Radio frequency-assisted hot air drying of pacific white shrimp (Litopenaeus vannamei): drying kinetics, product quality and composition evaluation

ABSTRACT

The influence of radio frequency-assisted hot air drying (RFHAD) on the drying kinetics, product quality and composition of Pacific white shrimp was evaluated and compared with radio frequency drying (RFD) and hot air drying (HAD). The results demonstrated that RFHAD showed faster drying rate and also ensured better product quality in terms of texture property, astaxanthin content, color quality, rehydration rate and microstructure of shrimp. Furthermore, RF wave could significantly reduce the formation of bitter amino acids (p < .05) and showed no significant difference in the content of umami amino acid. However, secondary structure changes were observed in RFHAD shrimp with a decrease in β-sheet and α-helix contents and increase in β-reverse fold, and RFHAD noticeably decreased the onset temperature, peak temperature and thermal enthalpy (ΔH) values of shrimp, which showed lower thermal stability. Therefore, the RFHAD method is a fast and efficient drying technology for shrimp with high-quality characteristic.

KEYWORDS:

• Radio frequency-assisted hot air drying
• Litopenaeus vannamei
• drying characteristics
• product quality
• structural

1. Introduction

The South American white shrimp (Penaeus vannamei) has high productivity, fast growth, strong adaptability, and easy cultivation, making it one of the main shrimp species for aquaculture worldwide (Lin et al., 2022). Shrimp has high nutritional value is rich in essential amino acids, astaxanthin, vitamins, mineral elements, proteins, etc., and has significant effects in supplementing dietary deficiencies and improving human health (Becerra et al., 2014; Chen et al., 2019). Shrimp protein is the most popular protein among all seafood proteins in terms of its amino acid structure because it contains high content of lysine (Li et al., 2023). Moreover, it is especially popular for its unique taste and flavor. Among the top three shrimp species with high aquaculture production worldwide, the South American white shrimp has continuously increased in production since its introduction to China in the early 1990s. In 2019, China’s marine aquaculture production of shrimp reached 1.14 million tons. The high moisture and protein content of the shrimp, along with its soft muscle tissue, made it susceptible to spoilage and limited in storage time at room temperature (Zhang et al., 2019). Currently, besides being consumed fresh or frozen, shrimp is commonly processed into dried shrimp. It has gained popularity among consumers because of its abundant nutrition, unique flavors and convenience.

Currently, traditional drying methods of shrimp mainly include sun drying and hot air drying. However, the former have drawbacks such as excessive hardness, poor rehydration, difficult chewing and unsatisfactory texture due to unsanitary drying environments and long drying times, while the latter consumes high energy, exposes the products to high temperatures, and may cause heat damage and excessive oxidation. Consequently, researchers have been exploring alternative approaches. In recent years, novel drying technologies have grown rapidly to meet current and future consumer demands, including microwave drying, infrared drying, radio frequency drying, vacuum freeze drying, etc (Shewale et al., 2021). Thereinto, radio frequency drying (RFD) is an emerging thermal technology typically referring to 1–100 MHz electromagnetic waves that penetrate in food materials to generate internal heat (Mahmood et al., 2022). According to the requirement of U.S. Federal Communication Commission (FCC), the RFD frequency bands which are allowed to use for food processing are 13.56, 27.12 and 40.68 MHz (Federal Communications Commission [FCC], 2020). RFD processing has been utilized in processing large dimension and bulk food materials, which is due to its large penetration, great heating uniformity, high energy efficiency and volumetric heating (Chen et al., 2019). Recently, RFD technology have been published applications in sterilization (Bermudez-Aguirre & Niemira, 2023; Jiao et al., 2018; Nohemí Soto-Reyes et al., 2022; Sosa-Morales et al., 2022), disinfestation (Hou, 2017; Sun et al., 2019; Xiao et al., 2022), thawing (Jiang et al., 2023; Liu et al., 2021), enzyme inactivation (Jiang et al., 2021), retaining freshness (Lara et al., 2021), and dehydration (Cao et al., 2021; Elik, 2021; Wei et al., 2023; Zhou et al., 2018a). Great heating uniformity and high efficiency could remarkably improve the drying efficiency of food products, with smaller effects on the quality in comparison with hot air drying. For agricultural product drying, some researchers found that the combination of RFD with HA drying could simultaneously improve drying rate, disinfect bacteria and minimise its effect on the product quality (Cao et al., 2021; Zhang et al., 2015). Research has been conducted on RFHAD of grain, fruits, vegetables and nuts. Özbek (2021) reported that RFHAD dehydration could reduce drying time by 45% for carrot powders with retention rates of 80.26% and 72.42% of beta-carotene and total phenolic content. Similar results have also been found by Xie et al. (2020) for corn seeds. However, studies on RFHAD technology applied on shrimp or other seafoods are limited. It has been reported that the drying rate of RFHAD is 1.1–1.4 times of that of HAD for tilapia fillets and RFHA dried samples showed higher rehydration rate and lower volume shrinkage rate (Cao et al., 2021). Zhang et al. (2021a) was conducted to investigate the effects of RFD tempering and freeze-thaw cycles on melanosis and quality of Pacific white shrimp. It focused on the aspect of RFD tempering on shrimp. Currently, very few researches comprehensively investigated the dried shrimp under RFHAD compared to other drying methods. Moreover, the quality and nutritional changes of shrimp during drying process cannot be ignored, such as the astaxanthin, protein, and free amino acids.

