Combined Effects of Gamma Irradiation and Blanching Process on Acrylamide Content in Fried Potato Strips

Abstract

Potato tubers were irradiated in ⁶⁰Co gamma station at different doses in order to investigate the effect of gamma irradiation on acrylamide formation in fried potato strips. Acrylamide content due to the irradiation treatment was reduced by 20–54% compared to a control after frying the irradiated tubers. While apply a blanching process, using warm tap water, to potato strips before frying has decreased acrylamide by 61%. A combination of gamma irradiation and a blanching process, which was applied in this work, showed a maximum decrease in acrylamide formation to reach 78% in fried potatoes.

Keywords:

• Acrylamide
• Gamma irradiation
• Blanching
• Fried potato

Introduction

The Swedish National Food Administration reported in 2002 for the first time the presence of acrylamide (CH₂CHCONH₂) in some carbohydrate-rich foods cooked at high temperatures. Acrylamide, which is an odorless white or colorless crystalline solid, formed during the heat treatment of carbohydrate-rich foods as a result of the Maillard non-enzymatic browning reaction between the amino acid asparagine and reducing sugars.^[1] Many researchers have reported that acrylamide was not detected in uncooked or boiled foods.^[2,^3]

Acrylamide has been found in different concentrations in a variety of popular foods, including potato chips, French fries, biscuits, coffee, and bread.^[4–6] It has a toxicological effect; it is classified as probable human carcinogen with a potential health risk and it is also known to be a neurotoxin and a genotoxin to rats, dietary acrylamide intake increased the risk of ovarian cancer in women.^[1,^7–10] Chen et al.^[11] provided a link between high cancer risks in adolescents when the acrylamide concentration is higher than 168 µg/kg, based on intake of French fries.

Different factors could affect the acrylamide formation in fried potato products: production and environmental factors, cooling and post-harvest processing, pre-treatment as blanching in warm water, temperature and times of frying, the temperature of the final stage of the frying process, types of fryers.^[1,²,^9] Due to the importance of acrylamide concentration reduction in cooked foods, many studies are aimed at reducing the precursors that form acrylamide in carbohydrate-rich food by different ways namely (1) choosing new kinds of potatoes with less reducing sugar or/and less amino acids, (2) changing the storage conditions by modifying the time, temperature, humidity, and aeration,^[12–14] (3) pre-treatment such as immersion in a salty or acidic solution^[1,⁹,¹⁵,^16] or in warm water.^[15,¹⁷,^18] The immersion studies did not investigate the probable effects of these additions on the final products during/and after frying, While blanching the potato strips in warm water for a few minutes before frying could improve the quality of the French fries.^[13,^17]

Irradiation can be used in the food industry for sprout inhibition, shelf life extension, insect disinfection, microbial decontamination, and improvement of hygienic quality of food,^[19–23] radiation processing of food can be undertaken for both export and domestic markets.^[24] The commercial food irradiation has been allowed after the recommendation of the wholesomeness of food irradiation by the joint FAO/IAEA/WHO.^[25] A high dose of gamma irradiation (more than 10 KGy) has been approved for the improvement of hygienic quality of different kinds of food and for microbial control.^[22–26] Dose in the range of 60–150 Gy has been found to be effective for sprout inhibition of potato by gamma radiation regardless of variety and storage temperature.^[23] Many researchers noted that high acrylamide content in potato strips can be an important reason for reducing acrylamide levels by a different way. Recently, a few studies investigated the effect of irradiation on the level of acrylamide in fried potatoes.^[28,^29] This present study represents an original work which aimed to reduce acrylamide concentrations in fried potato by irradiating the tubers followed by thermal treatment (blanching in warm tap water) in order to reduce the amount of precursors of the Maillard reaction.

Materials and methods

Samples of potato tubers cv. Spunta grown in Damascus, Syria were obtained from a local supplier, packed in cotton pouches. Then potato tubers were weighed as in the sampling plan and transferred into polyethylene plastic bags, labeled and identified with respective radiation. Each bag of potato tubers (5 kg) is considered as a replicate.

Treatment (Irradiation and Blanching)

Potato tubers samples were exposed to gamma radiation at doses of 0, 50, 100, and 150 Gy in a ⁶⁰CO package irradiator (dose rate 848 Gy/h). The irradiation was performed at room temperature. The absorbed dose was determined using alcoholic chlorobenzene dosimeter.^[30] For each treatment, three boxes of potato tubers were used. All analyses were performed on controls and treated samples immediately after irradiation.

