Stability Of Rape Seed Kizakinonatane

Published Date: 02 Nov 2017

Jin-Kui Ma, Hang Zhang, Tomohiro Tsuchiya, Yoshinobu Akiyama, Jie-Yu Chen*

Running Title: Stability of Kizakinonatane Oil During Frying

Correspondence: Professor Jie-Yu Chen, Faculty of Bioresorce Sciences, Akita Prefectural University, 241-438 Kaidobata-Nishi, Shimoshinjo-Nakano, Akita-shi, Akita, 010-0195 Japan

Tel.: +81-18-872-1857

Fax: +81 -18-872-1676

Email: [email protected]

Key words

Deep-fat frying; Canola oil; French fries; Oxidative stability; Kizakinonatane oil

Abbreviations: KO, Kizakinonatane oil; RBD, Refined, bleached and deodorized; CO, canola oil; AV, acid value; PV, peroxide value; CV, carbonyl value; TPC, total polar compounds

Summary

The frying performance of Kizakinonatane (Brassica napus) oil (KO), extracted via cold-pressing, during deep-fat frying of frozen French fries with/without replenishment was studied. Refined, bleached and deodorized (RBD) canola oil (CO) was used for comparison. The frying oils were used during intermittent frying of frozen French fries at 180, 200 and 220°C for 7 h daily over 4 consecutive days. Results show that acid value (AV), total polar compounds (TPC), carbonyl value (CV) and viscosity significantly increased (P<0.05) with frying time and temperature in both CO and KO oils. Changes in frying oil color were influenced by the type of oil. The KO exhibited a significantly (P<0.05) lower TPC and CV content than that of CO. Replenishment with fresh oil had significant effects on all chemical and physical parameters, except peroxide value (PV), during frying processes. The KO can therefore be considered a potential source of frying oil.

1 Introduction

Deep-fat frying is a popular and important procedure for both industrial and domestic preparation and manufacture of foods. The flavors, texture and appearance of food can be enhanced simply by immersing them in hot oil for the specified times required by various food types . In addition, when compared with other common culinary procedures, frying causes smaller nutrient losses and even enhances the food nutritive value, in a shorter cooking time .

During deep-fat frying, the oil is continuously used as a frying medium at elevated temperatures in the presence of air (oxygen), a metal container, and water. Under these conditions, a variety of chemical reactions take place, including oxidation, hydrolysis, oxidative polymerization, isomerization and cyclization . All of these chemical reactions lead to decomposition of the frying oil and formation of numerous volatile and nonvolatile products, which may have deleterious effects on human health . During the deep-fat frying process, oils are not only employed as a heat-exchange medium, but also contribute to the quality of fried products . Therefore, both oil quality and composition are of great importance.

Deep fried foods, such as French fries and tempura, are very popular in industries, restaurants and homes in Japan. Although canola oil still remains popular with Japanese consumers (40% of total Japan retail sales in 2011), oil products from other seeds are gaining more popularity . Kizakinonatane (Brassica napus) (registration No. Natane Norin 47), was developed by the Tohoku National Agricultural Experiment Station and claimed as a zero erucic Japanese rape variety. It is commonly cultivated throughout the northern part of the Tohoku region, especially in Akita Prefecture, because of its greater intrinsic cold tolerance and higher oil yielding ability compared with other domestic varieties . KO obtained by cold-pressing has received favorable consumer reviews because of its characteristic taste, superior fragrance and light yellow color. Consumers also tend to believe that cold-pressed oil has a higher nutritive value than conventionally produced oils because of the lack of heat and chemicals involved in the production process. Additionally, despite the widespread distribution of many types of oil, a particular oil is often selected for frying based on local availability and regional preferences.

To the best of our knowledge, there have been no reports concerning the oxidative stability of the KO during deep-frying. There is minimal data in the literature on the constituents of KO and this appears to be limited to the work of Embaby and co-workers , who determined the minor anti-nutritive components, such as glucosinolates, phytic acid, sinapine and total phenols, in Kizakinonatane seeds.

Asthe quality of frying oil is crucial to the nutritional quality and shelf-life of the final products, the current work is focused on investigating the oxidative stability of cold-pressed KO during deep-fat frying of French fries by measuring the acid value (AV), peroxide value (PV), total carbonyl compounds (represented by the carbonyl value, CV), total polar compounds (TPC), color and viscosity of the KO before and after frying. Frying cycles were also carried out with commercial CO for comparison. Most frying operations involving foods like potato chips, vegetables, and chicken are conducted at temperatures of 165-185°C. In this study, however, the high temperatures of 200 and 220°C have been applied in the investigation of the maximum frying performance of KO.

