Optimization of vacuum frying condition for producing silver carp surimi chips

Abstract In this study, we explored the feasibility of vacuum frying to produce crisp silver carp surimi chips. The influence of three process parameters (frying temperature, frying time, and slice thickness) on the quality parameters of vacuum‐fried surimi chips (oil uptake, crispness, and optical properties) was investigated. The experimental results showed the optimal conditions were chosen as 2‐mm surimi slice being vacuum‐fried at 118°C for 2.5 min. Under these conditions, the oil content, breaking force, and color difference to commercial potato chips were 24.33%, 15.21 N, and 14.03, respectively. Additionally, we also measured the water loss during vacuum frying and the oil quality changes during storage of surimi chips. Results demonstrated the rapid loss of water content of surimi chips during vacuum frying and oil deterioration was kept at acceptable low level up to 100 days. Taken together, our study supported the applicability of vacuum frying technology to produce high‐quality silver carp surimi chips.

Up to date, surimi-based products mainly include fish balls, crabsticks, fish cake, chikuwa, and narutomaki (Park, Lin, & Yongsawatdigul, 1997). However, these traditional surimi-based products are highly susceptible to microbial spoilage and hence need cold chain to preserve. Drying is one of the most common food preservation methods (Velescu, Tenu, Carlescu, & Dobre, 2013). Therefore, surimi chips are a promising format of surimi-based snack to be developed as an inexpensive, tasty, and easily available snack.
In addition, since chips might be one of the most popular snacks (Huang & Zhang, 2012), the development of surimi chips may also make the surimi-based foods more marketable.
In this study, we explored the feasibility of using the silver carp (Hypophthalmichthys molitrix) surimi as ingredient to produce crisp surimi chips by vacuum frying. The effects of three process parameters (including temperature, frying time, and slice thickness) on the oil uptake, crispness, and optical properties of vacuum-fried surimi chips were investigated. Additionally, we also measured the water loss during vacuum frying and the oxidation of the fatty acids during storage of surimi chips. Taken together, the results supported the use of the vacuum frying technology as a method for producing high-quality surimi chips.

| Materials and chemicals
Vacuum-packaged frozen Nemipterus virgatus surimi was provided by Haixin Foods Co., Ltd, and stored in freezer (−30°C). Surimi was thawed prior to the experiments. Palm oil and sucrose fatty acid esters were also supplied by Haixin Food Co., Ltd. Sodium chloride was purchased from China National Salt Industry Corporation. Cassava starch and soy protein isolates were purchased from Angel Yeast Co., Ltd.

| Preparation of surimi samples
Surimi was chopped at 1,800 rpm for 2 min using a silent cutter machine (JYL-D020, Joyoung Co., Ltd) before sucrose fatty acid esters, sodium chloride, cassava starch, soy protein isolates, and ice were added (as shown in Table S1) and chopping continued for another 2 min. After chopping, the sample was steamed at 95°C for 5 min and frozen again before sliced into slices of 1-5 mm using a meat slicer (300ST-12, Itop Kitchen Equipment Co., Ltd).

| Vacuum frying
Vacuum frying experiments were performed in a vacuum fryer (QS-05, Quanshi Food Machinery Co., Ltd). Surimi slices were placed in the frying basket, and the lid was closed. Once the desired oil temperature and vacuum level were reached, the basket was lowered into the oil and vacuum-fried for the required frying time before the surimi slices were lifted from the oil, and then, the oil was removed by centrifuge at 200 rpm for 2 min. Then, the vacuum-fried surimi chips were cooled down to room temperature on paper towel prior to further analysis.

| Oil content measurement
Vacuum-fried surimi crisps were subjected to Soxhlet extraction by hexane to measure the oil content according to the AOAC Official Method 972.28 (AOAC, 1995) using a SOX406 Fat Analyzer (Jinan Hanon Instruments Co., Ltd.). Briefly, fried samples were grinded into the size of 20 meshes using a mortar and then extracted by diethyl ether in a Soxhlet extraction barrel with dropping bottle at 65°C. The extracts were then dried in 103 ± 2°C until constant weight and weighted after cooling to air temperature. The oil content was expressed as g oil/100 g dry sample.

| Analysis of textural properties
Texture profile analysis (TPA) of the vacuum-fried surimi chips was carried out using TA.XT2i texture analyzer (Stable Micro Systems) equipped with a cylindrical probe P/50. The force was applied to the sample by using P/50 probe until the sample was cracked. The pretest speed, test speed and post-test speed were set at 2.0, 1.0, 10.0 mm/s. The distance was 3 mm. The trigger type was 'auto'.
The deformation ration in the tests was set as 30%, the interval stopping time was 5 s, and the trigger force was 5 g. The forcetime curve was recorded and analyzed using Texture Exponent 32 software (Stable Micro Systems). The maximum breaking force was recorded to reflect the crispness of surimi chips as previous report (Diamante et al., 2012).