During drying process, it mostly leads to proteins denaturation, which is from the native state to the disordered state due to the changes in non-covalent hydrogen bonding, hydrophobic and electrostatic interactions, or covalent cross linking (Belitz et al., 2009). Drying treatment of the native structure of the protein at different times and temperatures leads to significant changes in its nutritional value and stability. While the stability of proteins may be related to their secondary structure. Fourier transform infrared spectroscopy (FTIR) is a very powerful tool for obtaining protein secondary structure. The relative content of the secondary structure of the protein (α -helix, β -fold, β -turn, and uncoiled) can be obtained by properly fitting the amide I band of the original FTIR and analyzing its second derivative (Farrell et al., 2001). It has been reported that the β -turn and random coiling of proteins could promote the formation of thermal aggregates (Qi et al., 2018).

In this study, hot air drying was used to dry shrimp as control and comparative analysis of shrimps dried by RFHAD and RFD methods was profiled. The objectives of this study were to (1) investigate the drying characteristics of RFHAD, RFD and HAD; (2) compare the astaxanthin content, calorimetry study, free amino acids analysis, protein conformation and meat structure of dried shrimps obtained from RFHAD with those obtained after RFD and HAD; and (3) evaluate the feasibility of RFHAD applications in shrimps.

2. Materials and methods

2.1 Pretreatment of materials

Fresh shrimp was purchased from a local supermarket in Ningbo, Zhejiang Province. A total of 350 g of shrimps with uniform size and no mechanical damage were selected for each groups. After cleaning with tap water, they were boiled in 6% salt water for eight minutes, then removed and drained, which was the group of CK. The average initial moisture content (w.b.) of shrimps was 71.25 ± 1.13%, which was measured using a halogen moisture meter (ESH 31, Sunny hengping instrument Co., Ltd, China).

2.2 Radio frequency-assisted hot air drying (RFHAD)

The radio frequency-assisted hot air drying experiment was performed by using a 5 kW, 27.12 MHz pilot-scale RFD-system equipped with a hot-air system (temperature range of 40°C–80°C) and air vent. RFD system consists of two parallel-plate electrodes and the distance between the electrodes could be adjusted between 5 and 12 cm. Before the samples were transferred into the equipment, the RFD system should be started up for 0.5 h to reach steady state conditions. Approximately, 480 g ±2 g of shrimp samples were placed on the bottom electrode in a polypropylene container with uniform ventilation holes (18 cm × 12.5 cm × 7.5 cm). The shrimp samples were paved with four layers in the container (7.0 cm ± 0.3 cm of thickness). The temperature of the samples was monitored by a fiber-optic temperature sensor. The RFHAD process was performed at an electrode gap of 8.5 cm and air temperature of 60°C according to our preliminary experiments. The container was removed immediately from the RFHAD system at a interval time to measure the sample weight using an electronic balance. The RFHAD process was stopped until the moisture content of samples dropped below 15% ± 0.5% (wet basis). After drying, the shrimp samples were cooled at chamber temperature, and then packaged in a valve bag and finally kept at −20°C until further analysis.

2.3 Radio frequency drying (RFD) and hot air drying (HAD)

RFD was carried out under the same conditions (using the same sample weight and the number of flat layer) as RFHAD group without applying hot air using the same drying system as described in section 2.2. HAD was also carried out under the same conditions (using the same sample weight and the number of flat layer) as RFHAD group. The shrimp samples were placed in a forced HA oven at 60°C. The same procedures explained in section 2.2 were applied to get dried shrimp after RFD and HAD. RF and HA drying processes were terminated when the moisture content of shrimp reached 15% ± 0.5% (wet basis) for the following quality and composition experiments.

2.4 Drying characteristics

To interpret the drying characteristics of shrimps, drying rate (DR) diagram was calculated according to Elik (2021), which defined as water mass removed per unit time per unit mass of dried samples. The moisture ratio curve (MR), which refer the rate of surface evaporation of shrimps (Cao et al., 2021), indicating the ratio of sample moisture as compared with the initial moisture.

2.5 Color measurement

Surface and internal color of the dried shrimps was measured using a colorimeter (Ci6×, X-rite Pantone Co., Ltd, U.S.A.). In the Hunter Lab color system, L* value represents the lightness, with higher values indicating brighter intensity, and a* (green/red) and b* (blue/yellow) were measured (Gong et al., 2019). The measurement point is on the surface or internal of the second abdominal segment of the dried shrimp. Three dried shrimps were measured per group, with three repetitions for each shrimp. Shrimps after boiling in saltwater were used as the control in the calculation of ΔE. Total color difference (ΔE) and whiteness index (WI) were calculated as follows (Cao et al., 2021):

(1) $\mathrm{WI}=100-\sqrt{{\left(100-{\mathrm{L}}^{\ast }\right)}^2+{\left({\mathrm{a}}^{\ast }\right)}^2+{\left({\mathrm{b}}^{\ast }\right)}^2}$(1)

(2) $\mathrm{\Delta }\mathrm{E}=\sqrt{{\left(\mathrm{\Delta }{\mathrm{L}}^{\ast }\right)}^2+{\left(\mathrm{\Delta }{\mathrm{a}}^{\ast }\right)}^2+{\left(\mathrm{\Delta }{\mathrm{b}}^{\ast }\right)}^2}$(2)

where ΔL*, Δa* and Δb* represented the difference between the average L*, a* and b* values of the dried group and the control group, respectively.

