min were higher than those at 180 °C for2.5 min when “fresh” or “reused once” oil was applied.

FFA content of oil is related to oil hydrolytic rancidityinstead of oxidative reaction (Warner and Mounts 1993).FFA content increases with frying time (Mazza and Qi 1992;Xu et al. 1999). Also, deep frying via the “high heat shorttime” approach may facilitate a quicker and greater removalof water/moisture, compared to the “low heat long time”approach. Thus, a lower FFA content was detected in thecase of 180 °C for 2.5 min. However, with the increase ofthe number of times the oil had been used, such a differencebecame insignificant.

Oils contain FFAs as the result of hydrolytic rancidity,not oxidation. Hydrolytic rancidity develops when hydroly-sis of triglycerides occurs in the presence of moisture,resulting in fatty acids cleaved from triglycerides (Olias etal. 1993; Kristott 2000). FFAs are more susceptible to au-toxidation than esterified FAs, acting as pro-oxidants (Irwinand Hedges 2004; Choe 2007; Zhang et al. 2007). FFAshave hydrophilic and hydrophobic groups in the same mol-ecule (Choe and Min 2006; Choe 2007). The hydrophiliccarboxy group of FFAs does not easily dissolve in thehydrophobic oil phase but occurs on the oil surface, causinga reduced surface tension, elevated oxygen diffusion rateand accelerating oil oxidation. The FFA results of this studysuggest that a shorter handling time period after deep frying(i.e. left in the air for 0.5 h), deep frying at 180 °C for2.5 min and using “fresh” or “reused once” oil facilitatedreduced hydrolytic rancidity.

Total Extractable Phenolic Contents of Different PotatoFritters

The trend in TEPC values (Fig. 2) was generally oppo-site to that of Totox values (Table 1). In some cases, the

0

0.1

0.2

0.3

0.4

0.5

0.6

0.5 h 1 h 0.5 h 1 h 0.5 h 1 h

FF

A (

%, a

s o

leic

aci

d)

Fresh oil Reused once Reused twice

180 °C, 2.5 min 165 °C, 3 min

Fig. 1 Free fatty acid contents of potato fritters that were produced inthe absence of added phenolics and subjected to different deep-fryingand different post-frying handling conditions

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APE-containing samples (which had the lowest Totox val-ues) had the second highest TEPC values, whilst in othercases, the TEPC values of APE- and quercetin-containingsamples were similar and the highest among all the differentsamples. The trend of TEPC values depended on potatofritter formulation, the nature of the deep-frying medium(“fresh” or “reused”) and deep-frying and post-frying han-dling conditions.

The presence of phenolic compounds in the batter ofpotato fritters decreased oil oxidation at the cost of phenolicdegradation. The mechanism and reaction rate of each indi-vidual phenolic compound against oil oxidation could bedifferent and determined by the interactions between thephenolic compounds and the free radicals generated duringoil oxidation. The protective effect of the APE ingredientagainst oil oxidation was the sum of the effect from eachindividual co-existing phenolic compounds in APE (Fig. 3).The phenolic compounds present in APE belong to differentclasses including phenolic acids, cinnamic acids and flavo-noids. These phenolic compounds have different chemical

structure and polarity, and exist in different forms (aglyconform like quercetin, or glycoside form like rutin which isquercetin-3-O-rutinoside). These differences lead to the dif-ferent antioxidative powers in different environments(Torres et al. 1987; Rice-Evans et al. 1996; Murkovic2003). The difference in phenolic polarity is demonstratedin Fig. 3: the eluting order of phenolic compounds is “morepolar eluting earlier” by reverse-phase HPLC which wasused in this study.