Irradiated and non-irradiated potato tubers were peeled and cut into strips of 1 cm × 1 cm × 5 cm with a French fries-shaped cutter. Irradiated and non-irradiated strips were divided into two lots. One lot was fried (100 g of strips in each replicate) in 1 L of sunflower oil at 170 ± 0.5°C for 5 min in an electrical thermostated deep fryer (Molinex). The second lot was blanched using a water bath equipped with a thermostat. One hundred grams of strips from non-irradiated (control) and from irradiated potato strips were heated at 85 ± 0.5°C for 5 min in 500 mL tap water and then drained before frying at 170 ± 0.5°C for 6 min. One liter of sunflower oil was used for each replicate. The fried potatoes were stored at 4°C until analysis.

Chemical Analysis

Approximately 150 g of raw potato tubers (washed and peeled) were blended for 15 s in a laboratory blender, and were used in all the chemical analysis. Each sample was homogenized and analyzed in triplicates. To determine the moisture content, samples were dried at 105°C for 6 h. To measure the ash content, samples were incinerated in a muffle furnace at 550°C for 4 h, crude fat (as extractable component in Soxhlet apparatus), crude protein (as Kjeldahl nitrogen) were measured using standard methods.^[31] Total sugar was analyzed using Anthrone indicator method by measuring the absorbance at 620 nm with a T70 UV/VIS Spectrophotometer (PG Instrument Ltd). The reducing sugar was measured by iodometric determination of the unreduced copper remaining after reaction, and the concentration of reducing sugar was expressed as g glucose per 100 g powders of dried potato.^[31] Total volatile basic nitrogen (TVBN) in the sample was determined in terms of mg TVBN per kg potato.^[30]

Acrylamide Determination

Reagent and standard

Acrylamide (>99%) was obtained from Fluka, methanol analytical grade from Merck. Water used throughout the experiments was bidistilled deionized and passed through 0.22 µm filters. Strata X (SPE) cartridges were supplied by phenominex. Stock solution of acrylamide was prepared by dissolving 100 mg of acrylamide in 1 L of distilled water. A set of standards for high performance liquid chromatography (HPLC) analysis were prepared by appropriate dilution of the stock solution in concentrations from 3 to 35 mg/Kg with distilled water. All stock and standard solutions were kept at 4ºC.

Sample preparation

Fried potato strips (100 g) were ground and homogenized and then 10 g were weighed The sample was spiked with 50 mL of acrylamide solution (0.5 mg/Kg) to determine the percentage recovery of the method. The sample was suspended in 25 mL mobile phase (water-methanol 95:5,v/v) and vortexed 15 min for homogenizing it. Then the suspension was centrifuged at 10,000 rpm at 0°C for 20 min, 5 mL clear supernatant was collected and loaded through the Strata X (SPE) cartridges. The cartridges were preconditioned two times using 2 mL methanol and 2 mL of water at a rate of 2 mL per min. The acrylamide residues in the cartridges were eluted with 1 mL of water and collected. All resultant eluents were passed through a 0.45 µm filter and analyzed directly by the HPLC.^[16,31–33]

HPLC analysis

The quantification of acrylamide was performed using Agilent-1100 Series HPLC system, the chromatographic separations were performed on Agilent-ZORBAX-Eclipse XDB-C8 (4.6 × 150 mm) 5 µm column. The flow rate of the mobile phase (water-methanol 95:5,v/v) was 0.5 mL/min, the injection volume was 50 µL. The analysis was performed at an ambient temperature using Vis-UV detector at detection wavelength of 210 nm.

Statistical Analysis

The four irradiation doses and two heating treatments were distributed in a completely randomized design with three replicates. Data were subjected to the analysis of variance test (ANOVA) using the SUPERANOVA computer package (Abacus Concepts Inc, Berkeley, CA, USA, 1998). A separation test on treatment means was conducted using Fisher’s least significant differences (LSD) methods at 95% confidence level.^[34]

