Effect of Thermal Sterilization on the Selected Quality Attributes of Sweet and Sour Carp

Abstract

In this study, a traditional Chinese dish, a ready-to-eat sweet and sour carp was developed. The effects of sterilization temperature (110, 115, 121, and 130°C) on the selected properties, including heat penetration characteristics, as well as rheological, instrumental color, texture, and sensory attributes, were investigated at the same lethality value (F₀) of 4.5 min. The results showed that the heat penetration parameters of cooking value (C-value) and the ratio of C-value to F₀ value (C-value/F₀) exhibited an inverse relationship with processing temperature. The color and rheological properties of the sauce and the texture of the fish sterilized at 130°C were better preserved compared to those sterilized at other temperatures. The sensory attributes of the product sterilized at 130°C scored highest among the final products sterilized at the four temperatures. This study suggested that short-time sterilization at a high temperature can minimize the negative thermal impact on sweet and sour carp product, which would be feasible in the practical production.

Keywords:

• Carp
• Sweet and sour carp
• Thermal sterilization
• F₀
• C-value
• Sauce

INTRODUCTION

Carp (Cyprinus carpio) is a popular freshwater fish species worldwide that grows rapidly and easily.^[1] Although carp are abundant in China,^[2] most of them are consumed at home or in restaurants rather than being sold in supermarkets. Unlike other Chinese dishes, sweet and sour carp (SSC) has not been industrialized probably owing to the short shelf life. Thus, it is promising to develop a ready-to-eat SSC product by thermal sterilization which is the most commonly used preservation method.

On the other hand, thermal sterilization may affect the sensory and nutritional aspect of fish products. Ali et al.^[3] reported that the textures of sardines packed in aluminum cans were impaired by thermal processing. The texture of Indian mackerel also deteriorated after thermal sterilization.^[4] Kong et al.^[5] reported heating significantly changed the quality attributes of Salmon muscle, including color, shear force, cook loss, and shrinkage. Many methods (e.g., agitated retorting, small disk shaped products, and thin profile containers aiming to improve the heat penetration) have been utilized to reduce the negative impacts of heat treatment on the qualities of foods.^[6−8] Alternatively, foods can be sterilized at a high temperature to shorten the heating time to reduce the negative thermal impact on products. Sreenath et al.^[4] found that nutrients and textures could be better retained at 130°C at F₀ of 8 min. Therefore, the texture and sensory quality of SSC might be well preserved by sterilization at a high temperature. Thereby motivated, this study aims to design a ready-to-eat traditional SSC product packaged in retort pouches. The effects of thermal sterilization on the final qualities of SSC products, including fish texture as well as sauce color and rheological properties, were compared at 110, 115, 121, and 130°C at the same F₀ value of 4.5 min (recommend F₀ range for canned fish products: 4–6 min).^[9]

MATERIALS AND METHODS

Materials

Cultured carp (mean body weight: 1.0–1.2 kg) were purchased alive from a local fish market in Wuxi, China. The fish was cleaned by removing head, entrails, and tail then were washed in chilled potable water. Then the cleaned fish was cut into 3 cm × 3 cm × 1.5 cm blocks weighing about 13 ± 1 g per block. The fish blocks were salted in 4% (w/w) brine for 2 h at ambient temperature(25 ± 2°C), and then were dried at 50°C for 3 h in an ENGZI 101-1-BS drying oven (Shanghai Yuejin Machine Factory, China). The moisture of the dried fish blocks was balanced at room temperature for approximately 30 min, and then were fried in 180 ± 5°C hot oil for about 1.5 min.

Tomato paste containing 28% soluble solids was obtained from Xinjiang Xiaochu Food Co., Ltd. (Xinjiang, China). Flavoring substances, including sugar, vinegar and salt, were bought from a local supermarket (Wuxi, China). Xanthan gum was obtained from Danisco Co., Ltd. (Kunshan, China). Acetylated distarch phosphate was obtained from National Starch and Chemical Company (Shanghai, China). The sweet and sour sauce was prepared by mixing tomato puree (20 g/100 g sauce), sugar (10 g/100 g sauce), vinegar (3 g/100 g sauce), salt (0.5 g/100 g sauce), xanthan gum (0.3 g/100 g sauce), acetylated distarch phosphate (1.5 g/100 g sauce), and water (64.7 g/100 g sauce). Then SSC was prepared by mixing a fried fish block with the sweet and sour sauce at a weight ratio of 1:1.5 in a retort pouch (a three-layer polyethylene package, size: 10 cm × 12 cm × 105 μm). The total weights of the packaged products (pH: 6.4 ± 0.1) were 25 ± 2 g. The packaged SSCs were sealed by a DZ-280/2SE Vacuum-sealing machine (Shanghai Heyi Packaging machinery Co., Ltd., Shanghai, China) to remove air before heat processing.

