Nutritional quality variation in dried Pacific Oyster (Crassostrea gigas) using hybrid-pump dryer under different heating treatment

Abstract

This study explored the potential of using hybrid pump dryer (HPD) to utilize overproduction in aquaculture of oysters, especially during winter. HPD-dried oysters maybe used as amendments for kimchi, a traditional Korean side dish, for possible nutrient source and flavor enhancer. Oysters were subjected to different heating treatments and evaluated for proximate composition, quality characteristics, and antioxidant activities. Lower lipid and higher glycogen content were observed in HPD-dried oysters processed than the samples dried with hot air (HAD). HPD-dried oysters also exhibited lesser browning activity, better surface color, and higher antioxidant activities. Ash, protein, and water activity were slightly affected by heating treatment. VBN and TBARS were found to be higher in HAD-dried oysters, indicating faster spoilage. Applying heat pattern in drying resulted to improved quality characteristics and antioxidant activities and slower degradation of dried oyster products compared to their single-temperature-drying counterparts, especially those dried at high temperatures.

Introduction

Awareness and preference for a healthy lifestyle can lead to changes in consumer eating habits and tendency to choose food with high level of protein and the so-called “good fats” (Kang and Kim, 2013). Consumers seek more and more wholesome, edible foods which are easy to cook and eat, additive-free with long shelf-life, and well-maintained freshness, sensory, and nutritional properties. However, production of this type of food entails modifying formulations and processing technologies, which may increase the risks (e.g., quality deviation from specified standards), connected to its consumption (Oliveira et al., 2015). Such changes in food culture can influence consumption and the distribution structure of food (including marine products) as well as the development of processing technologies.

Drying is one of the oldest and most fundamental processing techniques in food industry. Advances in drying technologies are driven by the demand for enhanced and fortified products as improved quality increases the market value of goods (Chou and Chua, 2001; Deng et al., 2015). One of these innovations in drying technologies is ultrasound-assisted drying which has been proven to lessen drying time with elevated drying temperature and preserve nutrients, flavor, and color of biological materials like fruits and vegetables (Aydar, 2020; Horuz et al., 2017; Kowalski and Mierzwa, 2015).

Hot air drying (HAD) and freeze-drying (FD) are few of the major preservation techniques used for perishable products like seafood. HAD can be cost efficient but is prone to considerable quality losses. On the other hand, FD retains nutrient, color, flavor, and texture but FD’s high operating cost makes it impractical for use in medium-to-large-scale production (Deng et al., 2015). Because of these limitations, new hybrid drying technologies, with minimum environmental effects, are being developed to provide food with better quality (Chou and Chua, 2001). Hybrid pump dryers (HPDs) consist of a heat pump integrated to a vacuum dryer. In drying mechanics, an efficient heating system should reduce energy consumption. Vacuum heat pumps are environment-friendly and operate on relatively small amount of energy (Strommen et al., 2002). HPD utilizes low temperature energy sources that are usually present in atmospheric air and are called “ambient energies”. This “ambient energy” source is then passed through the evaporator (usually as moist) and is rapidly cooled, resulting in water to be condensed out to produce heat for drying. Consequently, the dry, heated air is continuously supplied to the product being dried while moisture is collected and recirculated throughout the HPD system (Perera and Shafiur Rahman, 1997). Recycling the recovered latent heat and sensible heat lead to the following: low energy consumption, high performance coefficient of heat pump, and greater thermal efficiency of vacuum dryer, which make HPD advantageous over traditional drying methods. As a result, food quality can be retained or improved, especially with heat-sensitive products due to HPD’s operability at low drying temperatures and independence from outside ambient conditions (Chou and Chua, 2001; Perera and Shafiur Rahman, 1997). HPD technology has been widely used in the aquaculture industry for many years, but its application in drying of oyster and other shellfish is less known.

Oysters are the most harvested bivalve worldwide. In South Korea, it is the most cultivated shellfish, with its production steadily progressing through the decades at 32,289–32,780 t according to Korea Maritime Institute (KMI). In 2016, around 80% (26,198 t) of marketed oysters have been sold raw, even in their non-spawning period. Oysters are considered to have low meat quality during non-spawning season. South Korea exports oysters in larger volumes as raw (20,396 t), canned (7,016 t), dried (506 t), and salted (349 t) goods during spawning season (KMI, 2019). Oysters are a good source of protein and glycogen, have low fat content, and are rich in minerals and omega-3 fatty acids, as well as, essential amino acids, and demonstrate antioxidant properties (Cai et al., 2019; Qian et al., 2008; Umayaparvathi et al., 2014). Despite this, ventures in the market of oysters have not been much, which are mostly sold in its raw form. As of now, very few or no studies have been reported on HPD-drying of oysters.