2.6 TPA determination

Five dried and rehydration shrimp were randomly selected to determinate the TPA as presented by Wang et al. (2023). A P/5 cylindrical probe was used with a downward speed of 5 mm/s, compression speed of 0.5 mm/s, and recovery speed of 0.5 mm/s. The deformation value was set to 50%, triggering force to 5 g, and compression depth to 5 mm. Evaluation parameters characterizing the texture of the shrimp, including hardness, springiness, adhesiveness, and chewiness, were obtained from the texture characteristic curve. Experiments were done in triplicates.

2.7 Rehydration rate calculation

The rehydration rate is represented by the recovery rate. It is calculated by dividing the weight of the sample after rehydration and drainage by the weight of the dried sample before rehydration. The rehydration was conducted by immersing the shrimp in boiling water at 100°C for 5 min and then draining the water. The calculation formula is as follows (Gong et al., 2019):

(3) $R_f=\frac{m_f}{m_g}\ast 100\mathrm{\%}$(3)

where R_f is the recovery rate (%), m_f is the weight (g) of the sample after rehydration and drainage, and m_g is the weight (g) of the dried sample before rehydration. Experiments were done in triplicates.

2.8 Astaxhantin content determination

The content of astaxanthin was performed by high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) via a Shimadzu LC-20A series system equipped with a photo-diode array (PDA) detector (Yang, 2015). The pretreatment procedure is as follows: 0.2 g samples were mixed with 1.5 ml of 60% acetonitrile and 0.4 g anhydrous sodium sulfate. And then the solution was sonicated for 30 min and centrifuged at 12,000 rpm for 5 min. The above extraction was repeated twice. The sample extracts were filled with nitrogen for 5 min and then resolved with 1.5 ml of acetonitrile. The solution was filtered through a 0.22 μm PVDF syringe filter and then was injected for HPLC analysis with C18 column (250 mm × 4.6 mm, 5 μm) at 40°C. The flow rate was 1.0 ml/min with a 10 μl injection volume and the monitoring was performed at 450 nm. The content of astaxanthin was calculated by the standard curve of Y = 29862X–201491 (R² = 0.9975).

2.9 Free amino acids determination and taste activity values analysis

The free amino acids of dried shrimp was carried out according to the method of Shi et al. (2022) with some modification. Briefly, 2 g of the dried shrimp and control samples were homogenized with 15 ml of 70% methanol and then for 40 min of ultrasound treatment. After centrifugation, 1 ml homogenate was filled with nitrogen for 5 min, then sealed and dried by a vacuum centrifugal concentrator until no visible liquid. one milliliter ultrapure water was used to dissolve the sample. To prepare the derviatives, 20 μl of positive leucine, 100 μl of triethylamine solution and 100 μl of phenyl isothiocyanate (PITC) solution were added to 200 μl of sample extracts. The vial containing the reaction mixture was incubated at room temperature for 1 h. The mixture was treated with 400 μl of n-hexane and then centrifuged at 12,000 rpm for 2 min. Ten microliters of underlying liquid was injected for HPLC (LC-100, Shanghai Wufeng Scientific Instrument Co., LTD, China) analysis.

Taste activity values (TAV) were determined according to Kato et al. (1989), which could demonstrate the impact of the umami components on taste. The calculation formula is as follows:

(4) $\mathrm{TAV}={\mathrm{C}}_1/{\mathrm{C}}_2$(4)

where C₁ denotes the concentration of a taste compound and C₂ is the minimum concentration at which said compound can be perceived.

2.10 Differential scanning calorimetry

The calorimetry study of the dried shrimp samples was tested by using a differential scanning calorimeter (DSC-1 STARe, Mettler-Toledo), which was adapted from Thorarinsdottir et al. (2002). The shrimp sample (approximately 10 mg) was weighed in an aluminum pan and heated from 25°C to 150°C at a rate of 20°C/min using an empty crucible as a reference. The onset, peak, and the conclusion temperatures (T₀, T_p, and T_c) together with the thermal enthalpy (ΔH) were recorded.

2.11 FTIR and protein secondary structure evaluation

FTIR spectroscopic measurements were carried out using a Bruker tensor 27 FTIR spectrometer using the KBr pressed disc method (Yang et al., 2020). After mixing the lyophilized protein powder was with an appropriate amount of KBr, it was grounded thoroughly and then pressed into transparent sheets, and Fourier transform infrared patterns were collected in a darkroom. Acquisition resolution was 4 cm⁻¹. Scan range was 400–4000 cm⁻¹. The number of sample scans was 32. The spectra were analyzed using Peakfit 4.12 software.

2.12 Scanning electron microscopy (SEM) observation

The morphological features of dried shrimp samples were conducted with a S2400N scanning electron microscope (Hitachi Co. Led, Japan). The pretreatment of dried shrimp was carried out according to the instruction outlined by Xuan et al. (2018). The samples were dehydrated, coated with gold-palladium to make the sample conductive before scanning by CCU-010 sputter coater (Safematic Co. Led, Switzerland), and then photographed at 1000, 3000 and 5000 magnifications with an accelerating potential of 10 kV.

2.13 Statistical analysis

All measurements were performed in triplicate and the results were reported as means ± standard deviations (SDs). One-way ANOVA with Duncan’s tests was performed by using SPSS v.20 (SPSS Inc., Chicago, IL, U.S.A.). Differences were considered significant at the level of p < .05.