Thermal Sterilization of SSC

Eighty pouches of SSC were randomly divided into four batches and then were separately loaded in the retort of a RHS-03-700 Full-automatic Rotary Multifunctional Sterilizer (Wenzhou Longqiang Dairy Machine Factory, Zhejiang, China). Thermocouple probes (model no. SSA12040 G700 TS, EllabA/S) were inserted into the cold spot of the fish blocks to record the core temperature, and the output was acquired every minute using an Ellab data recorder (model TM 9608, Ellab A/S). The pouches batches were processed at 110, 115, 121, and 130°C to a standardized F₀ value of 4.5 min. Each temperature was tested in triplicate. The initial temperature of the product was maintained at 30 ± 2°C. The pouches were cooled down to a core temperature of 40°C by pumping water into the retort to prevent the proliferation of thermophiles and the overcooking of the product. In this study, heating targets to eliminate Clostridium botulinum in the products, and the accumulated lethality during the entire processing (heating and cooling) can be calculated by equation (1) using a reference temperature of 121.1°C.

(1)

where t represents time (min), Z is the temperature sensitivity of the target microorganism (Z of Clostridium botulinum: 10°C), and T represents the temperature at any given time. Cook value (C-value) represents the degree of heat treatment with respect to nutrient degradation and textural changes that occur during processing, which can be calculated by a similar equation utilizing the reference temperature of 100°C and Z_c of 33°C. ^[10]

(2)

The commercial sterilities of the SSC products processed at the four temperatures were tested according to Chinese standard GB/T4789.26.^[11] Five pouches of the SSC product were randomly selected from each batch processed at different retort temperatures and were incubated at 37°C for 14 days. The incubated pouches were reopened under aseptic conditions, and one gram of each homogenization sample was transferred to a sterile tube containing 10 mL of BCP Glucose Broth and an inverted small tubule. The tubes were then incubated at 37°C for 48 h to observe the development of gases indicating the survival of microorganisms. The tubes in which no gases were developed were incubated for another 48 h at 37°C to exclude possible errors.

Rheological Properties of Sweet and Sour Sauces

All rheological measurements were carried out by an AR-1000 Rheometer (TA Instrument Ltd., Crawley, UK) using a parallel plate geometry (40 mm diameter, 1 mm gap).^[12] The temperature was kept at 25.0 ± 0.1°C by circulating water. Each sample was equilibrated for 2 min prior to measurement. All rheological measurements were performed in triplicate. The experimental rheological data were obtained directly from the TA Rheology Advantage Data Analysis software V.5.0.1 (TA Instrument Ltd., Crwaley, U.K.).

Continuous shear measurements

The continuous shear tests were performed at the shear rate from 0 to 150 s⁻¹. The rheological properties are described by the power-law model:

(3)

where τ is shear stress (Pa), K is the consistency index (Pa.sⁿ), γ is shear rate (s⁻¹), and n is the flow behavior index (dimensionless).

Frequency sweep measurements

Viscoelastic properties, including storage modulus (G’), loss modulus (G”), and loss angle (δ) were determined by small amplitude oscillatory shear flows at the frequencies from 0.6–20 rad/s.^[13] According to the strain sweep results (data not shown), the strain amplitude was selected as 2% to allow the linear viscoelastic measurements. The frequency dependences of G’ and G” can be approximately described by the following equations:

(4)

(5)

where K’ and K” are frequency exponents, ω is the angular frequency, and n’ and n” refer to the frequency exponents providing useful information for the viscoelastic nature of food materials.^[13]

Color Analysis of Sauce

The colors of the sauces from unsterilized samples and the samples sterilized at different temperatures were assessed using a Hunter-Lab colorimeter Ultrascan PRO (Hunter Associates Laboratory Inc., Reston VA, USA). The color of a sample is denoted by three dimensions, i.e., L*, a* and b*. L* denotes lightness from 100 (perfect white) to 0 (totally black), and a* and b* denote redness/greenness and yellowness/blueness, respectively. The L*, a*, and b* values of each finely homogenized sweet and sour sauce were tested by being loaded inside the sample holder. The instrument was standardized using a standard white tile (No. 21733001).^[14] Each sample was analyzed in triplicate.