Oil oxidation starts from the primary lipid oxidationwhich involves three steps: (1) initiation to form free radi-cals, (2) propagation of the free-radical chain reactions, (3)termination to form stable, non-radical species (Nawar1996; Frankel 1998, 2005; Choe and Min 2006). Secondaryoxidation reaction is a subsequent cause of unsaturated FArancidity. During this stage, alkoxyl radicals induce furtherbreakdown from the methyl end of a FA to small moleculecompounds (volatile compounds) via cleavage. The under-lying mechanisms involve hydroperoxide (LOOH) cleav-age, producing an alkoxyl radical (LO·) and a hydroxyl

0 1 2 3 4 5 6

C-180-1 h-freshA-180-1 h-freshR-180-1 h-freshQ-180-1 h-fresh

C-180-0.5 h-freshA-180-0.5 h-freshR-180-0.5 h-freshQ-180-0.5 h-fresh

C-165-1 h-freshA-165-1 h-freshR-165-1 h-freshQ-165-1 h-fresh

C-165-0.5 h-freshA-165-0.5 h-freshR-165-0.5 h-freshQ-165-0.5 h-fresh

C-180-1 h-onceA-180-1 h-onceR-180-1 h-onceQ-180-1 h-once

C-180-0.5 h-onceA-180-0.5 h-onceR-180-0.5 h-onceQ-180-0.5 h-once

C-165-1 h-onceA-165-1 h-onceR-165-1 h-onceQ-165-1 h-once

C-165-0.5 h-onceA-165-0.5 h-onceR-165-0.5 h-onceQ-165-0.5 h-once

C-180-1 h-twiceA-180-1 h-twiceR-180-1 h-twiceQ-180-1 h-twice

C-180-0.5 h-twiceA-180-0.5 h-twiceR-180-0.5 h-twiceQ-180-0.5 h-twice

C-165-1 h-twiceA-165-1 h-twiceR-165-1 h-twiceQ-165-1 h-twice

C-165-0.5 h-twiceA-165-0.5 h-twiceR-165-0.5 h-twiceQ-165-0.5 h-twice

Total extractable phenolic content (mg catechin equivalent / kg potato fritter)

Fig. 2 Total extractablephenolic contents of potatofritters that were produced inthe absence or presence ofadded phenolics and subjectedto different deep-frying anddifferent post-frying handlingconditions. Q quercetin, R rutin,A apple phenolic extract, Ccontrol; “165” or “180” refersto deep-frying temperature;“0.5 h” or “1 h” refers to thetime period for fritters in the airafter deep frying; once reuse oilonce, twice reuse oil twice

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radical (·OH). The high-energy alkoxyl radical (LO·) canthen further convert to different products via various path-ways, including re-attacking the lipid substrate (LH). Thehomolytic β-scission reaction is the most important reactionin secondary oxidation, generating compounds with lowmolecular weight (Frankel 1996, 1998). Once the free lipidradicals were initiated due to oil oxidation, various chainreactions would be triggered (Nawar 1996; Frankel 2005;Choe and Min 2006). During deep frying, various oil dete-rioration reactions occur, depending on light, oxygen, heat,pH, oil composition and other physical changes in oil, e.g.viscosity and foaming degree (Frankel et al. 2002; Choe andMin 2007; Sørensen et al. 2008). At temperatures from 175to 190 °C, the oil uptake can be remarkably affected bydeep-frying duration (Pokorny 1999; O’Connor et al. 2001).These explain the differences in oil deterioration caused bythe two deep-frying conditions, “180 °C for 2.5 min” and“165 °C for 3 min”.

Phenolics vary in the carbon skeleton and hydroxylationof the phenolic rings (Boudet 2007) and may suppress lipidoxidation via donating hydrogen atoms to lipid peroxylradicals to interfere with the initiation or propagation ofprimary oxidation (Cheung et al. 2007; Tsuzuki et al.2008). A difference in the stability of phenoxy radicalswould have caused different rates of propagation and sub-sequent oil oxidation reactions (Gordon 1990). The bonddissociation enthalpies of the O–H bond and the capacity todonate electrons of phenolics may be associated with thematrix medium, i.e. the absence or presence of co-existingphenolic compounds (e.g. “quercetin was added singly”versus “quercetin present as one of the phenolics in APE)(Pino et al. 2006; Pazos et al. 2007). Quercetin is generally amore potent antioxidant than rutin, but rutin may impartgreater iron-chelating effects (resulting in less lipid perox-idation) than quercetin (Omololu et al. 2011). This explains

the overall trend of “quercetin was more protective”, withfluctuation resulted from slightly increased protection ofrutin in some cases of this study.