In this study, the effects of HPD on physical quality and biochemical composition of oysters and its potential in oyster food processing industry (e.g., as an additive in kimchi-making) were investigated. Strommen et al. (2002) noted that drying temperature and time are the main factors affecting quality and nutritional components of dried products, especially in fish. And as the same principle can be applied to other seafood and shellfish, the oyster samples were subjected to different temperatures and durations using HPD as described in the methods section.

Materials and methods

Materials and reagents

Commercial size (shell length: 12–14 cm) Pacific oysters (Crassostrea gigas) were collected from an aquaculture farm in Tongyeong, South Korea and immediately kept at − 20 °C while being transported. The oyster samples were pre-washed, manually shucked, and drained of excess water upon arrival at laboratory. The shucked oysters were divided into five groups, spread out in trays, layered inside the chamber of hybrid pump dryer and subjected to different drying treatments (see Drying of oyster meat in methods section). Dried oyster samples were stored in vacuum containers at − 20 °C prior to further analyses.

Alcalase (EC. 3.4.21.62., specific activity: 2200 U/mg) was purchased from Novozymes China Biotechnology Co., Ltd. (Tianjin, China). Pepsin (EC. 3.4.23.1, specific activity: 2400 U/mg) and pancreatin (P7545, 8 × USP), ABTS, Trolox, FL (fluorescein disodium), AAPH (2,2`-azobis(2-methylpropionamidine) dihydrochloride), KOH (potassium hydroxide), and 1,10-phenanthroline were all supplied by Sigma-Aldrich (St. Louis, MO, USA). Both EV (estradiol valerate) and PROG (progesterone) injections were procured from Tianjin Anoric Bio-Technology Co., Ltd. (Tianjin, China). Sildenafil citrate was bought from a local pharmacy. Serum and enzymatic assay kits were purchased from Nanjing Jiancheng Bioengineering Institute (Nanjing, China). All other reagents used were HPLC-grade or analytical grade.

Methods

Drying of oyster meat

The oyster samples were dried in a hybrid pump dryer designed by A1 Engineering Co., Ltd. (Suncheon, South Korea) composed of insulated cylindrical drying chamber made of steel sheet (2 m diameter × 3.2 m length), 7.5 kW heat pump to supply heat in chamber, and liquid ring vacuum pump to control drying pressure in the chamber (see Fig. 1). Experiments were carried out at − 2,000 mmAq vacuum pressure with different heating treatment. The first three groups of oyster samples were subjected to constant heating at different temperatures (45 °C, 55 °C, 60 °C) for 24 h. The remaining groups were subjected to 2-step heating process; wherein, the fourth group was treated by heating at 55 °C for 10 h followed by heating at 45 °C for 14 h while the last group was treated by heating at 65 °C for 7 h followed by heating at 45 °C for 17 h. All temperature settings were programmed with ± 1 °C adjustment. Air velocity at dryer chamber was maintained at 1.2 m/s by controlling the blower speed. For comparison, separate group of oyster samples were dried using hot air dryer at 70 °C for 24 h.

Fig. 1.

Schematic design of hybrid pump dryer used in drying of oyster meat at different heating treatment

Proximate composition and water activity (A_w)

For proximate analyses, moisture content was determined with moisture analyzer (WBA-110 M, Daihan Scientific, Seoul, South Korea). Lipid content was extracted using Bligh and Dyer method (1959). Crude protein content was determined using semi-micro Kjeldahl method described in AOAC method 920.153 while ash content was evaluated according to AOAC method 938.08 (AOAC, 2000). A_w was measured at 25 °C using water activity meter (Aqua Lab Model 3 TE Series, Decagon Pullman, WA, USA).