Instrumental Texture Analysis of Fish Blocks

The textures were measured by a TA-XTplus Texture Analyzer (Stable Micro Systems, Co., UK) equipped with a cylindrical aluminum probe (diameter: 25 mm).^[15] After the pouches were removed, the fish blocks were equilibrated at room temperature and then compressed twice to 40% of their original heights. The textures were measured at the speed of 1 mm/s using a 5 kg load cell. The two compression cycles were separated by 5 s. Hardness, springiness, gumminess, and chewiness were calculated using the Texture Expert software, version 1.22 (Stable Micro Systems, Surrey, UK). All determinations were performed in triplicate.

Sensory Analysis of SSC

The sensory characteristics of SSC products, including color, flavor, texture (hardness, springiness, chewiness, and gumminess), and the overall acceptability were evaluated by a panel of 10 trained judges (five females and five males) with a nine-point scale.^[16] Five samples, including one unsterilized sample and four samples sterilized at 110, 115, 121, and 130℃, were distributed to each panelist following a random order. All the tests were performed in the sensory laboratory of Jiangnan University according to the international standards.^[17] The panelists were asked to assign a score to each characteristic (9 = extremely desirable, 1 = extremely undesirable) of the products, and only a score no lower than 4.0 was considered acceptable.

Statistical Analysis

Statistical analysis was performed using SPSS 18.0 (SPSS INC., Chicago, USA). The data were expressed as mean ± standard deviation. A difference higher than 0.05 between treatments was considered significant by the Duncan’s Multiple Range test.

RESULTS AND DISCUSSION

Thermal Sterilization of SSC

Spores and spoilage organisms in SSC may alter the product qualities. The spores of Clostridium botulinum in low-acid products (pH > 4.6) can be destroyed at the F₀ of 3 min, but more severe conditions are still in need to control spoilage organisms.^[18,19] Bell et al.^[9] studied the mass loss of precooked tuna muscle at the F₀ of 4 min during retorting and the storage in cans. Simpson et al.^[20] established a mathematical model for the thermal processing of seafood packed in retortable pouches at a minimum F₀ of 4.5 min. Thus, an equivalent lethality at 4.5 min may be suitable, at which SSC products were sterilized at four different temperatures (110, 115, 121, and 130°C). The F₀ versus C-value heat penetration curves of the SSCs sterilized at the four temperatures were shown in Fig. 1, and the penetration parameters were summarized in Table 1. Figure 1 showed that all the retort temperatures reached up to the aimed ones within three minutes, and the core temperatures of the samples, except for those treated at 130°C, also increased to the target ones respectively. Table 1 indicated that the SSCs processed at the four temperatures were commercially sterile. The total thermal process time (TPT) and the time of products sterilized at retort temperature (U_T) significantly shortened with rising sterilization temperature at the same F₀. When the temperature increased from 110 to 130°C, TPT and U_T dramatically reduced from 86.13 and 65.72 min to 18.15 and 0.48 min, respectively.

Table 1  The parameters of thermal sterilizations at different temperatures.*

Parameter	110°C	115°C	121°C	130°C
Come-up-time (min)	1.73 ± 0.07^c	1.91 ± 0.11^b	2.11 ± 0.06^b	2.92 ± 0.13^a
UT (min)	65.72 ± 0.35^a	14.55 ± 0.16^b	4.35 ± 0.08^c	0.48 ± 0.02^d
TPT (min)	86.13 ± 1.03^a	40.21 ± 0.94^b	23.09 ± 1.07^c	18.15 ± 0.87^d
g (°C)	2.14 ± 0.02^d	3.52 ± 0.13^c	4.09 ± 0.23^b	8.31 ± 0.27^a
C-value (min)	136.20 ± 0.85^a	72.12 ± 0.94^b	34.02 ± 0.13^c	22.12 ± 0.16^d
C-value /F0 ratio	30.27 ± 0.44^a	16.03 ± 0.48^b	7.56 ± 0.08^c	4.92 ± 0.11^d
Bacteria level	Commercial sterility	Commercial sterility	Commercial sterility	Commercial sterility

