Effect of ultrasound treatment on dehydration kinetics and physicochemical, microbiological, structural and rehydration characteristics of tilapia

ABSTRACT

The effect of ultrasound for 15 and 30 min (40 kHz, 130 W, 10% salt solution, 25°C) on dehydration kinetics and physicochemical, microbiological, structural and rehydration characteristics of tilapia cubes dehydrated at 35°C was evaluated. The ultrasound treatments decreased dehydration time in the range of 8.7–19.2%, in comparison with the control treatments (15 and 30 min, 10% salt solution, 25°C). Of the Wang and Singh, Lewis, Modified Page and Page models, the last one was the one that better described the dehydration kinetics. The ultrasound significantly (p < .05) increased the ash content and significantly (p < .05) decreased the a_w of the dehydrated tilapia cubes but did not have an effect on the microbiological characteristics. In addition, the tilapia cubes treated with ultrasound presented porous structures with large and irregular cavities, which increased the amount of moisture absorbed in the rehydration, in comparison with the control treatments.

RESUMEN

Se evaluó el efecto del ultrasonido por 15 y 30 min (40 kHz, 130 W, solución de sal al 10%, 25°C) en las cinéticas de deshidratación y características fisicoquímicas, microbiológicas, estructurales y de rehidratación de cubos de tilapia deshidratados a 35°C. Los tratamientos de ultrasonido disminuyeron el tiempo de deshidratación en el rango de 8.7-19.2%, en comparación con los tratamientos controles (15 y 30 min en solución de sal al 10%, 25°C). De los modelos de Wang y Singh, Lewis, Page Modificado y Page, el último fue el que mejor describió las cinéticas de deshidratación. El ultrasonido incrementó significativamente el contenido de cenizas y disminuyó significativamente la a_w de cubos de tilapia deshidratados, pero no tuvo efecto sobre las características microbiológicas. Además, los cubos de tilapia tratados con ultrasonido presentaron estructuras porosas con cavidades grandes y porosas, lo cual aumentó la cantidad de agua absorbida en la rehidratación, en comparación con los tratamientos control.

KEYWORDS:

• Tilapia cubes
• dehydration
• ultrasound
• physicochemical characteristics
• rehydration

PALABRAS CLAVE:

• Cubos de tilapia
• deshidratación
• ultrasonido
• características fisicoquímicas
• rehidratación

1. Introduction

Tilapia (Oreochromis niloticus) is a freshwater fish species which has widely been cultured on a global scale and likely to be the most important cultured fish in the twenty-first Century (Thangaraj et al., 2018). The world aquaculture production of Oreochromis niloticus in 2016 was 4.2 million metric tons and the fourth most cultured species (FAO, 2018). More and more people prefer to eat tilapia because it is delicious, tender, and rich in protein and various unsaturated fatty acids (Ye et al., 2017). However, the quantity of the tilapia used for processing is too small to meet the requirement of increased production, and it is a highly perishable product as it consists of up to 80% of water (Li, Li, & Li, 2009). Therefore, to solve the problems of high enzymatic and bacterial activity in fresh fish, the use of processing and preservation technology is necessary (Duan, Jiang, Wang, Yu, & Wang, 2011).

Currently, the tilapia and other aquatic product processing methods consist of drying and freezing processes. Freezing process technology requires a large initial investment, and higher costs of production and transportation. Drying is one of the most general methods in the aquatic product processing industry, which is widely applied in developing countries. It can not only reduce water activity, prevent food from spoilage and deterioration but also reduce the weight and volume of products and make storage and transportation easier (Hu, Duan, & Liu, 2016).

The hot-air drying belongs to heat drying and it is more common in food drying treatment, but prolonged hot-air drying and high drying temperature easily lead to problems such as loss of product flavor, poor color, loss of nutrients, reduced product rehydration rate, migration of soluble solute, great energy consumption in processing, etc. (Zhi-qiang, Li-jing, Min, & Sheng-lan, 2012).