Color measurement

A colorimeter (NE 4000, Nippon Denshoku industries Co., Ltd, Tokyo, Japan) was used to measure color of oyster samples, before and after drying, as described by Tian et al. (2012). For each oyster, the lightness (L^*), redness (a^*) and yellowness (b^*) were evaluated for each of the 6 sides; wherein, L^* measured the whiteness value, ranging from black (0) to white (100), a^* measured red when positive and green when negative, and b^* measured yellow when positive and blue when negative. Values of a^* and b^* ranged between − 60 and + 60. Whiteness index, which is an important quality characteristic for consumer preference in marketing and production of oyster meat (Lee et al., 2012), was measured using the following formula as described by Tian et al. (2012):

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

Glycogen content

The glycogen assay was carried out using an iodine glycogen method (Li et al., 2007). Ground, freeze-dried samples were dissolved in 60 volumes of 30% KOH and heated at 100 °C for 30 min. Next, 0.2 mL of hydrolyzed sample was transferred into a micro-cuvette and 1.3 mL iodine reagent (1.92 mL of 0.68 M I₂/KI in 500 mL saturated CaCl₂ solution) was added. The mixture was incubated at 20 °C for 40 min and absorbance was measured at 460 nm UV–Vis spectrophotometer (X-MA 6100PC, Human Corporation, Seoul, South Korea). Glycogen content was computed from extrapolation of calibration curve using standard solutions of glycogen (Sigma-Aldrich, St. Louis, USA).

Non-enzymatic browning

Non-enzymatic browning was assessed as described by Negi and Roy (2001). Five gram of dehydrated oyster sample was soaked in 100 mL of 60% ethanol for 12 h and filtered. Absorbance of the filtrates was recorded at 420 nm (SpectraMax M2, Molecular Devices, CA, USA) using 60% ethanol as blank.

TVBN (Total volatile basic nitrogen)

TVBN was determined by Conway micro-diffusion analysis after extraction with TCA (trichloroacetic acid) as described by Hasegawa (1987) with slight modification. Oyster samples (2 g) were homogenized with 8 mL of 5% TCA at 12,000 rpm for 2 min, allowed to stand at ambient temperature for 30 min and centrifuged (3000 rpm, 10 min). The supernatant was decanted and added with 5% TCA up to 10 mL and used as sample solution. A volume (1 mL) of this solution was pipeted into the outer ring of Conway unit. Next, 1 ml of saturated potassium carbonate (K₂CO₃) solution was pipeted into the outer ring while 1 mL of 1% boric acid was pipeted into inner ring. The Conway unit was closed with and gently swirled to mix the solutions in the outer ring and incubated at 37 °C for 1 h. After incubation, the inner ring solution was then titrated with 0.02 N HCl using mixed indicator solution (bromocresol green and methyl red in ethanol) until solution changed from green color to pink. Blank titration was also done using 5% TCA. TVBN was computed using the following formula:

$$TVBN\left(\frac{\text{mg}}{100\text{g}}\right)=\left(V_S-V_B\right)\times 0.28\times \left\{\frac{\left[\left(W_S\times \frac{M}{100}\right)+10\right]\times 100}{W_S}\right\}$$

where V_S = Volume of 0.02 N HCl used in titration of sample in ml. V_S = Volume of 0.02 N HCl used in titration of blank in ml. W_S = Weight of sample in g. M = moisture content of sample in %.

TBARS (Thiobarbituric acid reactive substances)

TBARS values in dried oysters were quantified using a food TBARS Assay kit (Oxford Biomedical Research, Rochester Hills, MI, USA). Samples (5 g) were homogenized in 5 mL deionized water (DI) in a 15 mL centrifuge tube to get a smooth suspension which was further diluted up to 10 mL with DI. Next, 1-mL aliquots were added to two separate screw-cap tubes, labeled A and B, and 1 mL indicator solution was added to tube A as test sample while 1 mL blank solution was added to tube B as blank. The solutions were mixed in vortex mixer (~ 1 min) and incubated at room temperature for 60 min. Each solution was centrifuged (8,000 rpm, 5 min) and the aqueous layer was carefully siphoned without disturbing the solid upper layer. Absorbance of the supernatants were measured at 532 nm (SpectraMax M2, Molecular Devices, CA, USA), the absorbance of blank (tube B) was subtracted from test sample (tube A) readings to obtain the TBA-MDA (thiobarbituric acid-malondialdehyde) adduct absorbance in each sample. TBARS values were computed from the extrapolated standard curve and expressed as mg MDA/100 g sample.