| Color measurement
Color of surimi chips was measured in terms of Hunter parameters

| Single-factor experiments
To optimize the process parameters for vacuum frying, initial singlefactor tests were conducted before response surface experiments to screen the appropriate influential ranges of variables, including temperature (95-135°C), frying time (1-5 min), and slice thickness (1-5 mm).

| Box-Behnken design (BBD) test
According to the results of single-factor experiments, a Box-Behnken response surface design with three independent variables was established (as shown in Table 1). Three independent variables were coded at three levels (−1, 0, and 1) on three responses (including oil content, breaking force, and chromatic aberration), which resulted in an experimental design of 17-run experiments using a Design-Expert software (version 8.0.6) ( Table 1). The analysis of variance (ANOVA) for the response surface quadratic model was also performed using Design-Expert software and is given in Table 2.

| Moisture content measurement
The water content of surimi chips was measured by the oven-drying method (AOAC Official Method 984.25) (Shiroma & Rodriguez-Saona, 2009). The water content was expressed as g water/100 g total.

| Determination of acid values (AVs) and peroxide values (POVs)
The acid value (AV) and peroxide value (POV) of the oils were determined using a titration method according to the AOAC Official

| Statistical analysis
All data were presented as means ± standard deviation. As for multiple group comparison, the significance of the differences among the treatment groups and their respective control groups was analyzed using Origin 8.0 software. Statistical significance was assessed by either Student's t test or one-way analysis of variance (ANOVA) followed by Tukey's multiple comparison. Differences between means were considered statistically significant if p < 0.05.
All the experiments were carried out at least in three independent experiments.

| Single-factor experiments for optimization of vacuum frying conditions
We first determined the effects of frying temperature on the quality characteristics of surimi chips. As shown in Figure 1A, the oil content . TA B L E 1 Box-Behnken experimental design and the response for 17 experimental runs  The effects of slice thickness were next to be explored. As seen in Figure 1C, the oil content gradually decreased, while the breaking force showed a general upward trend by increasing slice thickness up to 5 mm. In several previous studies investigating the effects of slice thickness on final oil content of potato crisps, a similar trend was also observed that increasing the potato crisps thickness decreased the TA B L E 2 Regression equations (for the coded variables) and ANOVA for fitted models final oil content (Krokida et al., 2000). Meanwhile, as expected, the breaking force increased with the increase in slice thickness, which is similar to previous findings reported by George O. Abong et al. (Abong, Okoth, Imungi, & Kabira, 2011). In addition, the ΔE declined to lowest point at 2 mm and then increased. Therefore, the central point of slice thickness for the following response surface optimization was set as 2 mm.

| Response surface methodology for optimization of vacuum frying conditions
Since the frying is a process involving complex heat transfer and chemical reactions (Gertz, 2014), to reflect the possible interaction effects arising between factors, response surface methodology was also employed in this study. The design matrix of response surface methodology experiments is shown in Table 1. Table 1   Similarly, when the frying time was set, the oil content was found to decrease with increase in frying temperature from 105 to ~112°C; then, it showed rapid uptrend when the frying temperature continued to increase. Figure 2A The color difference to commercial potato chips (ΔE) ranged from 13.3203 to 20.1633 was recorded in our tests. As shown in Figure 2C, frying temperature and slice thickness greatly affected the ΔE, which was decreased by increasing the frying temperature and slice thickness. In contrast, only a little change in the ΔE was observed when frying time extended from 2 to 4 min. This was also reflected by ANOVA.
Taken together, these results are in accordance with single-factor test and the ANOVA. Based on our purpose to achieve low oil uptake, low ΔE, and low breaking force, the optimal condition of vacuum frying for producing surimi chips was next determined by Design-Expert software, which was chosen as 2.24-mm surimi slice being vacuumfried at 117.85°C for 2.70 min. Upon this condition, the theoretical oil uptake, breaking force, and color difference to commercial potato chips (ΔE) were predicted to be 23. 67%, 15.37 N, and 13.51, respec-tively. The suitability of the models to predict the responses was next tested under this selected optimum condition. As shown in Table 4, considering the actual operation convenience, the optimal conditions for adjusting the condition were chosen as 2-mm surimi slice being vacuum-fried at 118°C for 2.5 min. The experimental results were found to be in agreement with the theoretical responses, suggesting the obtained model in this study was accurate and adequate to guide the production of vacuum-fried surimi chips.

