Effects of ultrasound-assisted frying on the physiochemical properties and microstructure of fried meatballs

Abstract

The aim of this study was to investigate the effects of power ultrasound (0, 200, 400, 600 and 800 W, 20 kHz) on the quality of fried meatballs. The frying time and frying temperature were also considered as fixed factors. The meatballs were fried for 8, 12 and 16 min at 120, 140 and 160 °C. Results showed that the ultrasonic groups saved about 1 to 3 min than the control group to 80 °C. Hardness, springiness, cohesiveness and chewiness of fried meatballs were significantly different among different ultrasonic treatments (P < 0.05). As for colour, the ultrasonic treatments could significantly increase the L*values. Cooking yield was from 82.58% to 85.50% in ultrasonic treatments at 120 °C for 8 min. High moisture retention and cooking yield were shown in fried-assisted ultrasound treatments and were consistent with the microstructure of scanning electron microscopy (SEM). Our data show that ultrasound-assisted frying could improve the quality of meatballs.

Graphical Abstract

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Introduction

Ultrasound is a form of mechanical vibration energy with the frequency higher than 20 kHz which human ear can not be detected (Fulya & Gülden, 2015). When sounds go through the liquid media, compression wave interacts with liquid media resulting in gas dissolution and acoustic cavitation. The size of these cavities is bigger with subsequent circles of ultrasound and finally become overthrow releasing high pressure as well as temperature (Jayasooriya et al., 2004). In recent years, several studies have investigated the effects of ultrasonic power on meat quality (Kang et al., 2016a, 2017a,b). As for fresh meat, ultrasonic treatment with a frequency of 25 kHz and an intensity of 2 W cm⁻² was applied in semimembranosus muscles resulting in tender meat during aging (Dickens et al., 1991). Ultrasound (2200 W, 15 kHz) could effectively tenderize the pork loin and decrease shear force by 72.3% after 48 h aging (Yeung & Huang, 2017). It was reported that beef M. semitendinosus treated by ultrasound of 20 kHz could significantly increase myofibrillar fragmentation index (MFI) and decrease Warner-Bratzler shear force (WBSF) (Wang et al., 2018). Jayasooriya et al. (2007) reported that meat treated with ultrasound could improve tenderness while no significant effect was shown on drip loss and colour. In terms of meat processing product, ultrasound-assisted cooking resulted in a higher cooking yield and moisture retention as well as a lower cooking loss (Reynolds et al.,1978). Dolatowski et al. (2001) found that the juiciness of chicken samples was enhanced by the ultrasound treatment. In addition, Pohlman et al. (1997) suggested that ultrasound-assisted cooking could significantly decrease the cooking time. However, some researchers found that ultrasound did not significantly affect meat quality (Joanna et al., 2008; Smith, 2011; McDonnell et al., 2014). The rate of NaCl diffusion in pork tissue increased with increasing temperature and power ultrasound intensity. Ojha et al. (2016) reported that ultrasonic treatment had a low influence on the mass transfer of salt into pork samples at low ultrasonic power. These different results may be due to different ultrasonic power, treatment time, animal species and muscle types.

Deep-fat frying (DFF) is one of popular food preparation techniques which are defined as a food heating processing that immerses food into the oil with a high temperature ranged from 150 to 200 °C. DFF can result in popular fried products with unique taste and crispy texture as well as distinctive aroma. Meatball is one typical fried product with golden colour as one of crucial factors to evaluate the quality of meatballs (Choe & Min, 2007). Generally, meatballs are properly fried at the temperature of 150 to 180 °C (Choe and Min, 2007). However, during frying processing, lighter colour was shown at the edge when frying temperature was lower than proper temperature. The surface of meatballs was over-fried while the inside was insufficiently fried in a high temperature which reduced meat quality and cooking yield. The texture and sensory properties of meat were influenced when its moisture was progressively decreased during frying (Kita et al., 2007).

However, few studies have been performed to use ultrasound-assisted frying to improve the quality of meatballs. Therefore, the objective of this study was to investigate the effects of ultrasonic power on the physicochemical properties, sensory evaluation and microstructure changes of meatballs. Meanwhile, it is urgent to develop a new green and high-efficiency technology to improve the yield and taste quality in a shorter cooking time as well as a lower cooking temperature. Thus, frying temperature and frying time were also considered as experiment factors in this experiment.