Results and discussion

Effect of Gamma Irradiation on Chemical Composition of Raw Potato

Characterization of potato tubers are listed in Table 1. No significant differences in dry protein, moisture, ash, crude protein, total carbohydrate, starch, and reduction sugar were observed in potato tubers post irradiation, these results, related to the effect of gamma irradiation on proximate analysis, are consistent with previous.^[30] Immediately after irradiation, TVBN value of non-irradiated control sample of potato was 114 mg/kg potato. TVBN values decreased significantly (p < 0.05) according to irradiation doses (Table 1). The TVBN is related to protein breakdown,^[35] and the observed increases may be attributed to the formation of ammonia or other basic compounds. It is obvious that as irradiation doses increased, high energy is absorbed by the food substrate given rise to generation of new volatile compounds through oxidation,^[35] or by breaking the chmical bonds of poly-sup-products to soluble sup-products of low molecular weights.^[36]

TABLE 1 Effect of different irradiation doses on the composition of raw and irradiated samples of potato tubers (on dry weight basis)

Parameters	Irradiation doses (Gy)
	0	50	100	150
Dry matter (%)	20.11 ± 1.30	18.44 ± 2.44	19.71 ± 1.31	19.31 ± 1.62
Moisture (%)	79.89 ± 0.11	81.55 ± 2.44	80.29 ± 1.31	80.69 ± 1.62
Ash (%)	0.99 ± 0.06	1.03 ± 0.05	0.98 ± 0.05	1.01 ± 0.09
Crude protein (%)	10.92 ± 1.13	11.04 ± 1.05	10.08 ± 0.83	10.96 ± 1.51
Total carbohydrades (%)	82.99 ± 0.94	80.62 ± 0.92	82.52 ± 0.62	79.98 ± 1.28
Reducing sugar (%)	0.83 ± 0.003	0.86 ± 0.03	0.83 ± 006	0.88 ± 0.003
Starch (%)	76.59 ± 3.42	75.86 ± 6.11	79.49 ± 6.17	77.85 ± 1.72
Volatile basic nitrogen (mg/Kg)	114.24 ± 13.76	75.43 ± 38.61	68.24 ± 16.73	39.66 ± 14.08