ABTS radical scavenging activity

The ABTS assay was done following the slightly modified method of Re et al. (1999). ABTS stock solution (7 mM in DI) was mixed and reacted with 2.45 mM potassium persulfate in the dark at room temperature for 12 − 16 h. The mixture was then diluted with phosphate buffer solution (PBS, 50 mM, pH 7.4) to get 0.70 ± 0.02 absorbance at 734 nm. Sample solutions were also diluted accordingly. Next, 150 μL of diluted ABTS solution was mixed with 50 μl sample in a 96-well plate and incubated in the dark for 60 min at 30 °C. Absorbance was measured at 734 nm (Spectromax M2, Molecular Devices, CA, USA). PBS (50 mM, pH 7.4) was used as blank while a calibration curve was plotted from trolox standard solutions (20 to 180 μM). The ABTS radical scavenging activity was expressed as trolox equivalents (TE in μmol/μmol sample).

GST (Glutathione-S-transferase) activity

GST activity was evaluated according to Habig and Jakoby (1981) with some modifications. Sample solutions (10 μL) were mixed with 180 μL of 1 mM CNDB (1-chloro-2,4-dinitrobenzene) in PBS containing 4% (v/v) ethanol and 2 μL of 100 mM reduced GSH (L-glutathione). The CDNB-GSH conjugate formed was determined by measuring the absorbance at 340 nm (Spectromax M2, Molecular Devices, CA, USA) for 10 min with minute-intervals. GST activity was expressed in μmol CDNB-GSH/mg protein.

SOD (Superoxide dismutase) assay

SOD activity was done using competitive inhibition assay method by Veskoukis et al. (2016) with minor modifications. Sample homogenates (10 μL) were mixed with 740 μL phosphate buffer (67 mM, pH 7.4), 100 μL 0.5 mM xanthine and 100 μL 0.1 mM cytochrome c solution in separate microcentrifuge tubes using vortex mixer. Next, 190-μL aliquot of each mixture was placed in 96-well plate and 10 μL bovine xanthine oxidase (diluted 1/50 from 25-unit stock) was added. Absorbance was measured at 550 nm for 1 min. SOD activity was computed based on the molar extinction coefficient of cytochrome c (19.2 L mol⁻¹ cm⁻¹).

Statistical analysis

Data were analyzed using one-way analysis of variance (ANOVA) with Statistical Packages for the Social Sciences (IBM SPSS Statistics 23.0) software (SPSS Inc., USA). Significant differences (p < 0.05) among means (n = 3) were identified using Duncan’s multiple range tests.

Results and discussion

Effect of drying treatment on proximate composition of oysters

Proximate compositions of dried oyster meat are shown in Table 1. Only moisture was greatly affected by different heating treatments using HPD and HAD. Oyster samples subjected to constant heat (at 45 °C, 55 °C, and 60 °C) with HPD have lower moisture content (21.6%, 23.1%, and 22.6%, respectively) than the samples dried at 70 °C using HAD (26.6%). Meanwhile, samples treated at 55 °C-10 h/45 °C-14 h and 65 °C-7 h/45 °C-17 h heating patterns with HPD were found to have highest moisture content (28.4% and 27.1%, accordingly). Relative variations were observed in other dried oyster components, such as: glycogen (17.8–19.5%), crude protein (30.1–32.7%), lipid (12.0–14.7%), and ash (3.9–4.6%), regardless of the heat treatment applied. Kim et al. (2001) reported lower values (crude protein: 22.1%, lipid: 3.4%, and ash: 3.9%) for the same species of oysters (dried in a convection dryer at 45 °C prior to fermentation) used in their study.

Table 1.

Proximate composition (%) of dried oyster using hybrid pump dryer (HPD) at different conditions and hot air dryer (HAD). Data presented as mean ± SD (n = 3)

Dryer used	Heat treatment	Moisture	Crude protein	Total lipid	Ash	Glycogen
HPD-No heat pattern	45 °C(24 h)	21.6 ± 0.3a	32.7 ± 0.9b	12.9 ± 1.5a	3.9 ± 0.3a	17.8 ± 1.3ab
	55 °C(24 h)	23.1 ± 0.7b	30.1 ± 0.7a	12.2 ± 1.8a	4.6 ± 0.3b	16.4 ± 2.5ab
	60 °C(24 h)	22.6 ± 0.5b	30.5 ± 1.1ab	12.0 ± 1.4a	4.6 ± 0.2b	16.2 ± 2.0ab
HPD-Heat pattern	55 °C(10 h) + 45 °C(14 h)	28.4 ± 0.2d	31.8 ± 1.2ab	13.3 ± 1.7a	4.1 ± 0.2ab	19.5 ± 2.8b
	65 °C(7 h) + 45 °C(17 h)	27.1 ± 0.2c	32.7 ± 1.4b	13.6 ± 1.7a	4.1 ± 0.4ab	18.9 ± 1.8ab
HAD	70 °C − 24 h	26.6 ± 0.4c	32.2 ± 1.5ab	14.7 ± 1.9a	4.3 ± 0.3ab	15.2 ± 1.5a