2 Materials and Methods

2.1 Materials

Commercially refined regular CO was obtained from Nisshin OilliO Group Ltd (Tokyo, Japan). Unrefined rapeseed oil was supplied by Akita New Bio Farm Co., Ltd (Akita, Japan). Frozen par-fried French fries in an institutional pack were purchased from a local supermarket and stored at -18°C until use. All chemicals and solvents used in the study were of analytical reagent grade and purchased from Wako Pure Chemical Industries Ltd (Osaka, Japan).

2.2 Methods

2.2.1 Frying Procedure and Oil Sampling

Two methods were carried out to fry the frozen French fries: without replenishment (Frying Experiment I) and with frequent replenishment of oil (Frying Experiment II). The frying was conducted in a restaurant style stainless steel electric fryer TF-40A (Taiji & Co., Ltd., Kanagawa, Japan). In these experiments, 4 kg of fresh oil (CO or KO) was heated for 7 h daily for 4 days at 180, 200 and 220°C. When the temperature of the oil reached the appointed temperature, a batch of 100 g of frozen French fries was fried for 3 min at 22 min intervals for a period of 7 h day-1 on 4 consecutive days. This is equivalent to frying 17 batches per day and, therefore, 68 batches for the whole frying process. During the process, 200 mL of heated oil was drawn after every 3.5 h of frying and kept frozen at -18°C until analysis. At the end of each frying day, the fryer was switched off, covered with the fryer lid and left to cool overnight. In frying experiment I, no fresh oil was added to the fryer throughout the entire frying process. In frying experiment II, experiment I was replicated, except that 400 mL of fresh oil was replenished on every second day of frying. In industrial frying (i.e., high frying load), the general recommendation for food to oil ratio is 1:6. Nevertheless, since the main purpose of this study is to compare the frying stability of KO to that of CO, the food to oil ratio was set at 1:40, which is common to catering/restaurant applications (i.e., low frying load) .

2.2.2 Determination of Acid Value

The AV is the number of milligrams of potassium hydroxide necessary to neutralize the free acids in 1 g of sample. AV of the oil samples were determined in triplicate using an automatic potentiometric titrator AT-500N (Kyoto Electronics Manufacturing, Kyoto, Japan) according to the AOCS Official Method Cd 3d-63 . All AV analysis results were expressed in mg KOH g-1 oil.

2.2.3 Determination of Peroxide Value

PV was measured with an AT-500N following the Japan Oil Chemist’s Society (JOCS) Official Method 2.5.2.1 . Briefly, 5 g of oil sample was dissolved in a mixture of acetic acid and isooctane (3:2 v/v ratio) and then left to react with a saturated potassium iodide solution. The released iodine was then titrated with 0.01 mol/L sodium thiosulfate solution. All PV analysis results were expressed in meq O2 kg-1 oil.

2.2.4 Determination of Carbonyl Value

The CV of the oil samples was determined according to the JOCS Official Method Tentative 13-2003 Carbonyl Value (Butanol Method) . All CV analysis results were expressed in μmol g-1.

2.2.5 Determination of Total Polar Compounds (TPC)

The TPC estimation of oil samples were based on the dielectric constant change, measured directly in the hot oil, with a food oil monitor FOM 310 (Ebro Electronics, Inglostadt, Germany), according to the manufacturer’s instructions.

2.2.6 Determination of Color

The color index of oil samples were determined by measuring the spectrophotometric absorbance at 490 nm using a UV-3100PC spectrophotometer (Shimadzu Co., Kyoto, Japan). Oil samples were placed in a standard disposable cuvette (1 cm optical path) and warmed at 25°C in a water bath for 30 min prior to measuring the spectrophotometric absorbance. The spectrophotometer absorbance was zeroed against air.

2.2.7 Determination of Viscosity

The flow characteristics of oil samples were measured using a sine-wave vibro viscometer (SV-10; A&D Company, Ltd., Tokyo, Japan). Measurements were performed at 25°C. Viscosity analysis results were expressed in mPa·S.

2.3 Statistical Analysis

All measurements were carried out at least in triplicate. Data were analyzed by one-way analysis of variance (ANOVA) and regression analyses using SPSS 19.0 statistical software (SPSS, Inc., Chicago, IL, USA). Statistically significant differences between means were determined by Duncan’s multiple range tests. Statistically significant differences were determined at the P<0.05 level.