Material and methods

Materials and chemicals

Pork back fat purchased from a local market was stored at 4 °C for 2 h. Beef flank muscles from both sides was collected from a slaughterhouse at 48 h postmortem (Zhongyuan Haoyue Halal Food Industry Co., Ltd., Kaifeng, Henan, China). After removing all visible fat and connective tissue, the outside flat was vacuum packaged and stored at −20 °C. Other condiments were from the pilot laboratory of National Meat Research Center, Nanjing Agricultural University Nanjing, China.

Preparation of fried meatballs

The preparation of fried meatballs was performed using the method of Huang & Mittal (1995) with slight modifications. The outside flat was thawed at 4 °C for 12 h. Seasoning was prepared by mixing 1.5% salt, 1.5% sugar, 0.3% sodium tripolyphosphate, 0.3% black pepper, 15% ice water and 10% starch based on the total weight of meat. The pork back fat and beef outside flat were minced in a mincer (TC, 12E, SIRMAN, Venezia, Italy) using a 10-mm diameter sieve. Then the minced meat and seasoning were put into a laboratory chopper (BZBJ-15, Expro Stainless Steel Mechanical and Engineering Co., Ltd., Hangzhou, Zhejiang, China) for 90 s at 2789 g and all ingredients were mixed thoroughly. The mixed meatball stuffing was shaped into 80 g meatballs with a diameter of 5 cm by meatball shaper (T-473, SAVORLIVING, Shanghai, China). The prepared meatballs were put in an ultrasound fryer (Tianhua Ultrasonic Electronic Instrument Co., Ltd, Jining, China). The meatballs mass to oil mass ratio was 1:5 (W/W). Three main factors including ultrasound power, frying temperature and frying time were set to the meatball frying processing. The factors were set as followings: the ultrasonic power was 0, 200, 400, 600 and 800 W, the frying time was 8, 12 and 16 min, and three levels of frying temperature (120, 140 and 160 °C) were considered. Each treatment with six replicates was prepared. The meatballs were cooled down to a room temperature (25 °C) before further analysis.

Central temperature detection

The central temperatures of fried meatballs were determined by a pricking thermometer (108-2, Testo SE &Co. K GaA, German). During frying process, thermocouples were placed in the geometric centre of the meatball and the temperatures were recorded in every 2 min.

Cooking yield and moisture retention

After the fried meatballs were cooled to 25 °C, the paper towel was used to wipe the oil from the surface of meatballs. The cooking yield was measured by the ratio of cooked weight to raw weight according to the method of Park et al. (2017). Water content was determined by GB 5009.3-2016 National Food Safety Standard Determination of Water in Food. Moisture retention was expressed as the rate retained moisture in cooked meatball per 100 g of raw meatballs. The cooking yield and the moisture retention were calculated by using Formulas 1 and 2

Colour

The colour of fried meatballs was determined by a colorimeter (Chroma Meter CR-400, Konica Minolta, Sensing, Inc., Sakai, Japan) referred to Gao et al. (2014). The parameters were set as follows: illuminant of D₆₅, observation angle of 10 ° and 8 mm aperture. Colour difference meter was calculated with a white blank (L* = 96.86, a* = −0.15, b* = 1.87) before recording the L* (lightness), a* (redness), and b* (yellowness) values. Each prepared sample was determined three times in different locations on the surface of meatballs.

Texture profile analysis

A texture analyzer (XT Plus, Stable Micro systems Ltd, Godalming, UK) was used to analyze texture. Testing parameters were set according to Dondero et al. (2006) with slight modifications. The parameters were set as follows: probe of 50 (50 mm stainless cylinder), testing speed 1.0 mm s⁻¹; a compassion rate of 50%; pre-test speed and post-test speed, 5.0 mm s⁻¹; withdrawal speed, 1.0 mm s⁻¹. Samples were put on the centre of the TPA plate and the test interval between two compression was 5 s. Cubic samples (1 cm × 1 cm × 1 cm) from the centre of meatballs were prepared by using a double edge knife. Hardness, springiness, cohesiveness and chewiness were recorded. The results were conducted by Texture Expert Exceed 2.64a inner micro TPA. Six replications were prepared for each treatment.