Repeated use of frying oil appeared to be another factorwhich influenced oil oxidation status. For the “reused” oils,more free lipid radicals existed, thus requiring a greateramount of phenolic antioxidants to scavenge these freeradicals and interrupt the chain reactions of oil oxidation(Chang et al. 2003; Wanasundara and Shahidi 2005; Frankel2005; Choe and Min 2006). Furthermore, it is obvious thatleaving the hot deep-fried fritters in the air for a longerperiod increased oil deterioration. Increased exposure tothe air (oxygen and moisture), especially when the potatofritters were just deep fried and their temperatures were stillhigh, would have accelerated the oxidation process viaforming thermally oxidised compounds and the hydrolyticrancidity process via forming pro-oxidant FFAs (Choe andMin 2006).

Conclusions

Adding small amounts of phenolic antioxidants to the batterof potato fritters is a feasible approach to improve thenutritional value and to reduce oil deterioration of deep-fried potato fritters. The impacts of added phenolics onpotato fritters depend on the type of phenolic ingredient,deep-frying conditions and post-frying handling of fritters.Apple phenolic extract that contains various phenolic com-pounds at different concentrations appeared to be moreeffective than quercetin or rutin in batter formulation. Forfresh or “reused once” oil, deep frying potato fritters at 180 °Cfor 2.5 min (“high heat short time” approach) is advantageousin terms of reducing oil deterioration, whereas for oils usedmore than once, deep frying of potato fritters at 165 °C for

0 5 10 15 20 25 30 35 40 45 50

mA

u0

20

40

60

80

100

120

312

7

6

5

411109

817

13

1214

15

1618

19

Time (min)

Fig. 3 HPLC chromatograms(λ0280 nm) of apple phenolicextract. Peak 1 3-O-p-coumar-oylquinic acid; 2 procyanidindimer B1; 3 catechin; 4 un-known; 5 5-O-caffeoylquinicacid; 6 procyanidin dimer B2; 7epicatechin; 8 p-coumaric acid;9 procyanidin trimer C1; 10myricetin 3-O-rutinoside; 11,21 ferulic acid; 12 myricetin3-O-glucoside; 13 phloretin2-O-xylo-glycoside; 14 querce-tin 3-O-glucoside; 15 phloretin2-O-glycoside; 16 quercetin3-O-rhamnoside; 17 kaemp-ferol 3-O-glucoside; 18quercetin; 19 phloretin

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3 min (“low heat long time” approach) is recommended.Reducing the time period of exposure to the air after deepfrying and before consumption, e.g. from 1 to 0.5 h, canreduce the oil oxidation and increase the retained phenoliccontent.

Deep-fried foods with batter formulated with a low con-centration of phenolics may satisfy the requirements ofconsumers for healthier fast foods. Importantly, this noveland convenient product retains the desirable sensory char-acteristics of deep-fried foods. Future work should be di-rected towards profiling the volatile secondary oxidationproducts and the changes in fatty acid composition usingtechniques such as near-infrared spectroscopy and gas chro-matography–mass spectroscopy. A quantitative sensoryacceptability evaluation on the phenolic-containing potatofritters is also recommended.

Acknowledgments The authors acknowledge technical assistancefrom Mr. David Jin (Plant & Food Research) at the beginning of thisproject and reviews from Dr. Geoffrey I.N. Waterhouse (The Univer-sity of Auckland). Miss Dongni Xue from The University of Aucklandthanks the internship placement at Plant & Food Research, Mt. Albert,in the summer of 2011–2012.

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