Similar letters within a column are not significantly different as indicated by Duncan’s multiple range test (p < 0.05)

Glycogen content in dried oysters decreased as applied drying temperature was increased. Additionally, higher glycogen contents were found in oyster samples dried using HPD with two-step heat patterns (19.5% and 18.9% for 55 °C-10 h/45 °C-14 h and 65 °C-7 h/45 °C-17 h, respectively) than the samples dried constantly at single-point temperatures, as shown in Table 1. Meanwhile, HPD-dried oysters contained lower lipid contents than the HAD-dried samples. However, oysters dried in two-step heat patterns contained higher lipid than the oysters dried at single-point temperatures. No distinct trends were observed in protein and ash contents of dried oyster samples although higher crude protein and lower ash contents were found in oysters dried using HPD at 45 °C and dried with employed two-step heat patterns (55 °C-10 h/45 °C-14 and 65 °C-7 h/45 °C-17 h) than the samples dried constantly at 55 °C and 60 °C. These changes in proximate compositions of dried oysters could be attributed to the breaking of non-covalent chemical bonds caused by processing (in this study, drying), leading to changes in water molecules, proteins, polysaccharides, and lipids. Specifically, this breaking of non-covalent bonds leads to water loss during drying, protein denaturation and, consequently, to unfolding of protein chains and exposing of free carboxylic and amino groups (Deng et al., 2015).

Overall, employing two-step heat patterns seemed to maintain, if not improve, the proximate composition of dried oyster products. These observations are confirmed by Chua et al. (2000) whose study found out that applying two-stage heating pattern in HPD drying can improve the drying quality of a wide variety of agricultural products and increase drying efficiency up to 87% while reducing color change in the final product.

Color changes in oyster meat dried under different heating treatment

Changes in muscle color are related to browning reactions, degree of structural changes in protein and their derivatives, as well as myoglobin concentrations (Rahman, 2006). Total color difference (ΔE) quantifies the overall deviation of a given sample’s color when compared to a reference sample. This means that the smaller ΔE value, the closer the samples’ color to each other. In this study, 100 was used as reference value for whiteness index, which is an essential quality index for oyster meat (Lee et al., 2012). Color measurements of dried oyster samples are presented in Table 2. Samples dried using HPD at 45 °C, 55 °C, and 60 °C were found to have lightness (L^*) at 27.3, 36.1, and 38.8; redness (a^*) at 2.4, 2.9, and 3.2; and yellowness (b^*) at 8.6, 13.6, and 13.9, respectively. ΔE values decreased from 70.7 to 60.4 as drying temperature increased. Meanwhile, applying heat pattern slightly improved the color quality of dried oyster meat, as demonstrated by higher L* (37.2 and 39.5, for 55 °C-10 h/45 °C-14 h and 65 °C-7 h/45 °C-17 h) and lower ΔE values (63.2 and 60.9, for 55 °C-10 h/45 °C-14 h and 65 °C-7 h/45 °C-17 h) observed in the dried samples compared to the oysters constantly heated in single temperatures using HPD and HAD. The low redness (a, wherein positive reading = red and negative reading = green) and yellowness (b, wherein positive reading = yellow and negative reading = blue) values found could be attributed to the planktons consumed by oysters (Lee et al., 2012), which gave them the slightly orange-brown color when dried.

Table 2.

Surface color of dried oyster by hybrid pump dryer (HPD) at different conditions and hot air dryer (HAD). Data shown as mean ± SD (n = 6)

Dryer used	Heating treatment	Surface color3
		L	a	b	ΔE
HPD-No heat pattern	45 °C(24 h)	27.3 ± 1.9a	2.4 ± 0.5ab	8.6 ± 1.4a	70.7 ± 1.8c
	55 °C(24 h)	36.1 ± 1.8b	2.9 ± 0.4bc	13.6 ± 1.7bc	62.9 ± 2.3ab
	60 °C(24 h)	38.8 ± 2.5b	3.2 ± 0.5c	13.9 ± 1.3c	60.4 ± 4.1a
HPD-Heat pattern	55 °C(10 h) + 45 °C(14 h)	37.2 ± 2.2b	1.9 ± 0.2a	14.2 ± 1.8c	63.2 ± 3.5ab
	65 °C(7 h) + 45 °C(17 h)	39.5 ± 1.4b	3.4 ± 0.1c	17.2 ± 1.3d	60.9 ± 1.9a
HAD	70 °C(24 h)	38.7 ± 2.9b	2.2 ± 0.3a	10.9 ± 1.7ab	66.7 ± 3.7bc