Sensory evaluation

Sensory evaluation was analyzed on treatments of different ultrasonic power (0, 200, 400, 600 and 800 W). Oil temperature was set to 160 °C and frying time was 12 min. Thirty trained panelists half males and females aged from 20 to 45 was from students and staff from the pilot laboratory of National Meat Research Center. These panelists were trained for a period of 2 months in 1 h sessions that occurred twelve times a month (36 h in total). Trained panelists was followed the protocols established by the Standard GB/T 16291.1-2012. Five meatballs from each treatment were assessed every time. Each fried meatball was cut into cubic 2 × 2 × 2 cm and placed in a small white plate in the taste room. Room temperature was set as 25 °C with a bright light. Meatballs were served in random order to panelists seated in individual places. Mineral water and crackers were provided to rinse their mouth between evaluations. Appearance, hardness, taste and springiness of fried meatballs were evaluated. Before evaluating, panel members would receive instructions recording details of each indicator from point 1 to 9 (1 = white and light appearance, extremely hardness, easily chewed and lack of meat flavour, less springiness respectively; 9 = golden and darker appearance, extremely tender, chewy and full of meat flavour, extremely springiness respectively).

Scanning electron microscopy (SEM)

SEM was examined using the method described by Álvarez et al. (2012) with a slight modification. Beef meatballs were fried with different ultrasonic power of 0, 200, 400, 600 and 800 W respectively. Samples were fried at the temperature of 160 °C for 12 min. Samples from the centre of the meatballs were cut into 1 mm slices and fixed for 48 h at 4 °C in 2.5% (v:v) glutaraldehyde in 0.1 m phosphate buffer saline (PBS, pH 7.3). The slices were then transferred to 0.1 m phosphate butter (pH 7.3) in 1% (v:v) osmium tetroxide (O₂S₄) for 5 h. After washing three times with PBS (pH 7.3), slices were dipped in incremental concentrations of ethanol (50%, 60%, 70%, 80%, 90%, 95% and 100%, each for 30 min) to dehydrate and then immersed in acetone to further dehydration. Samples were stored in the oven for 15 to 20 min at 50 °C and then mounted on aluminum sample holders (Luster Alpin Mount, West Chester, PA, USA). Finally, mounted specimens were sputtered coated with gold-palladium for 2 min at 10 mA in a Denton 503 High Vacuum Evaporator with10 kV. The fixed slices were examined by a scanning electron microscope (Hitachi-S-3000N, High Technologies Corp., Tokyo, Japan).

Statistical analysis

SAS 9.2 statistical software was used for the data analysis. The three-factor factorial design was performed in this study. Ultrasonic power, frying time and frying temperature were set as fixed effects in a Mix-model analysis. LSD (least significant difference) was adjusted by Bonferroni method for multiple testing in the Mix-model analysis. Duncan's multiple range test was used for multiple testing in the main factor of ultrasound. Significance was determined at the level of P < 0.05.

Results and discussions

Central temperature detection

To compare the effects of traditional DDF and ultrasound-assisted frying on the frying rate of meatballs, oil temperature was set to 160 °C. The central temperature of fried meatballs among different ultrasonic power of 0, 200, 400, 600 W and 800 W is shown in Table 1. The central temperature of fried meatballs samples was improved by increasing frying time. When meatballs were treated with ultrasound, the internal temperatures of samples were significantly higher compared to meatballs fried without ultrasound (P < 0.05). High central temperatures were presented in high power of ultrasound treatments at the beginning but it remained constant at the end of frying between different ultrasound treatments. The internal temperatures were similar among power ultrasound of 200–600 W groups when samples were fried for 12 to 16 min. The frying time was shorter when central temperature of meatballs reached 80 °C in ultrasonic-assisted frying treatments than meatballs fried without ultrasound treatment. It suggests that ultrasonic processing groups could decrease frying time when meatballs were fried at the same temperature. These results are in accordance with Pohlman et al.(1997) who showed that the ultrasound-assisted cooking could decrease cooking time compared to traditional cooking. The ultrasonic wave is mechanical vibration energy which contributes to forming effective stirring and flow in media (Chemat et al., 2011). These effects accelerate convection and heat conduction from oil media to meatballs and eventually heat energy produced by electric heating wires could be spread and distributed quickly. Similarly, the heat energy produced by the thermal effect of the ultrasound can be absorbed by propagating medium resulting in temperature arises (Got et al.,  1999). The resulting heat was increased with the improvement of the ultrasonic power. The fried beef meatballs were accomplished when meatballs were fried for about 7 to 12 min. To compare the interaction of ultrasonic power, frying temperature and frying time on fried beef meatballs, the frying time was set to 8, 12 and 16 min. The final temperature of meatballs is shown in Fig. 1.