3. Results and discussion

3.1 Drying characteristics

The drying curves and drying rates of pacific white shrimp which subjected to RFD, RFHAD and HAD are shown in Figure 1. Compared with HAD group, RFD pasteurization corresponded to a remarkably decreased moisture content. As seen in Figure 1(b), the drying rate of shrimp treated by RFHAD was highest, then follows by the RFD and HAD groups, which was similar to the results of drying curves. In the HAD stage, two phases of the drying process were demonstrated as follows. In the first 40 min, the drying rate of shrimp rapidly increased to 12.5%. In this phase, the moisture of shrimp was mostly as the free water and the temperature of material was similar to the wet-bulb temperature of the air. Then, in the next phase, gradual decreases in drying rates were observed due to the less energy was absorbed along with the evaporating of moisture. For RFD and RFHAD groups, three phases of the drying process were observed. The first peak value of drying rate for RFD and RFHAD groups was found at 25 min and 15 min, respectively. Then, the second peak value was 60 min and 50 min. The third phase was similar to the tendency of HAD group. It could be found that RFHAD accelerate the arrival of peak value of drying rate and shortened the drying time to the target moisture content of 15%. Similar results have been reported for in-shell walnuts, pepper and so on (Liu et al., 2021; Mao & Wang, 2022).

Figure 1. (a) Drying curves of RFD, RFHAD and HAD. (b) Drying rates of pacific white shrimp when subjected to RFD, RFHAD and HAD.

3.2 Texture and color difference

Table 1 shows the texture and color difference analysis results of pacific white shrimp after RFD, RFHAD and HAD treatments. A decrease in hardness, springiness, fracturability, chewiness and gumminess occurred for RFHAD group, but no significant difference was found (p > .05). And there is a slight increase in cohesiveness and resilience of shrimp treated by RFHAD. However, some researchers have published that RFD maintained a higher hardness of tilapia fillets for thawing in comparison with control group (Zhang et al., 2021b). Lan et al. (2020) observed that the hardness and chewiness in pacific white shrimp treated by RFD were higher than that treated by RT and WT. These phenomenon was probably due to the minimal damage to the myogenic fibers under shorter treatment time of RF wave. However, longer drying times resulted in similar effects on the texture of shrimps as HAD treatment. Ashtiani et al. (2023) has also been published that hardness of the dried mushroom was significantly higher than that of the fresh sample, which was due to the effect of drying on shrinkage and case hardening.

Table 1. Effect of different drying technologies on the texture and color difference of pacific white shrimp.

Group	Hardness (g)	Springiness (mm)	Cohesiveness	Fracturability	Chewiness (mJ)	Gumminess	Resilience	L	a	b
HAD	3650.69 ± 1295.54^a	0.85 ± 0.07^a	0.37 ± 0.05^a	3451.28 ± 1296.94^a	1457.36 ± 1037.52^a	1662.99 ± 1080.05^a	0.05 ± 0.01^a	64.83 ± 3.08^a	9.93 ± 1.45^a	10.44 ± 1.41^a
RFD	3322.44 ± 1361.45^a	0.89 ± 0.07^a	0.37 ± 0.10^a	3798.33 ± 1367.28^a	1192.02 ± 794.85^a	1297.17 ± 823.63^a	0.05 ± 0.01^a	63.90 ± 4.93^a	10.12 ± 1.98^a	10.19 ± 2.01^a
RFHAD	3141.42 ± 1204.31^a	0.80 ± 0.12^a	0.41 ± 0.07^a	2915.95 ± 864.14^a	956.55 ± 413.83^a	1212.89 ± 567.01^a	0.07 ± 0.03^a	63.20 ± 2.88^a	10.15 ± 1.51^a	11.20 ± 3.30^a

Different low letters in a column represent significant difference (p < .05) within each treatment.

Color is one of the important factors that affect the sensory quality of products and consumers’ purchase intention (Cao et al., 2021). As shown in Table 1, there is no significant difference among the three drying groups (p > .05). In the literature, some different results concerning color difference for RFHAD have been reported. Cao et al. (2021) found brighter and less deformation of the surface of fish fillet treated by RFHAD compared with HAD but over-heating resulted in darker of surface color of fish skin treated with RFHAD. These differences could be attributed to the different pretreatments, drying temperature, equipment, food composition and physical quality (Doymaz, 2017; Rao et al., 2014).

3.3 Rehydration rate

The rehydration ratio (R_f) is a crucial indicator for evaluating the quality of dehydrated food, which reflects its ability to restore freshness. Higher R_f after drying demonstrated better quality dried product (Dev et al., 2011). According to Figure 2, R_f of shrimps treated by HAD was lowest among three groups, which was due to the longer drying time induced irreversible cellular rupture and remarkably shrinkage. R_f of shrimps treated by RFHAD showed significantly higher than that treated by HAD (p < .05). Swelling effect due to more rapid heating rate and higher effective moisture diffusivity of shrimps treated by RFHAD could be responded for these phenomenon, which is in consistent with above results. Moreover, less surface shrinkage and loose and porous structure were found in RFHAD shrimps, in which case that higher porosity was created and higher water absorption capacity was obtained (Özbek, 2021). It also has been found that the R_f wave could reduce the membrane permeability and changed starch and pectin which reduced hydrophilicity (Liu et al., 2003). Similar results were reported for RFHA-dried carrot cubes (Gong et al., 2019), tilapia fillets (Cao et al., 2021), apple slices (Shewale et al., 2021), and kiwifruits (Zhou et al., 2018b).