3 Results and Discussion

3.1 Changes in Acid Value

Changes in the AV of the oil samples in experiment I are given in Fig.1A. The AVs significantly increased (P<0.05) with increasing frying temperature and time in all treatments. The AVs of KO sharply increased from an initial value of 1.69 mg KOH g-1 oil to 5.5, 6.97 and 6.57 mg KOH g-1 oil after 28 h of frying at 180, 200 and 220°C, respectively. As the AV of fresh KO was similar to the discard level of 2.5 mg KOH g-1 oil set in Japan before frying, it is not surprising to note that all the KO samples quickly reached a level of 2.5 mg KOH g-1 oil after only 7 h of frying. In comparison, the AVs of CO samples gradually increased from an initial value of 0.09 mg KOH/g oil to 1.02, 1.32 and 2.05 mg KOH g-1 oil after 28 h of frying at 180, 200 and 220°C, respectively. After 28 h of frying the AVs of all CO samples were still below the level set for discard.

Changes in AVs of all the treatments in frying experiment II are shown in Fig. 1B. The results indicate that the AVs of all oil samples significantly increased (P<0.05) with elevated temperatures and prolonged frying time. At the end of 28 h of frying, the respective AVs at 180, 200 and 220°C were found to be 0.85, 1.07 and 1.28 mg KOH g-1 oil for CO samples, and 5.97, 7.28 and 8.73 mg KOH g-1 oil for KO samples.

The high initial AV level of KO could be attributed to the fact that free fatty acids and moisture in cold-pressed KO were not removed as they did not undergo the refining process . As more free fatty acids are released in oil, it becomes more susceptible to thermal oxidation under elevated frying temperatures, which is an indicator of the instability of oil. Therefore, the commercial CO showed a lower susceptibility to the hydrolysis of fatty acids than cold-pressed KO during the frying processes. This observation is in agreement with that reported in previous similar studies .

The slope of regression also showed that the extents of hydrolytic deterioration of CO in experiment I, as measured by AV content, were 1.2, 1.3 and 1.7 times faster during frying at 180, 200 and 220°C compared with those in experiment II, respectively. Apparently, replenishment played an important role in the changes in AV. In other words, frequent replacement of frying oil with fresh oil seems to slow down the hydrolysis of frying oil . However, in KO samples, the increment rates were 0.9, 1.0 and 0.8 times, respectively. This could be attributed to the higher initial AV and moisture existing in fresh KO samples, noting that the presence of free fatty acid and moisture in frying oil is believed to catalyze further hydrolysis of triglycerides . The free fatty acid in oils is formed from hydrolysis, oxidation due to free radical formation and cleavage of double bonds during frying. The AV was considered a good indicator of frying oil quality. However, others do not consider AV to be a good measure of frying oil quality because free fatty acids react to produce further volatile compounds and polymers .

3.2 Changes in Peroxide Value

The PV, which represents the total hydroperoxide content, is considered one of the most common quality parameters of oils during production, storage and marketing . The PVs of frying medium KO in both experiment I and II were dramatically decreased (P<0.05) during the preliminary stages of frying, followed by a slight increase and then a decrease as frying continued (Fig. 2). This observation might be explained by the fact that the high initial level of hydroperoxides, which resulted from poor quality seeds and its accumulation in the fresh cold-pressed KO because of a lack of refining processes , were rapidly destroyed by high temperature during frying. The decrease in PV during the initial stages of frying indicates that the decomposition rate of hydroperoxides surpassed their rate of formation, and that the prolonged frying time could also have played a role in decomposing the hydroperoxides.

Although the same temperatures were applied to both KO and CO samples, the results obtained from the CO samples showed a sharp increase in PV (P<0.05) during the initial stages of frying, followed by a decrease (Fig. 2A). Further frying led to a new slight increase in PV. Similar pattern changes were also observed in experiment II. An increase during the early stages of frying was observed, followed by a decrease after further frying (Fig. 2B). The results in experiment II also indicate that frequent replenishment with fresh oil has little impact on the PV during deep-frying, similar to the case for experiment I.

An increase in the PV during the frying process implies the formation of peroxides through oxidation. However, peroxides are unstable compounds and will break down to a variety of nonvolatile and volatile products under high temperatures , thus leading to the formation of fewer hydroperoxides as frying continues. The late increase in PV could be attributed to new hydroperoxides that were formed once more during the cooling period . The same patterns of change were reported in many previous deep-fat frying studies .