Cooking yield and moisture retention

As shown in Table 2, the main effect of frying time (FT1) and frying temperature could significantly affect the cooking yield of meatballs (P < 0.05). The interaction of frying temperature×frying time (UP×Ft) and the cross effects of all factors (UP×Ft×FT1) were also significant (P < 0.05). Cooking yield was shown to be significantly different in frying time and frying temperature. The results of cooking yield among the five ultrasound treatments are presented in Fig. 1. Ultrasound could significantly improve the yield of fried meatballs compared to traditional frying treatment (P < 0.05).

Moisture retention of fried meatballs is given in Table 3 and Fig. 2. The main effect of frying time (FT1), frying temperature (Ft) and their interactions (Ft × FT1, UP × Ft × FT1) all showed significant effects on the moisture retention of fried meatballs (P < 0.05). As mentioned before, removing moisture from the product is one of the important aspects of frying process. The content of moisture retention is related to the quality of cooking yield. High cooking yield and the higher moisture retention was shown in groups treated with ultrasound compared to samples without ultrasound (P < 0.05, Fig. 2). These results are consistent with Pohlman et al. (1997) who found that ultrasound (20 kHz, 1000 W) could result in higher water retention and lower cooking loss in beef longissimus.

The water in the muscle is formed by the bound water, immobilized water, and free water. Immobilized water accounts for 80% of moisture in muscle cells (Liu et al., 2015). Zou et al. (2018a) found that ultrasound increased the water holding capacity (WHC) and the water binding strength ability of spiced beef. When meatballs were fried with ultrasound, ultrasonic mechanical effects could change more free water to immobilized water thus the moisture retention was increased. Ultrasonic cavitation is one of the main effects of myofibrillar destruction (Guan & Ruicheng, 2010). The cavitation effect produced by ultrasound can effectively destroy the structure of myofibrils which could contain more water (Turantaş et al., 2015). Myofibrillar proteins are destroyed in frying process which contributed to form a sticky network. The strong meatball network could contain more water. During traditional deep frying process, meatballs are immersed in hot oil which increased the surface temperature of meatballs rapidly. The heated bubbles in oil caused the expansion and contraction of meatballs immersed in the oil (Lalam et al., 2013). This phenomenon may create fine microscopic channels in the foodstuff which contributed to facilitating moisture removal. The moisture of meatball surface was evaporated meatballs in the first few minutes that resulted in lower cooking yield and moisture retention (Lawrie & Ledward, 2006). When meatballs were fried-assisted with ultrasound, a tough crust was formed at a faster speed which contributed more moisture in the centre of meatballs. The thickness of the crust surface increased during frying processing leading to decreasing heat and vapour transfer rate from the meatball.

Texture profile analysis

The results of ultrasonic power and cooking time on the textural properties of fried meatballs are shown in Tables 4–5. For hardness, springiness and cohesiveness, the main effect of frying time (FT1), frying temperature (Ft) all showed significant effects (P < 0.05). The cross effects of all factors (Ft × FT1, UP × Ft × FT1) showed significant effects (P < 0.05). For chewiness, the main effect of frying temperature (Ft) was significant (P < 0.05) while the main effect of frying time (FT1) and ultrasonic power (UP) was not significant (P > 0.05). Their cross effects of all factors (UP×Ft×FT1) showed significant effect on chewiness (P < 0.05). As shown in Table 5, the hardness and cohesiveness of fried meatballs in ultrasonic treatment groups were significantly lowered compared to meatballs without ultrasound (P < 0.05). Meatballs fried by ultrasound at 800 W showed significantly higher hardness than those with ultrasound power of 200 W. It indicates that high power of ultrasound could increase meatball hardness deeply.