Figure 2. Rehydration rate of pacific white shrimp after subjected to three different drying technologies.

3.4 Astaxanthin content

Astaxanthin is the main pigment found in crustaceans, usually present in its free form or esterified and bound to macromolecules like proteins (carotenoproteins). Breaking down this complex results in a significant color change, as carotenoids are released (Hernández Becerra et al., 2014). Figure 3 showed the astaxanthin content of shrimp after subjected to HAD, RFD and RFHAD. According to the results, the astaxanthin content of RFHA-dried shrimp was highest (68.35 ± 2.69 μg/g DM), then the RFD group (66.78 ± 1.87 μg/g DM) and HAD group (61.06 ± 1.88 μg/g DM) which was in consistent with the results of color difference mentioned above (higher a and b for RFHAD). But there is no significant difference among three groups (p > .05). In comparison with fresh shrimp, the astaxanthin content of RFHA, RF and HA dried shrimp decreased approximately 63.09%, 63.94% and 67.03%, respectively. Literature reported that the majority of astaxanthin (approximately 75%) in cooked shrimp is degraded during the process of traditional sun drying (Hernández Becerra et al., 2014). Because astaxanthin is highly susceptible to heat degradation. Temperature and time are the two main factors that affect the thermal degradation of astaxanthin. Hence, RFHAD method has a certain effect on retaining the astaxanthin content in shrimp. Several works have reported the influence of drying method on astaxanthin degradation of dried shrimp (Niamnuy et al., 2008; Hernández Becerra et al., 2014; Lin et al., 2023;). The recommended drying temperature was lower than 70°C and excessive temperature could adversely affect the astaxanthin content and color of dried shrimp (Niamnuy et al., 2008). Lin et al. (2023) reported similar findings that the astaxanthin content of shrimp dried by medium and short-wave also decreased and the best drying temperature was 60°C based on the index of astaxanthin content.

Figure 3. Astaxanthin content of pacific white shrimp (in dry basis) after subjected to three different drying technologies.

3.5 Free amino acid content

Amino acids and their derivatives are important indicators of food nutritional quality and have a significant impact on the flavor characteristics of food (Gao et al., 2021). The effect of different drying technologies on the free amino acid (FAA) content and TAV values of pacific white shrimp are shown in Tables 2 and 3. In theory, the content of FAA in the shrimp has relationship with the proteolytic intensity of myofibrillar proteins. As shown in Table 2, the major FAAs presented in shrimp induced by different drying treatments were similar, which the main FAAs was Gly, followed by Pro and Arg. Results showed that the contents of Gly, Pro and Arg for RFD and HAD groups were significantly decreased (p < .05) and there is no significant difference between them (p > .05). Furthermore, lower content of Gly, Pro and Arg of RFHAD shrimps were observed in comparison with RFD and HAD groups. Probably the reason was the more pronounced protein aggregation induced by the thermal denaturation under RFHAD treatment. Tables 2 and 3 also demonstrated that RF wave could significantly reduce the formation of bitter amino acids (p < .05) and showed no significant difference with CK group in the content of umami amino acid. Similar studies have also been reported by Wu (2022) for chicken soup treated by RF and IR. Wu (2022) published that IR and RF pretreatment significantly reduced the formation of bitter amino acids (p < .05). Meanwhile, the umami amino acid of Glu became one of the main components of free amino acids in chicken enzyme hydrolysate. This might be attributed to the thermal denaturation of proteins, which results in protein aggregation and hided some active sites of hydrolytic enzymes, leading to a significant decrease in bitter amino acid content. Additionally, thermal denaturation of proteins might also induce the exposure of new enzyme cleavage sites, resulting in an increase in the proportion of umami amino acids.

Table 2. Effect of different drying technologies on the free amino acid content of pacific white shrimp (n = 3, dry basis).