*Results are presented as mean ± standard deviation (n = 3). Values that have different superscript letters differed significantly with each other (p < 0.05). U_T: time in minutes for sterilization at retort temperature; TPT: total process time; g: final temperature deficit; C-value: cooking value; C-value/F₀ ratio: C-value to F₀ value ratio.

Figure 1  Heat penetration characteristics showing F₀ and C-value of SSC in pouch sterilized at different sterilization temperatures. (a) 110°C; (b) 115°C; (c) 121°C; (d) 130°C.

Although thermal sterilization destructs microbes, it also degrades nutrients, changes (usually softens) textures, and inactivates enzymes.^[10] C-value that evaluates the impact of thermal processing on food should be minimized at any given lethality.^[21] In this study, C-values exhibited an inverse relationship with processing temperature, which were maximal and minimal at 110 and 130°C, respectively. The C-values at 130, 121, and 115°C reduced by 83.76, 75.02, and 47.05% respectively compared to that treated at 110°C.

The ratio of C-value to F₀ (C-value/F₀) correlates with the texture, color, and the sensory parameters of sterilized foods and can be used as an indicator to identify the process conditions that augment quality retention.^[4] The C-value/F₀ ratios of the SSCs processed at 110, 115, 121, and 130°C were listed in Table 1, which followed a descending order of 110 > 115 > 121 > 130°C. The results suggested that thermal processing temperature influenced C-value /F₀ ratio, and the better quality of the SSC sterilized at a higher temperature at the same F₀ results from the shortened time, which was in agreement with the results of Durance^[22] that the quality of canned foods could be boosted by controlling the retort temperature.

Rheological Properties of Sweet and Sour Sauces in SSC

Clarifying the rheological performance about fluid and semisolid foodstuffs is crucial to understand and design the flow processes in quality control, stability measurement, and storage.^[12] Thus, it is of great importance to study the flow behaviors of the sauce surrounding the fish block in SSC. The apparent viscosities of the sauces after thermal treatment all decreased with increasing shear rate (Fig. 2), indicating that all the sauces were of shear-thinning behaviors and could be classified into pseudoplastic fluids.^[23] Then the data were fitted to the power-law model utilizing equation (3), yielding the corresponding parameters (Table 2). The K value and the n value of the unsterilized sauce were highest and lowest, suggesting that the viscosity and the shear-thinning property were highest and greatest, respectively. The K values of all the sterilized sauces were lower than those of the untreated one, indicating that thermal sterilization lowered the apparent viscosity of the sauce probably because of the degradation of macromolecules (e.g., xanthan gum, modified starch) in the acidic sauces during thermal treatment.^[24] The K value of the sauce sterilized at 130°C was highest and was close to that of the unsterilized one, implying that a higher temperature that shortened the sterilization time could minimize the thermal impact on the fluid properties of sauces.

Figure 2  The shear rate dependence of apparent viscosity of sauces in SSC.

The viscoelastic properties of the sauces in SSC were shown in Fig. 3. The storage modulus (G’) and loss modulus (G”) of all samples increased with increasing angular frequency. Moreover, all the δ values were lower than 45°C, indicating that G’ values of all the samples were higher than their G” throughout the entire frequency range.^[12] The modulus (G’ and G”) of all the sterilized samples were lower (lowest at 110°C) than those of the unsterilized sample, which may result from the thermal degradation of polysaccharides as discussed above. The power-law parameters of the unsterilized sauce and the sauces sterilized at different temperatures were shown in Table 2. The K’ values of all the samples were apparently higher than their K” values, suggesting that these sauces were predominantly elastic than viscous. Table 2 also showed that K’ and K” of the unsterilized sauce were highest, suggesting that the unsterilized sauce had a stronger elastic structure than the sterilized ones.^[13] The unsterilized sample was of the minimal n’ and the maximal n” among all the sauces, indicating that thermal sterilization increased the elastic sensitivities and decreased the viscous sensitivities of the sauces. Besides, K’ and K” of the sample treated at 130°C were highest among the four sterilized sauces. Accordingly, sterilizing sauces at 130°C minimized the thermal effects on the steady-state viscous properties and the dynamic viscoelastic properties. A naked-eye observation also showed the sauce sterilized at 130°C did not flow away readily by adhering to the fish block firmly, which outweighed the sauces treated at other temperatures. The results were consistent with the rheological data tested by the rheometer (vide supra), revealing that 130°C is optimal for sterilizing SSC products.