The difference of meat tenderness is mainly determined by the myofibrillar network fraction and connective tissue fraction (Dolatowski et al., 2007). During frying process, Sahin et al. (2005) found frying time had an important influence on the hardness of chicken nuggets. High heating and long time treatments could induce myofibrillar protein denaturation which promoted the cross-linking of protein. The strong protein crosslinkage contributed to the improved hardness of meatballs. The changes in meat product tenderness were mainly due to the thermal changes of myofibril and collagen in the meat (Bertola et al., 2010). Ultrasound could prove that the breakdown of muscle cells of meat tissue due to its mechanical effect. Jayasooriya et al. (2007) indicated that ultrasound was capable of causing the physical disruption of bovine Semitendinosus and Longissimus muscles through ultrasound cavitation. When meatballs were fried with ultrasound, the destructive nature of cavitation to myofiber structure could decrease the hardness of meatballs and improve meat tenderness. It was reported that ultrasonic treatment (2200 W) for 6 min can effectively improve the tenderness of pork loin (Dolatowski et al., 2007). Unstable bubbles and cavities grew slowly in size and oscillate within media that produced high shear and pressure to disturb the integrity of muscle fibres and contributed to the tender meat.

Colour

The colour of surface meatballs was shown in Table 6 and Table 7, the main effect of frying temperature (Ft), frying time (Ft) and ultrasonic power (UP) showed significant effects on the L*values (P < 0.05). The interaction of all three factors (UP × Ft, UP × FT1, Ft × FT1, UP × Ft × FT1) was significantly different on L*values (P < 0.05). As for a* values, The main effect of frying temperature (Ft), ultrasonic power (UP) and their interaction (UP × Ft, UP × FT1, Ft × FT1, UP × Ft × FT1) showed significant differences in a* values of fried meatballs (P < 0.05). When ultrasound was applied as an effect during frying processing (Fig. 3), the L*values from ultrasonic power processing groups were significantly higher than the group fried without ultrasound (P < 0.05) while a* values were significantly higher than samples fried without ultrasound (P < 0.05). The colour of centre meatballs is shown in Table 8, the main effect of frying time, frying temperature, ultrasonic power and their interactions all showed significant effects on the L*values (P < 0.05). Frying time could significantly decrease the a* values and increase the b* values (P < 0.05) while no significant effects were shown among frying temperature and ultrasound power (P > 0.05). The changes in L*values could be caused by caramelization reactions and the non-enzymatic browning between reducing sugars and free amino acids.

In the frying process, Maillard and caramelization have an influence on pleasant flavour and attractive colour. Meatballs could generate golden colour and attractive flavour compounds as the cooking time and the cooking temperature raised (Trevisan et al., 2016). Non-enzymatic browning reactions are temperature-dependent. The degree of non-enzymatic browning reactions deepens severely along with the rise of temperature and time. The Maillard reaction produced browning surface and eventually reduced the L*values of meatball samples. When meatballs were fried with ultrasound, ultrasound could induce electric heating waves which were transferred to heat energy partially responsible for the increase of oil temperature. This may owe to the non-enzymatic browning and superficial caramelization reactions of the fried beef meatballs. The lightness was known to be positively correlated to the content of moisture (Özkan et al., 2003). It is suspected that ultrasound-assisted frying could initially expedite the dehydration of meatball surface. In the same way, the increase of a* and b* values can be attributed to the fact that part of the frying oil was adhered to the crust.