Similar letters within a column are not significantly different as indicated by Duncan’s multiple range test (p < 0.05)

L, lightness; a, red color; b, yellow color; ΔE, relative color difference index

Changes of A_w, browning reaction, and VBN in oysters dried under different heating treatment

Table 3 shows A_w, browning reaction, and VBN of HPD-dried and HAD-dried oyster samples. The water activity (A_w) values in dried oysters in this study were all below the maximum value (0.85) set by USFDA for successful inhibition of growth of pathogenic microorganisms (FDA, 2001). The determined A_w values (0.65–0.81) from dried oyster samples and did not vary greatly, except for the lowest A_w observed (0.65) in HPD-dried samples at 60 °C. Higher A_w (0.94) was reported by Kim et al. (2001) for the same species of dried oysters (using convection dryer at 45 °C prior to fermentation). Moisture content and water activity influence other organoleptic and physico-chemical properties of the dried product (Deng et al, 2015; Rahman, 2006).

Table 3.

Water activity (A_w), browning reaction, and volatile basic nitrogen (VBN) of dried oyster by hybrid pump dryer (HPD) at different conditions and hot air dryer (HAD). Data shown as mean ± SD (n = 3)

Dryer used	Heating treatment	Aw	Browning reaction	VBN (mg/100 g)
HPD-No heat pattern	45 °C(24 h)	0.81 ± 0.05b	0.13 ± 0.02a	22.1 ± 3.6a
	55 °C(24 h)	0.73 ± 0.03b	0.17 ± 0.01b	25.8 ± 1.4ab
	60 °C(24 h)	0.65 ± 0.02a	0.22 ± 0.02c	30.1 ± 2.9bc
HPD-Heat pattern	55 °C(10 h) + 45 °C(14 h)	0.80 ± 0.04b	0.15 ± 0.02ab	21.4 ± 1.8a
	65 °C(7 h) + 45 °C(17 h)	0.74 ± 0.05b	0.18 ± 0.03b	28.5 ± 2.1bc
HAD	70 °C(24 h)	0.78 ± 0.06b	0.43 ± 0.02d	32.7 ± 2.6c

Similar letters within a column are not significantly different as indicated by Duncan’s multiple range test (p < 0.05)

Meanwhile, browning activity in dried oysters increased with higher drying temperature (Table 3). Significantly lower browning activities (0.13–0.22) were noted in HPD-dried oysters than HAD-dried samples (0.43). Browning reactions occur at 80–90 °C and increase with time and temperature (Rahman, 2006). In addition, oysters dried with two-step heat patterns showed slightly lesser browning than their single-temperature counterparts. These results demonstrated that HPD drying effectively inhibited browning activity compared to HAD drying and employing a two-step heat pattern slightly improved the resistance of dried oysters against browning reaction. Therefore, using HPD with two-step heat patterns could also prevent color and textural modifications, off-flavors, and possible decrease in nutritional value and solubility which are related to browning reactions (Rahman, 2006).

VBN can be produced, either by reducing bacteria or through endogenous enzymatic activity, as protein is broken down into smaller molecules. VBN is also used as spoilage index for fish and seafood products. VBN values determined in dried oyster samples were all below 35 mg% (Table 3), which is the maximum limit for VBN content in fish and seafood products to be considered safe for consumption (EC, 1995). VBN content in HPD-dried oysters using two-step heat pattern were lower (21.4 mg% and 28.5 mg% for 55 °C-10 h/45 °C-14 h and 65 °C-7 h/45 °C-17 h) than the samples dried without heating pattern (22.1 mg%, 25.8 mg%, and 30.1 mg% for 45 °C, 55 °C, and 60 °C, respectively) and HAD-dried oysters (32.7 mg%). Kim et al. (2001) evaluated VBN levels during fermentation process of oyster-salted fish mixture and found an increasing trend with longer fermentation time. VBN content in oyster-salted fish mixture ranged from 14.8 to 34.8 mg until 40 days of ripening while the control sample (without added oyster extract) showed a rapid increase of 19.7–68.9 mg from the first 5 days of aging to the next 20 days, and then a gradual increase up to 40 days of fermentation. These results showed that HPD drying, especially with two-step heat pattern enhanced the final quality of dried oysters by decreasing water and browning activities, and reducing production of VBN, which are all associated to quality changes and degradation.