		Amino acid content/(mg·g^−1)
Number	Amino acid	CK	HAD	RFD	RFHAD
Umami amino acid (UAA)
1	Asp	0.320 ± 0.070^b	0.152 ± 0.102^a	0.157 ± 0.013^a	0.251 ± 0.011^a,b
2	Glu	3.830 ± 0.995^b	1.954 ± 0.579^a	2.180 ± 0.441^a	3.385 ± 0.322^b
Sweetness amino acid (SAA)
3	Ser	0.766 ± 0.124^a	0.432 ± 0.170^a	0.639 ± 0.077^a	0.525 ± 0.286^a
4	Gly	31.370 ± 2.689^c	17.464 ± 0.712^b	20.831 ± 1.408^b	11.162 ± 2.466^a
5	Thr*	0.821 ± 0.045^b	0.324 ± 0.088^a	0.378 ± 0.072^a	0.603 ± 0.266^a,b
6	Ala	6.028 ± 0.864^b	2.976 ± 0.148^a	3.866 ± 0.732^a	5.806 ± 0.957^b
7	Pro	20.067 ± 2.915^b	14.723 ± 1.629^a	13.539 ± 2.751^a	12.181 ± 3.051^a
Bitterness amino acid (BAA)
8	His**	0.760 ± 0.226^b	0.407 ± 0.134^a	0.437 ± 0.045^a	0.578 ± 0.119^a,b
9	Arg**	19.411 ± 1.064^c	12.132 ± 0.986^a,b	13.017 ± 1.011^b	9.763 ± 2.335^a
10	Tyr*	1.198 ± 0.116^b	0.837 ± 0.096^a	0.787 ± 0.076^a	0.696 ± 0.191^a
11	Val*	1.373 ± 0.216^a	0.921 ± 0.208^a	1.030 ± 0.116^a	1.028 ± 0.324^a
12	Met*	0.272 ± 0.024^a	0.183 ± 0.014^a	0.185 ± 0.001^a	0.327 ± 0.160^a
13	Cys-Cys	0.446 ± 0.012^b	0.147 ± 0.002^a	0.136 ± 0.002^a	0.149 ± 0.005^a
14	Ile*	0.655 ± 0.097^a	0.411 ± 0.086^a	0.494 ± 0.038^a	0.547 ± 0.214^a
15	Leu*	1.459 ± 0.206^a	1.163 ± 0.300^a	1.185 ± 0.105^a	1.067 ± 0.373^a
16	Phe*	0.570 ± 0.087^a	0.458 ± 0.040^a	0.458 ± 0.059^a	0.466 ± 0.134^a
17	Lys*	1.111 ± 0.073^b	0.680 ± 0.104^a	0.636 ± 0.100^a	1.124 ± 0.382^b
ΣUAAs		4.149 ± 1.044^b	2.106 ± 0.681^a	2.336 ± 0.445^a	3.636 ± 0.324^b
ΣSAAs		59.052 ± 3.769^c	35.919 ± 1.388^a,b	39.254 ± 4.008^b	30.278 ± 6.768^a
ΣBAAs		7.084 ± 0.811^b	4.800 ± 0.701^a	4.912 ± 0.467^a	5.404 ± 1.773^a,b
Σ flavor amino acid (UAAs+SAAs)		63.201 ± 4.797^b	38.026 ± 0.714^a	41.590 ± 4.279^a	33.914 ± 7.087^a
ΣEAAs*		6.261 ± 0.723^a	4.141 ± 0.734^a	4.367 ± 0.456^a	5.162 ± 1.844^a
ΣHEAAs**		0.760 ± 0.226^b	0.407 ± 0.134^a	0.437 ± 0.045^a	0.578 ± 0.119^a,b
ΣAAs***		90.455 ± 6.653^b	55.365 ± 1.815^a	59.957 ± 5.603^a	49.658 ± 11.143^a

*Essential amino acid, **Semiessential amino acid, ***Total amino acids.

Table 3. Effect of different drying technologies on the TAV values of pacific white shrimp (n = 3).

Taste compounds	Taste characteristics	Threshold /(mg·100 g^−1)	CK	HAD	RFD	RFHAD
Asp	Umami/+	100	0.320 ± 0.070^b	0.152 ± 0.102^a	0.157 ± 0.013^a	0.251 ± 0.011^a,b
Glu	Umami/+	30	12.766 ± 3.316^b	6.514 ± 1.932^a	7.265 ± 1.470^a	11.283 ± 1.072^b
Ser	Sweetness/+	150	0.511 ± 0.083^a	0.288 ± 0.114^a	0.426 ± 0.052^a	0.350 ± 0.191^a
Gly	Sweetness/+	130	24.130 ± 2.068^c	13.434 ± 0.548^b	16.024 ± 1.083^b	8.586 ± 1.897^a
Thr*	Sweetness/+	260	0.316 ± 0.017^b	0.125 ± 0.034^a	0.145 ± 0.028^a	0.232 ± 0.102^a,b
Ala	Sweetness/+	60	10.046 ± 1.441^b	4.960 ± 0.247^a	6.443 ± 1.221^a	9.677 ± 1.595^b
Pro	Sweetness/+	300	6.689 ± 0.972^b	4.908 ± 0.543^a	4.513 ± 0.917^a	4.060 ± 1.017^a
His**	Bitterness/−	20	1.266 ± 0.376^b	0.678 ± 0.224^a	0.729 ± 0.075^a	0.963 ± 0.198^a,b
Arg**	Bitterness/−	50	38.822 ± 2.129^c	24.264 ± 1.973^a,b	26.035 ± 2.022^b	19.525 ± 4.670^a
Tyr*	Bitterness/−	/
Val*	Bitterness/−	40	3.433 ± 0.541^a	2.302 ± 0.519^a	2.575 ± 0.291^a	2.570 ± 0.809^a
Met*	Bitterness/−	30	0.905 ± 0.079^a	0.611 ± 0.047^a	0.617 ± 0.002^a	1.090 ± 0.533^a
Cys-Cys	Bitterness/−	/
Ile*	Bitterness/−	90	0.728 ± 0.108^a	0.457 ± 0.096^a	0.549 ± 0.042^a	0.608 ± 0.237^a
Leu*	Bitterness/−	190	0.768 ± 0.109^a	0.612 ± 0.158^a	0.624 ± 0.055^a	0.561 ± 0.196^a
Phe*	Bitterness/−	90	0.633 ± 0.097^a	0.509 ± 0.044^a	0.509 ± 0.065^a	0.518 ± 0.149^a
Lys*	Bitterness/−	50	2.223 ± 0.147^b	1.361 ± 0.208^a	1.271 ± 0.199^a	2.249 ± 0.764^b

*Essential amino acid, **Semiessential amino acid.