Table 2  Power law parameters for apparent viscosity, G’ and G” of the sauces from SSC product.*

	K(Pa.s^n)	n	R^2	K’ (Pa.s^n)	n’	R^2	K” (Pa.s^n)	n”	R^2
Unsterilized	13.92 ± 0.13^a	0.186 ± 0.002^c	0.942	73.64 ± 0.21^a	0.141 ± 0.001^d	0.997	14.83 ± 0.01^a	0.236 ± 0.002^a	0.942
110°C	6.96 ± 0.06^d	0.276 ± 0.002^a	0.989	41.86 ± 0.04^c	0.175 ± 0.002^a	0.997	11.24 ± 0.01^d	0.197 ± 0.001^b	0.989
115°C	8.70 ± 0.02^c	0.267 ± 0.003^b	0.965	42.78 ± 0.02^c	0.168 ± 0.001^c	0.995	11.53 ± 0.02^c	0.201 ± 0.003^b	0.965
121°C	10.21 ± 0.03^b	0.271 ± 0.002^b	0.989	50.62 ± 0.43^b	0.167 ± 0.003^c	0.996	12.86 ± 0.05^b	0.175 ± 0.003^c	0.989
130°C	10.26 ± 0.04^b	0.272 ± 0.003^b	0.984	51.72 ± 0.07^b	0.171 ± 0.001^b	0.996	12.87 ± 0.01^b	0.176 ± 0.002^c	0.984

*Results are presented as mean ± standard deviation (n = 3). Values that have different superscript letters differed significantly with each other (p < 0.05). K: consistency index; n: flow behavior index; R²: correlation coefficient; K’ and K”: frequency exponents; n’ and n”: viscoelastic nature.

Figure 3  Frequency dependence of (a) G’, (b) G’’, and (c) δ of sauces in SSC.

Color Analysis of Sweet and Sour Sauces in SSC

The visual appearance of foods, manifested as color, determines the opinion held by consumers concerning food quality. The color of tomato products might change during thermal treatment, which might affect customer acceptance.^[25,26] L*, a*, and b* of the unsterilized sauce was higher than those of the sterilized ones (Table 3), revealing that the color of tomato sauce changed during thermal sterilization. Apaiah and Barringer^[27] and Nisha et al.^[28] reported that the decrease of color values after thermal treatment might be ascribed to the lycopene degradation and non-enzymatic browning or Maillard reactions. Besides, the decreases of L*, a*, and b* values were slowed down at higher treatment temperatures, which might be attributed to the shortened treatment time at the same F₀ value. The color values of the sauces treated at 121 and 130°C that exceeded those of the other two sterilized ones were close to that of the unsterilized one, indicating that the two temperatures better preserved the sauce color.

Table 3  Color values L*, a* and b* of sweet and sour sauces in SSC product.*

	Unsterilized	110°C	115°C	121°C	130°C
L*	32.77 ± 0.02^a	32.19 ± 0.08^c	32.22 ± 0.08^c	32.56 ± 0.05^b	32.51 ± 0.12^b
a*	17.13 ± 0.02^a	15.90 ± 0.05^c	15.98 ± 0.07^c	16.44 ± 0.11^b	16.53 ± 0.14^b
b*	12.25 ± 0.05^a	11.38 ± 0.08^b	11.56 ± 0.12^b	12.18 ± 0.07^a	12.16 ± 0.18^a

*Results are presented as mean ± standard deviation (n = 3). Values that have different superscript letters differed significantly with each other (p < 0.05).