ABTS radical scavenging activity and lipid oxidation in dried oysters

Drying treatment and temperature affected ABTS radical scavenging activity (RSA) of dried oysters, as shown in Table 4. Antioxidant activity of dried oyster samples against ABTS radicals decreased as drying temperature increased. Moreover, samples dried using HPD exhibited higher activity against ABTS radicals (38.7–45.9 μmol TE/μmol sample, for constantly heated oysters, and 41.5–45.3 μmol TE/μmol sample, for oysters dried with heating pattern) than the samples dried using HAD (37.2 μmol TE/μmol sample). Interestingly, applying two-step heat pattern enhanced ABTS RSA of dried oysters compared to their constantly dried counterparts. ABTS RSA can determine both hydrophilic (water-soluble) and lipophilic (lipid-soluble) antioxidants which can help in delaying the auto-oxidation and degradation of lipids and polyunsaturated fatty acids (Re et al., 1999). ABTS RSA results corresponded to the TBARS values detected in the oyster samples subjected to different drying treatments (Table 4). TBARS shows the extent of lipid oxidation by measuring the formation of secondary oxidants, which are responsible for the unpleasant odor and off-flavors related to spoilage, rancidity, and loss of fat-soluble vitamins and pigments. TBARS contents increase with prolonged storage period even at 4 °C, if storage conditions are not adequate (Choi and Lee, 2016; Rahman, 2006). The TBARS values in dried oyster samples increased with higher drying temperature. HPD-dried oysters were found to have lower TBARS content (0.71–1.19 mg MDA/kg) than the HAD-dried samples (1.25 mg MDA/kg). Moreover, oysters dried constantly in one temperature have lower TBARS values (0.71–1.05 mg MDA/kg) compared to the samples dried with two-step heating pattern (0.78–1.19 mg MDA/kg). These results could have resulted from the high drying temperature combined with higher moisture and lipid content found in oysters dried using HAD and using HPD with two-step heating pattern. Lee et al. (2012) found that lipids from C. gigas were rich in eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and other polyunsaturated fatty acids (PUFAs), which make them prone to oxidation (Rahman, 2006). Additionally, the higher ABTS RSA observed in HPD-dried oysters may have prevented lipid degradation and oxidation during drying, resulting to lower TBARS values.

Table 4.

ABTS and TBARS of dried oyster by hybrid pump dryer (HPD) at different conditions and hot air dryer (HAD). Data presented as mean ± SD (n = 3)

Dryer used	Heating treatment	ABTS (μmol TE/μmol sample)	TBARS (mg MDA/kg)
HPD-No heat pattern	45 °C(24 h)	45.9 ± 2.7d	0.71 ± 0.04a
	55 °C(24 h)	44.5 ± 1.9 cd	0.83 ± 0.03b
	60 °C(24 h)	38.7 ± 1.7ab	1.05 ± 0.05c
HPD-Heat pattern	55 °C(10 h) + 45 °C(14 h)	45.3 ± 2.8 cd	0.78 ± 0.02ab
	65 °C(7 h) + 45 °C(17 h)	41.5 ± 1.7bc	1.19 ± 0.03d
HAD	70 °C(24 h)	37.2 ± 2.5a	1.25 ± 0.06d

TE means Trolox equivalent while MDA stands for malionedialdehyde

Similar letters within a column are not significantly different as indicated by Duncan’s multiple range test (p < 0.05)

Changes in oxidative enzyme activities in dried oysters

Reactive oxygen species (ROS) are generated by external stimuli such as smoking and environmental pollution, as well as in vivo metabolism of oxygen. While some of these ROS may play a crucial role in body functions, most are removed by the inherent antioxidant defense mechanism of the body; and under normal physiological conditions, there is equilibrium in the production of these free radicals and their elimination by the body’s intrinsic antioxidants (Zhang et al., 2015). Oxidative stress occurs when this equilibrium is disturbed; resulting to accumulated oxidized and damaged biomolecules (Deponte, 2013). The in vivo antioxidative mechanism involves action of innate antioxidant nutrients and enzymes, which are thought to be target-specific and entail genetic factors that are still not yet fully understood (Shin et al., 2015). Such enzyme systems include superoxide dismutase (SOD), catalases, glutathione peroxidase (GPX), glutathione S-transferase (GST), aldo-ketoreductases, and DNA repair enzymes (Knapen et al., 1999).