3.6 Differential scanning calorimetry

Differential scanning calorimetry (DSC) are considered as a convenient and reliable thermo analytical method for monitoring the physico-chemical property changes of materials along with the increasing temperature (Dinani et al., 2015). And DSC could accurately reflect the thermodynamic changes during the denaturation process of protein. Figure 4 gathers the DSC thermograms of shrimp being dried by HAD, RFD and RFHAD, and endothermic characteristics (T₀, T_p, T_c and T_c-T₀) and gelatinization enthalpy (ΔH) were determined to describe their thermal properties. According to Figure 4, one apparent major endothermic peak was observed for dried shrimp. And it seems that drying treatment could decrease the size of endothermic peak based on the transition temperature range (T_c–T₀), following the order of CK > RFHAD > HAD > RFD. It has been reported that the presence of intermediate chemical forms of proteins could explain the broadening of peaks (Xue et al., 1999). These intermediate forms referred to different chemical structures formed during the process of protein changes in DSC process. The existence of these intermediate forms introduced more variations and complexity, leading to broader peaks. Therefore, the increase in peak width could be attributed to protein denaturation and reaction processes (Chandrapala et al., 2011; Dinani et al., 2015). This also implied that during the RF drying process, there was a lower quantity of denatured proteins, resulting in narrower peaks.

Figure 4. DSC thermograms of dried shrimp with different drying methods.

As shown in Table 4, T₀ and T_p response was significantly decreased under RF wave (p < .05), and there is no significant difference between RFD and RFHAD groups (p > .05). For T_c response, the RF dried shrimp showed lowest value (141.76 ± 0.29°C), then following by RFHAD group (146.24 ± 1.85°C), HAD group (152.86 ± 1.36°C) and control group (170.74 ± 1.89°C), which is significantly different between three groups (p < .05). Ling et al. (2019) published that the T_p commonly indicated the thermal stability of protein. Hence, the thermal stability of protein for shrimp was significantly affected by RFHAD (p < .05).

Table 4. Effect of different drying technologies on the thermal properties of pacific white shrimp.

Group	T0 (°C)	Tp (°C)	Tc (°C)	Tc–T0 (°C)	ΔH (J/g)
CK	124.48 ± 0.87^c	151.13 ± 1.52^c	170.74 ± 1.89^d	46.27 ± 2.76^b	1289.00 ± 36.59^c
HAD	115.67 ± 1.40^b	134.03 ± 1.21^b	152.86 ± 1.36^c	37.19 ± 0.93^a	479.67 ± 72.53^a,b
RFD	105.78 ± 1.27^a	127.33 ± 1.19^a	141.76 ± 0.29^a	35.98 ± 1.07^a	498.67 ± 11.93^b
RFHAD	107.04 ± 0.54^a	127.15 ± 0.53^a	146.24 ± 1.85^b	39.20 ± 2.40^a	408.67 ± 7.51^a

T₀, T_p, T_c and ΔH represented the onset temperature, peak temperature, end completion temperature and enthalpy, respectively. Different low letters in a column represent significant difference (p < .05) within each treatment.

Table 4 showed a comparison of the ΔH of the shrimp dried by HA, RF and RFHA treatments. The results demonstrated that ΔH was significantly affected by the drying methods (p < .05). Thereinto, the ΔH of shrimp dried by RFHA decreased to approximately 68.30% in comparison with control shrimp, and then following by 62.79% for HAD group and 61.31% for RF group. Hence, a significant decrease of the ΔH values for RFHAD groups could be thanks to the synergistic effect of RF wave and temperature. Furthermore, the lower ΔH of shrimp dried by RFHA could be correlated with the increase in redness and decrease in lightness (shown in Table 1), which is in consistent with the results of Dinani et al. (2015). The probable explanations for the decrease of ΔH values for RFHAD group could be given in two aspects. The first aspect was the phenomenon of protein denaturation under the RF wave. The denaturation could induce the changes in the secondary (α-helix and β-sheet formations), tertiary (alterations in hydrogen bonds) or quaternary (assembly of subunits) structure of proteins. As the degree of protein denaturation increased, the amount of native-like proteins decreased. DSC could reflect the thermal denaturation of native proteins. Therefore, the higher the degree of protein denaturation, the lower the ΔH value in the DSC trace (Kazemi et al., 2011). The second aspect was due to the exothermic reactions caused by protein denaturation during the DSC thermal analysis process. The ΔH value was a comprehensive value that included exothermic reactions resulting from protein aggregation reactions, as well as other possible endothermic reactions, such as hydrogen bond disruption. Therefore, an increased degree of protein denaturation and aggregation might lead to a decrease in the denaturation enthalpy (ΔH value) of the shrimp (Ibanoglu & Ercelebi, 2007). Overall, one or a combination of the aggregation reactions resulting from protein denaturation, as well as other endothermic and exothermic reactions, could affect the enthalpy value of shrimp dried by RF in comparison with HA.