Table 4  Instrumental texture profile analysis values of fish blocks from SSC product.*

	Hardness/g	Springiness	Gumminess	Chewiness
Raw fish	2014.35 ± 14.10^b	0.77 ± 0.03^b	911.09 ± 7.78^b	773.37 ± 47.99^b
Fried fish	3291.99 ± 45.38^a	0.79 ± 0.11^a	1565.39 ± 34.77^a	1023.59 ± 27.19^a
110°C	980.56 ± 12.76^f	0.61 ± 0.05^e	439.88 ± 11.71^e	316.91 ± 16.50^e
115°C	1179.11 ± 47.34^e	0.65 ± 0.01^e	568.44 ± 73.57^e	393.63 ± 41.74^e
121°C	1576.45 ± 83.22^d	0.71 ± 0.01^d	813.14 ± 4.28^d	539.81 ± 20.74^d
130°C	1758.70 ± 24.47^c	0.74 ± 0.02^c	853.52 ± 10.56^c	672.54 ± 35.70^c

*Results are presented as mean ± standard deviation (n = 3). Values that have different superscript letters differed significantly with each other (p < 0.05).

Instrumental Texture Analysis of Fish Blocks

Texture, one of the most important quality parameters of fish products, may be affected by thermal sterilization. The changes in the texture of fish blocks during thermal processes were measured and the results of texture profile analysis (TPA) were listed in Table 4. The hardness value of the unsterilized fried fish was higher than that of the raw materials, which might be associated with the decreased moisture of fish muscles after drying and frying increased the density of fish muscle fiber.^[29,30] In addition, the heat denaturation of myofibrillar and sarcoplasmic proteins also toughened fish muscles.^[5,31] The hardness values of the four sterilized samples were lower than those of the raw materials and the fried fish blocks, which might be attributed to the heat-induced muscle decomposition and fragmentation during sterilization.^[5] Moreover, the springiness, gumminess, and chewiness of the sterilized samples descending from 130 to 110°C were also lower than those of the fried fish block. Considering that traditional fried SSC fish blocks are commonly consumed directly without sterilization, the thermally sterilized sample with a similar texture will be easily acceptable. The above results exhibited that the texture of the sample sterilized at 130°C was most analogous to that of the unsterilized fried block, indicating that higher sterilization temperatures are conducive to maintaining the texture.

Sensory Analysis of SSC Product

The sensory scores given by the judges were presented in Table 5. The unsterilized sample scored higher compared to the sterilized ones, suggesting that thermal sterilization decreased the sensory quality of the products. Among the sterilized samples, the product sterilized at 110°C scored lowest, whereas the color of the product processed at 130°C scored highest. Besides, the colors of the products treated at 121 and 130°C did not differ significantly, which was in accordance with the instrumental results. The flavor was not influenced by the processing temperature. However, the texture of the fish blocks and the overall acceptability of the products were evidently influenced by the sterilization temperature, and the texture of the product processed at 130°C scored highest. The results revealed that shortening the heating time of SSC products at the same lethality level boosted the sensory qualities, which was consistent with the variations of C-value/F₀ ratio discussed above.

Table 5  Sensory score of SSC product.*

	Color	Flavor	Texture	Overall acceptance
Unsterilized	8.2^a	8.0^a	8.5^a	8.4^a
110°C	6.6^b	7.6^a	4.6^d	5.7^d
115°C	6.8^b	7.6^a	5.8^c	6.9^c
121°C	7.8^a	7.8^a	7.4^b	7.4^b
130°C	8.0^a	7.9^a	8.2^a	8.1^a

*Results are presented as median (n = 10). Values that have different superscript letters differed significantly with each other (p < 0.05).

CONCLUSION

In summary, sterilization at different temperatures at the same F₀ value of 4.5 min affected the selected quality attributes of SSC. Thermal processing parameters, including C-value and C-value/F₀, decreased with increasing sterilization temperature. The color, rheological properties, texture, and sensory scoring of the product sterilized at 130°C outweighed those treated at 110, 115, and 121.1°C. It is feasible to prepare the traditional Chinese dish SSC into a ready-to-eat product at a high sterilization temperature, aiming to ensure better color, texture, and sensory attributes.

FUNDING

This research was financially supported by the earmarked fund for China Agriculture Research System (CARS-46).