Because hydroperoxides are unstable at high temperature during frying, many researchers have stated that the PV can serve only as an indicator of the initial stages of oxidative change but is not reliable and suitable for the assessment of used frying oils .

3.3 Changes in Carbonyl Value

The CV measures the secondary oxidation products generated from the degradation of hydroperoxides, such as aldehydes and ketones, which are more stable than peroxides. Because these secondary products are considered to be the major contributors to rancidity and unpleasant flavors and decrease the nutritional value of fried foods , the determination of the CV is very important for evaluating the quality of frying oils.

The changes in CV during frying experiment I over 4 consecutive days of frying at 180, 200 and 220°C are shown in Fig. 3A. The results show that the CVs of KO samples were significantly increased (P<0.05) from 2.63 to 15.99, 19.36 and 47.21 μmol g-1 at the end of the frying process, which indicates that even after 28 h of frying at 220°C, none of the KO sample CVs exceeded the discard level of 50 μmol g-1 set in Japan . A rapid increase (P<0.05) in the CVs of all CO samples was observed with increasing frying time. The CVs of all CO samples increased from 1.99 to 57.43, 61.89 and 65.12 μmol g-1 during the 28 h of frying at different temperatures. The slopes of regression show that changes in the CVs of CO samples were 4.6, 3.7 and 1.4 times faster during frying at 180, 200 and 220°C, respectively, than for the KO samples. These results clearly show the lower levels of CV in KO samples compared with CO samples.

Many factors affect the formation and accumulation of carbonyl compounds during frying, including the frying temperature and time, type of frying foodstuffs and frying oil, antioxidants in fresh oil and refining process . In the present study, as illustrated by previous research , the low levels of CV in cold-pressed KO samples was partly due to the fact that cold-pressed oil contains a high level of minor compounds that could be acting as antioxidants during frying, thus resulting in more oxidatively stable oils than the corresponding refined oils. Nevertheless, Tonani et al reported that low levels of CV in recovered oil were mainly due to the vaporization of carbonyl compounds with steam generated from frying foodstuffs.

As expected, similar patterns of change in the CV were observed during experiment II. The CVs of all CO samples markedly increased (P<0.05), from 1.99 to 43.6, 52.92 and 61.72 μmol g-1, with increasing frying temperature and time, while the CVs of all KO samples showed moderate increases from 2.62 to 12.27, 14.45 and 38.11 μmol g-1 during frying (Fig. 3B). The lower increment of the CV shown in experiment II compared with experiment I was mainly a result of the frequent replenishment with fresh oil.

3.4 Changes in Total Polar Compounds

TPCs consist of degradation products, nonvolatile oxidized derivatives, polymeric and cyclic substances both formed during frying and those from food material contaminants present in the frying oil. The determination of TPC is one of the most reliable methods for the assessing the extent of deterioration in frying oils . The TPC content indicates the total amount of new compounds formed during frying that have higher polarity than triacylglycerols, thus providing a good indicator of the quality of used frying oils. The maximal values for TPC in many European countries have been set between 24 and 27% , while Japan has not established a specific level for polar compounds to date.

The initial levels of TPC were found to be 1.0 and 3.0% for CO and KO samples, respectively. In this study, the contents of TPC increased almost linearly with frying time for all the oils, with high correlation coefficients (R2>0.94) (Fig. 4). Assuming the maximum permissible amount of TPC is 24%, the time required to reach this amount was used as a measure of frying stability (t24). As shown in Fig. 4, the highest t24 value observed in experiment I was KO frying at 180°C (82.7 h), followed by temperatures of 200 (58.2 h) and 220°C (26.5 h). The highest t24 value was found in CO frying at 180°C (24.8 h), followed by temperatures of 200 (22.4 h) and 220°C (18.6 h). A similar pattern of change in TPCs was also observed in frying experiment II. The CO fried at 180°C showed the highest t24 value (33.1 h) while the highest t24 value was found in KO that was fried at 180°C (89.3 h). The results show that the COs exhibited a frying stability that was significantly lower than those of KOs.

These results of a lower TPC percentage indicate a high stability of the frying oil KO with regard to the changes in triacylglycerols that occurred during the frying processes. The lower TPC formation in KO samples was likely due to the fatty acid composition and the ratio of oleic to linolenic acid, which have been reported to affect the formation of TPC. It has been reported that decreasing the linolenic acid and increasing the oleic acid in frying oil would lead to a lower TPC formation during deep-frying compared with corresponding normal oils .