Glutathione (GSH) and related enzymes play a role in the metabolism and elimination of cytotoxic compounds and ROS (Knapen et al., 1999). Its reduced form (γ-L-glutamyl-L-cysteinylglycine) acts as both oxidant and reductant and converts a variety of electrophilic substances into less biologically active compounds (Deponte, 2013). GST acts as catalyst in the conjugation of GSH with proelectrophilic metabolites produced from foreign substances when they enter the body. This conjugation consequently reduces toxicity of lipophilic molecules or electrophiles by increasing their water solubility and converting them into non-reactive compounds that can be easily discharged through urine or bile (Deponte, 2013; Veskoukis et al., 2016). The GST and SOD activities of dried oyster samples were shown in Table 5. Lower GST levels (65.7–69.2 and 67.2–68.2 μmol/mg protein, for samples dried without heat pattern and with heat pattern, respectively) were exhibited by HPD-dried oyster samples than the HAD-dried samples (72.0 μmol/mg protein) at 70 °C. Since GSTs play a significant role in initiating the detoxification of potential alkylating agents like peroxides and disulfides, GST activity is often used as biomarker and indicator for oxidative stress (Knapen et al., 1999; Zhang et al., 2015). The results in GST enzyme activity of dried oysters corresponded with TBARS values which indicated lipid oxidation. The high GST in HAD-dried oysters implied greater oxidative stress and, thus, higher degree of lipid oxidation as shown by its TBARS value (Table 4). HAD-dried oysters contained high lipid content (Table 1) which could mean high PUFA content and, in turn, could attract radicals and induce lipid oxidation (Lee et al., 2012; Rahman, 2006).

Table 5.

Glutathione S-transferase (GST) and superoxide dismutase (SOD) of dried oyster by hybrid pump dryer (HPD) at different conditions and hot air drier (HAD)

Dryer used	Heating treatment	GST (μmol/mg protein)	SOD (U/mg protein)
HPD-No heat pattern	45 °C(24 h)	65.7 ± 0.9a	15.0 ± 2.6b
	55 °C(24 h)	66.8 ± 0.4ab	13.9 ± 1.2ab
	60 °C(24 h)	69.2 ± 1.1c	12.7 ± 1.5ab
HPD-Heat pattern	55 °C(10 h) + 45 °C(14 h)	67.2 ± 0.6ab	14.6 ± 1.8ab
	65 °C(7 h) + 45 °C(17 h)	68.2 ± 1.1bc	12.4 ± 2.7ab
HAD	70 °C(24 h)	72.0 ± 1.6d	10.2 ± 3.5a

¹Data are shown as mean ± SD (n = 3)

²Means within a column with the same letter are not significantly different as indicated by Duncan’s multiple range test (p < 0.05)

The superoxide radical (O₂^·−) only reacts with nitric oxide to produce peroxynitrite and with itself in a process called dismutation. SOD is an antioxidant enzyme in cells that catalyzes the dismutation of superoxide radical (O₂^·−) and its conversion into molecular oxygen and hydrogen peroxide to protect macromolecules from oxidation and prevent tissue damage (Gonzales et al., 2005; Verlecar et al., 2008; Veskoukis et al., 2016). SOD activity in HPD-dried samples decreased as temperature treatment increased (Table 5). Oyster samples dried with two-step heating pattern had lower SOD activity (12.4–14.6 U/mg protein) than the samples dried constantly in single temperature settings (12.7–15.0 U/mg protein). Meanwhile, lowest SOD activity was found in HAD-dried samples (10.2 U/mg protein). These observations could be due to the decrease in active enzymes caused by degradation from high temperature treatment (Verlecar et al., 2008).

This study confirmed our hypothesis that HPD is better than HAD in preserving oysters with minimum changes in their physical and chemical quality attributes. Significant differences were observed in HPD-dried oyster samples in terms of surface color, browning reaction, lipid and glycogen content, total volatile basic nitrogen, and lipid oxidation after drying, and antioxidant activities. Meanwhile, water activity and protein and ash contents were relatively similar in both HPD-dried and HAD-dried oysters. Employing HPD in preserving oysters may further improve distribution and consumption and help small and medium-scale culture farms expand into large-scale production.

Declarations

Conflict of interest

The authors declare no conflict of interest.