Effect of Gamma Irradiation and Blanching on Acrylamide Formation

Figure 1 represents the variation of acrylamide concentration of irradiated potato tubers at low doses: 0, 50, 100, 150 Gy and potato irradiated at the same doses and then blanched in warm tap water. The concentrations of acrylamide at the first treatment by gamma irradiation at 0, 50, 100, 150 Gy were 4551 ± 456, 3629 ± 248, 3310 ± 273, 2073 ± 247 μg/Kg, respectively. At the second combined treatment (irradiated and blanched) the acrylamide concentrations were 1768 ± 323, 1487 ± 106, 1339 ± 54, 1010 ± 141 μg/Kg, respectively. Gamma irradiation in the raw potato tuber induced a significant decrease in acrylamide formation in frying; this decrease was of 20, 27, and 54% in irradiated potato by 50, 100, and 150 Gy, respectively, compared to the control. Blanching un-irradiated potato strips decreased acrylamide significantly by 61% the acrylamide concentration compared to the control. Blanching irradiated potato at the same radiation doses as 50, 100, and 150 Gy above caused a decrease of 67, 70, and 78% respectively, in acrylamide concentration, compared to un-irradiated un-blanched potato (control). Figures 2 and 3 represent typical chromatograms of acrylamide and they show the decrease in the peaks area due to the applied dose of gamma irradiation (Fig. 2) and also the decrease in peaks area in the second combined treatment (Fig. 3) of irradiation dose and blanching with warm water. The acrylamide concentration in our study has ranged within the value that was indicated by Vinci et al.,^[1] Ou et al.,^[6] Brunton et al.,^[37] Mestdagh et al.,^[38] and Pedreschi et al.^[39] Gamma irradiation is widely used to inhibit sprouting in potato to extend the shelf life and to make it available during all seasons as reported by researchers.^[19] The low dose of gamma irradiation has a good effect on the quality of potato^[40] Only few studies had investgated the effect of gamma irradiation on acrylamide content in fried potatoes.^[28] The inhibition of potato sprouting in addition it would decrease the reducing sugars during storage and consequently reduce the concentration of acrylamide by influencing the Maillard reaction.^[28] The obtained results showed that gamma irradiation caused a decrease in acrylamide concentration proportionally to the applied dose (Fig. 1). No significant differences in reducing sugars of potato tubers were observed in the current investigation due to irradiation (Table 1), In this work, we did not investagate the effect of gamma irradiation on reducing sugars during long storage. On the other hand, Lu et al.^[23] indicated that gamma irradiation induced an increase in glucose content by the degradation of starch in potato tuber which could in turn increase the acrylamide formation during frying. The different effects seen of the gamma irradiation on the potato tubers could be attributed to the type of potato and to its chemical properties. So it can be assumed that gamma irradiation affected other acrylamide precursors—principally Asparagine—which had the ability to reduce the acrylamide content in the fried potato. Previous studies were interested in the reduction of the acrylamide precursor by blanching the potato slices in preheated water (50–90ºC) for different periods (2–70 min), this pre-treatment could inactivate enzymes; modifiy texture; preserve color, flavor, and nutritional value; removing trapped air and reduce sugars and/or asparagine and then could reduce the acrylamide formation (51–81%).^[13,¹⁵,¹⁸,^41] Other studies suggested reducing the formation of acrylamide in fried potato and crisps by using some additives such as organic acids (citric, acetic, lactic acid), salts (NaCl, CaCl₂, MgCl₂), hydrocollids, enzymes, agent’s inhibitors as reported by Vinci et al.,^[1] Kalita and Jayanty,^[9] Ou et al.,^[16] and Zeng.^[32] According to our knowledge, there are no studies that evaluated impacts of these additives on the fried potato and on the oil used in frying, which may influence the quality and the safety of fried potato. Therefore, it is suggested that blanching the potato strips solely by warm water is more likely to be a safer procedure compared to the additives. Gamma irradiation could break the protein and produce more amino acids,^[42] and in some kinds of food like seeds; the total carbohydrate contents decreased with increasing high dose of gamma irradiation which caused higher metabolic activities and hydrolyzing enzyme activity.^[43] The carbohydrates could breakdown to simple extractable form by gamma irradiation, but in our study the reducing sugars were not affected by gamma irradiation. An undesirable effect of irradiation in the formation of lipid oxides by the reaction of membrane lipids and other lipids in foods with oxygen radicals produced by gamma irradiation.^[44] Blanching potato strips by warm water able to leach the starch, the reducing sugars, lipids, and other soluble component of low molecular weight. As known, gamma irradiation has an important role in breaking some molecules in potato tubers, and also the blanching process in warm water has an auxiliary role in removing starch, reducing sugars and the sub-product which is formed by cutting.^[18] As we mentioned previously, the acrylamide forms by the reaction between reducing sugars and asparagine (Maillard reaction), so any change or modification between these components could probably affect the acrylamide concentration. More experiments are taking place to investigate the variation in asparagine and sugars concentrations before and after irradiation and blanching in order to confirm our hypothesis about the decrease of acrylamide content in fried potato strips. The results of these will be published. It is important to expand the research by examining the impact of storage, on the changes in the chemical composition of the irradiated potato, measuring the asparagines and reducing sugar content and their effects on acrylamide formation before and after each treatment. Thus, optimizing the gamma irradiation dose and the blanching process could significantly reduce the acrylamide content in fried potato.

FIGURE 1 Acrylamide concentrations in fried potatoes in two different treatments. (A) The first treatment (irradiating potato tubers at 4 low gamma doses before frying*). (B) The second treatment (combined treatment of irradiation potato tubers at 4 low gamma doses and then blanching the potato strips by warm water before frying).

FIGURE 2 Chromatogram represents the variation of acrylamide content in the fried potatoes at the first treatment (potato tubers irradiated with low doses of gamma irradiation [(1) 0 Gy; (2) 50 Gy; (3) 100 Gy; (4) 150 Gy] before fraying).

FIGURE 3 Chromatogram represents the variation of acrylamide content in the fried potatoes at the combined treatment (potato tubers irradiated with low doses of gamma irradiation [(1) 0 Gy; (2) 50 Gy; (3) 100 Gy; (4) 150 Gy] and then blanching the strips by warm tap water before fraying).

Conclusion

Gamma irradiation was capable of reducing the acrylamide formation by 20–54%, whereas the combination of gamma irradiation with blanching the potato strips in warm tap water prior to frying reduced the acrylamide concentration by 67–78%. The treatment of tubers by gamma irradiation in order to prevent sprouting could be considered as an effective process to reduce the acrylamide content in fried potato, and a further simple technique of blanching the potato strips prior to frying accounted for a more significant decline in the acrylamide level. Hence, the combination of the two treatments could be potentially useful on an industrial scale to lower acrylamide in fried potato strips.

ACKNOWLEDGMENTS

The authors wish to express their deep appreciation to the Director General of Atomic Energy Commission of Syria, the Head of Chemical Department, and to A. Odeh, and A. Alkaid.