3.5 Changes in Color

The color of frying oil is one of the major parameters of acceptance to be evaluated, because color development is an indication of oxidation, polymerization, formation of carbonyl compounds and other chemical changes .

The changes in color for all treatments in experiments I during 4 consecutive days of frying are given in Fig. 5A. The results show that the optical density of CO samples significantly increased (P<0.05) with increasing frying temperature and time. The optical density of CO samples increased from an initial value 0.06 to 0.78, 0.95 and 1.67 after 28 h of frying at 180, 200 and 220°C, respectively. The KO samples show a high initial optical density value of 1.19 and the lowest optical density of KO samples were found to be 0.41, 0.25 and 0.25 after 10.5, 7 and 3.5 h of frying at 180, 200 and 220°C, respectively. The optical density values were found to be 1.02, 1.15 and 1.36 at the end of frying at 180, 200 and 220°C temperatures. A similar pattern of change in optical density of all oil samples fried in experiment II was observed, compared to that oils fried in experiment I (Fig. 5B). The optical density values of CO were found to be 0.67, 0.92 and 0.96 while the values for KO were 0.81, 0.80, and 1.15 after 28 h of frying at 180, 200 and 220°C, respectively.

The increased optical density indicates that the oil samples became darker with increasing frying time and temperatures. This could be due to the oxidative reactions, the formation of polymers and browning pigments from food . As described by Blumenthal in his "frying oil quality curve", frying oil usually goes through five phases during the degradation process . Therefore, an explanation for the observation of an early decrease in the optical density in KO samples is that frying oil underwent a breaking-in period (mainly due to rapid breakdown of carotenoids and chlorophylls) during which the frying oil became quite clear. The high optical density value in the fresh KO samples could be attributed to the presence of carotenoids and chlorophylls that remained without the refining process .

Oil color can be greatly influenced by a variety of factors including the type of frying oil, the amount and type of food being fried. Since changes in oil color can be caused by more than one chemical process, monitoring the color change alone is not an accurate and reliable assessment of the overall quality of frying oil .

3.6 Changes in Viscosity

The changes in viscosity in experiment I are shown in Fig. 6A. The results show that viscosity significantly increased (P<0.05) with frying time for the samples at all measurement temperatures. Although with a higher initial viscosity, the KO samples showed a lower increase in viscosity than the CO samples after 17.5 h of frying. All oil samples fried in experiment II seemed to follow a similar pattern of change in viscosity, although the CO samples were found to have a higher increase in viscosity after 17.5 h of frying at 200 and 220°C (Fig. 6B). The initial higher viscosity of KO samples than that of CO samples was due to the impurities in unrefined cold-pressed oil.

The increase in viscosity of frying oils has been related to thermal oxidation and polymerization reactions. These reactions result in the formation of high molecular weight polymer compounds, leading to an increase in viscosity . It has also been reported that viscosity of oils may be related to differences in saturated fatty acids, with consequently higher melting points . Since the viscosity is the result of thermal polymerization, the excessive increases in viscosity observed in KO in experiment II fried at 180°C at the end of frying were unexpected, as oils were fried at a lower temperature and should have been less susceptible to thermal polymerization, compared to that of oil fried at higher temperatures.

4 Conclusions

Based on the results for AV, PV, CV, TPC, color and viscosity in this study, there was an appreciable distinction of frying stability between commercial RBD CO and cold-pressed KO. The oil type is crucial to oil thermal stability because cold-pressed KO showed a clearly lower degree of degradation in terms of TPC and CV when it was subjected to deep-fat frying of frozen French fries at 180, 200 and 220°C. On the other hand, the parameters for CO were higher after the frying process. However, it should be noted that the acid and peroxide values and viscosity in fresh KO were higher than those in the CO. Meanwhile, the results also showed that frequent replenishment with fresh oil and lower frying temperatures retarded the deterioration of the oils during frying and prolonged the useful life of frying oils..

In summary, noting the increasing popularity of cold-pressed KO as a new variety of oil from the northeastern part of Japan, the present study has demonstrated that KO can be considered a potential source of frying oil. Nevertheless, KO is a new product and further investigations are needed for a better understanding of the constituents of cold-pressed KO and the possible mechanisms involved in its oxidative stability during frying.

5 Conflict of interest

We declare that there are no conflicts of interest.