3.7 Fourier-transform infrared spectroscopy

FTIR is a commonly used method that provides information about protein conformation. The amide I band, located at 1600–1700 cm⁻¹, reflects the protein’s secondary structure, with characteristic peaks induced by N-H stretching vibrations. Spectral wavelengths ranging from 1600 to 1640 cm⁻¹, 1640 to 1650 cm⁻¹, 1650 to 1660 cm⁻¹, 1660 to 1680 cm⁻¹ and 1680 to 1700 cm⁻¹ are displayed as the amide I region, corresponding to β-sheet, random coil, α-helix, β-turn and β-reverse fold, respectively. Figure 5 depicted a comparison of the relative content of protein secondary structure of shrimp dried by HA, RF and RFHA. And the relative contents of secondary structure are summarized in Table 5. Results demonstrated that more than half of secondary structures of dried shrimps was β-sheet and β-turn. For HAD shrimp, there was a significant decrease in β-sheet content and increase in β-reverse fold (p < .05). RF drying changed the secondary structures of shrimps with significant decrease in β-sheet and α-helix contents and increase in β-reverse fold (p < .05). No significant changes were found for the random coil and β-turn contents of secondary structure (p > .05). Literature have been reported that the secondary structure is related with the various types of hydrogen bonds (Ling et al., 2019). Therefore, RF drying showed a disruption on the hydrogen bonds to some extent, leading to the conversion of α-helice to β-reverse fold in the secondary structure. It has been published that RF wave increased the random coil and decreased other secondary structures of protein isolated from rice bran and thermally sterilized SPI solution (Ling et al., 2019; Ren et al., 2018). However, Wu (2022) found that RF treatment could transform a higher proportion of α-helice, β-turn, and random coil structures in proteins into β-sheet of concentrated chicken soup. Nie et al. (2022) published that the vacuum freeze-drying increased the percentage of β-sheet and random coil, and decreased other three secondary structure contents of extracted gelatins of tilapia. Therefore, difference in drying method, protein type and protein state could lead to different results in the secondary structures.

Figure 5. Relative content of protein secondary structure of dried shrimp by different drying technologies.

Table 5. Effect of different drying technologies on the secondary structures of pacific white shrimp.

Group	β-sheet (%)	Random coil (%)	α-helix (%)	β-turn (%)	β-reverse fold (%)
CK	31.69 ± 1.12^c	12.88 ± 0.26^a	15.52 ± 0.66^b	26.76 ± 0.24^a	13.15 ± 0.83^a
HAD	28.23 ± 0.55^b	12.63 ± 1.43^a	15.45 ± 0.43^b	25.29 ± 3.80^a	19.71 ± 1.99^b
RFD	25.22 ± 2.16^a,b	12.98 ± 0.39^a	13.06 ± 0.22^a	26.91 ± 0.57^a	21.84 ± 1.74^b
RFHAD	24.67 ± 0.38^a	12.58 ± 0.55^a	13.46 ± 0.06^a	28.07 ± 0.58^a	21.22 ± 0.40^b

3.8 Surface morphology

Figure 6 showed the surface morphology of shrimp dried by HAD, RFD and RFHAD. As shown in Figure 6(a), untreated control was mainly composed of linear and relatively smooth fiber with loose structure. RF heating showed no significant changes in voids between the fibers and the surface roughness of the fibers. However, for HAD group, the influence of heating periods can be found easily by checking the wrinkle degree of the fiber. The damage level of RFD and RFHAD shrimps was less than that of the HAD, which indicating that muscle fibers were more severely damaged by heating. This phenomenon may be attributed to the denaturation and aggregation of muscle protein triggered by over long time thermal treatment. Thus, our results indicated that RF wave showed less influence in the microstructure of shrimp.

Figure 6. Effect of different dng technologies on surface morphology of shrimp: (a) Control, (b) HAD, (c) RFD and (d) RFHAD.

4. Conclusions

Comparing the drying characteristics, product quality and composition of Pacific white shrimp (Litopenaeus vannamei) in HAD, RFD and RFHAD, RFHAD is found to be an effective combined heating technology with higher efficiency and produces high quality dried shrimp product. RFHAD showed faster drying rate and also ensured better product quality in terms of texture property, astaxanthin content, color quality, rehydration rate and microstructure when compared to RFD and HAD. Furthermore, RF wave could significantly reduce the formation of bitter amino acids (p < .05) and showed no significant difference with CK group in the content of umami amino acid. However, a significant decrease of the T₀, T_p and ΔH values for RFHAD groups, which showed lower thermal stability, could be thanks to the synergistic effect of RF wave and temperature. Moreover, RF drying showed a disruption on the hydrogen bonds to some extent, leading to the conversion of α-helice to β-reverse fold in the secondary structure. Further studies to improve thermal stability of shrimp using RFHAD are in progress. This study indicated the RFHAD was an alternative method for drying of shrimp with higher drying efficient and high-quality characteristic.

CRediT authorship contribution statement

Xiaoting Xuan: Investigation, Analysis, Writing – original draft, Writing – review & editing. Ning Yu: Formal analysis, Writing-review & editing. Tian Ding: Methodology, Writing-review & editing. Hai-Tao Shang: Validation, Writing-review & editing. Yan Cui: Validation, Writing-review & editing. Xudong Lin: Writing-review & editing. Lin Zhu: Conceptualization, writing-review & editing.

Acknowledgement

The author acknowledges Science and Technology Projects of Ningbo (2022S153), Special Program for the Construction of Ningbo City’s Enterprise Innovation Consortium (2022H006) for their financial support to this research.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The work was supported by the Special Program for the Construction of Ningbo City’s Enterprise Innovation Consortium [2022H006]; Science and Technology Projects of Ningbo [2022S153].