Fat and the oxidative polymerization of carbohydrates significantly affect the final colour of cooking meat. It was reported that the a* values were increased with frying time and frying temperature (Özkan et al.,  2003). During thermal process, oil was adhered to the crust of fried meatballs and then a red tone was shown on the crust. The a* values were mainly affected by the content of pigment existed in muscle as well as light reflection and oxidation. Cavitation of ultrasound is capable of dissociating of molecular bonds and producing hydroxyl radicals (▪ OH) that have a significant influence on lipid oxidation (Miller, 2007). Kang et al. (2016b, 2017a,b) demonstrated that ultrasound produced ▪ OH was capable of contributing to accelerated lipid oxidation and protein oxidation. The increase of a* values could be due to the occurrence of oxidative denaturation of myoglobin to the oxygenated myoglobin. Ultrasonic power is capable of producing cavitations within the muscles and resultant extreme pressures induce mechanical disruption to produce oxygenated myoglobin from myoglobin by oxidation (Jayasooriya et al., 2004).

Scanning electron microscopy (SEM)

The scanning electron micrographs (SEM) of fried meatballs with different ultrasonic power for 12 min at the temperature of 160 °C are shown in Fig. 4. The samples fried without ultrasound appeared to be randomized with void spaces, cracks and a rough surface in Fig. 4 (a). When meatballs were fried with ultrasound, the network of fried meatball presented a regular porosity state and a complexity of microstructure in Fig. 2 (b–e). Void spaces existed in no ultrasound and ultrasound-assisted frying groups. Void space in irregular size existed in meatballs in traditional frying processing while some void spaces were covered that may be caused by myofibril cross-linking in the presence of ultrasound. During the frying process, the formation of porous structure in heat treatment could improve the meatballs evaporation. This structure of meatballs is consistent with the moisture loss of meatballs. The myofibril framework of samples treated with ultrasonic power was destroyed severely by the mechanical effects of ultrasound and eventually fibril network cross-linking was firmly formed. The difference in microstructures might partly explain the decrease of texture including hardness. As described by Pohlman et al. (1997), cooking assisted with ultrasonic power resulted in tender meat. Therefore, the microstructure changes from the mechanical effect of ultrasound could reflect the water retention and physicochemical properties resulting from ultrasound.

Sensory evaluation

To investigate the influence of ultrasound during frying process on the eating quality of meatballs, ultrasonic power was set as 0, 200, 400, 600 and 800 W, and fried for 12 min at the temperature of 160 °C (Table 9). As ultrasonic power increased, the appearance scores of meatballs increased significantly. The ratings of appearance fried with ultrasound were significantly improved than the group fried without ultrasound (P < 0.05). As mentioned above, instrumental colour of meatball surface was significantly different in the L* parameter among different ultrasonic treatments. A better colour was also shown in sensory evaluation. The decrease of hardness was consistent with texture measurement. It indicates that fried-assisted ultrasound could result in a tender meat. Scores of taste were significantly higher in ultrasound treatment (P < 0.05). Jambrak et al. (2009) claimed that ultrasound could change the properties of protein. Cavitation of ultrasound may contribute to release amino acids to form flavour substance as reported in cooked spiced beef with ultrasound (20 kHz, 120 min) by Zou et al. (2018b). Springiness is a subjective indicator to evaluate the quality of meat products (Zhuang & Savage, 2014). No significant difference was shown on springiness in fried meatballs during frying process (P > 0.05). These results suggested that meatballs fried-assisted ultrasound could show a better acceptability than the treatment without ultrasound.

Conclusions

This study was conducted to the ultrasonic power (0, 200, 400, 600, 800 W) on the quality of fried meatballs. The meatballs were fried for 8, 12 and 16 min at the temperature of 120, 140 and 160 °C, individually. The results showed that ultrasound had a positive effect on the decrease of frying time and the improvement of cooking yield. High moisture retention was presented in ultrasound treatment. As for colour, the L* values increased while a* values decreased as the increase of ultrasonic power. The cross-linking of myofibril could cover void spaces and form fibril network cross-linking when meatballs were fried assisted-ultrasound which was related to water retention and texture. For sensory evaluation, the quality of meatballs treated by ultrasound was superior to the group without ultrasound. Therefore, fried-assisted ultrasound could be a potential processing technology to improve the quality meat products.

Acknowledgments

This report was supported by National Key R&D Program of China (2016YFD0400700, 2016YFD0400703) and National Key R&D Program of China (2018YFD0400101, 2018YFD0400101).

References