| Determination of the water loss during vacuum frying
Next, we evaluated the water loss of surimi chips during vacuum frying. As shown in Figure 3, there is a rapid water loss in the initial stage of vacuum frying before leveling off after around 25-40 s. This F I G U R E 3 The moisture content of vacuum-fried surimi chips during vacuum frying, as affected by frying temperature and time (a); and slice thickness and frying time (b) phenomenon was also revealed by previous studies (Krokida et al., 2000), which may be due to the depletion of nearly all free water and the formation of crust to stop the water evaporation. Moreover, results also revealed that decreasing the slick thickness accelerated the water loss, while increasing the frying temperature (in the range of 95-105°C) had little positive influence on the water loss.

| Determination of the acid value and peroxide value of surimi chips during storage
Lipid oxidation during storage is one of the major factors causing the deterioration of the sensorial proprieties of chips. Therefore, to explore the oil quality stability of surimi chips produced by vacuum frying, we also measured the changes in acid value and peroxide value of surimi chips during the storage. Here, we adopt nitrogenfilled packaging, a common method used for commercial chips package to store fried surimi chips. As shown in Figure 4, both the acid value ( Figure 4A) and peroxide value ( Figure 4B) of nitrogen-filled packaged silver carp surimi chips were kept at low level for nearly 100 days. According to the regulations of frying fats and oils widely adopted in various nations (e.g., the limitation of acid value was as 2 [e.g., in Finland] -4.5 [e.g., in the Netherlands]) (Paul & Mittal, 1997), the results indicated that nitrogen-filled packaging is a suitable method for preventing lipid oxidation of vacuum-fried surimi chips during its storage.

| D ISCUSS I ON
The global market for healthy snacks is steadily growing, enhanced by the increasing demand of consumers for convenience products with high nutritive and sensory qualities (Sun-Waterhouse, 2011).
However, to the best of our knowledge, producing surimi chips has not been reported. Therefore, development of new surimi-based snack products (such as surimi chips) is needed. Here, we explored the vacuum frying as a useful technique for making nutritional snack of crispy surimi chips with minimum oil content.
Vacuum frying is a frying technology that is performed under the pressure below atmosphere levels (Pandey & Moreira, 2012).
Utilization of vacuum frying instead of conventional atmospheric frying has been proven to provide better heat and mass transfer rates. This method has been reported to be successfully applied to make fruits chips and preserve their natural flavors and aromas, and result in a good crispy texture.
In particular, previous studies have pointed out that vacuum frying appears to be a suitable method for chips production due to its low temperature and low oxygen level when operated in a closed system, which can significantly lower the boiling point of water and lead to the products with low oil content, desired sensory and nutritional quality, and less undesirable carcinogenic toxins (Dueik & Bouchon, 2011). Here, our results also demonstrated vacuum frying of surimi slices is a promising process enabling production of surimibased crisps with low oil content at 24.33% at optimal condition.
In addition, the results of the experiment also showed that color of vacuum-fried surimi chips was comparable to control commercial potato chips, which may make the surimi chips more acceptable to consumers. Meanwhile, the water loss curve also supported the dehydration process during the vacuum frying was quickly achieved, which might contribute to the decrease in energy consumption (Maity, Bawa, & Raju, 2014). Indeed, previous studies have suggested that the moisture loss during the vacuum frying determines the oil absorption via affecting the extent of crust formation and the volume that is available for oil infiltration (Dueik, Robert, & Bouchon, 2010;Shyu et al., 2005). Besides, we also determined the oxidative stability of lipid of vacuum-fried surimi chips. The results indicated that using nitrogen-filled packaging to store vacuum-fried surimi chips could keep both the acid value and peroxide value at acceptable low level up to 100 days, showing its stability might not be an obstacle to developing commercial surimi chips.
F I G U R E 4 The acid value (a) and peroxide value (b) of surimi chips during storage up to 100 days. Nitrogen-filled packaged vacuum-fried surimi chips were stored at 5°C (blue line), 25°C (red line), and 45°C (black line) to test the oxidative stability of lipid Admittedly, the present study was conducted with small sample size, and thus, before final recommendation of this technology for its industrial use, confirmations are required by using increased sample sizes.

| CON CLUS ION
In conclusion, our study here supported that the vacuum frying technique could be utilized for producing high-quality fried surimi-based chips with low oil content and similar color to commercial potato chips. Frying temperature, frying time, and slice thickness demonstrated significant effects on the quality attributes of vacuum-fried surimi chips. Agriculture and Forestry University (CXZX2018063). We also would like to thank the Haixin Foods Co., Ltd., for providing the fish surimi and other ingredients.

CO N FLI C T O F I NTE R E S T
The authors declare no conflict of interest.

E TH I C A L S TATEM ENT
This study does not involve any human nor animal testing.