On the other hand, novel drying techniques are emerging from a research perspective that may yet prove to have a positive impact on the food industry both in terms of product quality and energy efficiency. These novel methods exploit different physical phenomena to enhance already commercial drying techniques, as in the case of ultrasound (Moses, Norton, Alagusundaram, & Tiwari, 2014).

The use of ultrasound in the drying of foods has been carried out in two ways: using ultrasound pre-treatments prior to conventional drying and using air-borne ultrasonic drying (Magalhaes et al., 2017). When the ultrasound is used as a pre-treatment, the product is immersed in a liquid medium (distilled water or osmotic solution) with ultrasound application. The dipping of fish in salt solutions is named cold blanching and such treatment is recommendable, because it inhibits enzyme reactions and improves the sensorial quality of the product when it is followed by drying in hot air or cooking in steam (Dhanapali et al., 2013). The ultrasound as pre-treatment reduces the internal resistance during the drying process since it causes structural changes on the products, such as micro-channels formation (Ricce, Rojas, Miano, Siche, & Duarte Augusto, 2016).

Among tilapia dishes, in Mexico as well as other countries, the tilapia ceviche is one of the most popular dishes. Typically, ceviche is made by marinating uncooked tilapia in vinegar or citrus juice or a mixture of lime and other citrus juices, and seasoning them with salt, cucumber, onion, pepper, and other spices. Other major recipe variables include the temperature (either room or refrigerator) and time of marination (Mathur & Schaffner, 2013).

To our knowledge, there is no scientific information available on the application of ultrasound in the production of tilapia cubes dehydrated at low temperature, which could be used as the main ingredient for some dishes as ceviche. Therefore, the objective of this study was to evaluate the effect of ultrasound pretreatment on dehydration kinetics and physicochemical, microbiological, and rehydration properties of tilapia cubes dehydrated with low-temperature air.

2. Materials and methods

2.1. Materials

Frozen tilapia fillets were purchased from local supermarkets, transported with ice packs, and stored at −20°C until used. Fillets were thawed for ~30 min at room temperature. Thawed fillets were cut into cubes of 0.7 cm on a side.

2.2. Ultrasonic treatment

Tilapia cubes samples (200 ± 1 g) were placed in 1000 mL beakers with 800 mL of 10% w/w saline solution at 25°C. For ultrasound treatment, a 40 kHz ultrasound bath (Branson, Model MTH-3510; a power of 130 W; a tank capacity of 5 L; internal dimensions of 290 × 150 × 150 mm; an acoustic energy density of 0.026 W/cm³) was used. Beakers containing the tilapia samples were placed directly into the ultrasound bath at 25°C to receive the ultrasound treatment for 15 (US-15) or 30 min (US-30). For control treatments, beakers with tilapia samples into 10% w/w saline solution were placed into a water bath at 25°C during 15 (C-15) or 30 min (C-30). Then, the tilapia samples were removed from the beakers, superficially blotted with absorbent paper, and weighed to proceed to dehydration experiments.

2.3. Dehydration study

The tilapia samples (200 g) were dehydrated in a cabinet drier (Venticell 111-Standard, Germany) at 35 ± 2°C under an air velocity of 1.3 m s⁻¹ and relative humidity of 55 ± 5%. In this dryer, air was flowing horizontally through tilapia cubes. The velocity of air passing through the system was measured by a CEM DT-618 thermo-anemometer (Shenshen Everbest Machinery Industry, Co., Ltd, Nanshan, Shenzhen, China). For dehydration, tilapia cubes were uniformly spread in a single layer on a rectangular tray formed by an aluminum frame (size 40 cm × 30 cm) and a plastic mesh where the distance between wires was 1.3 mm. The samples of tilapia cubes were removed of dryer at time intervals of 60 min during the dehydration process and their weights were recorded with a digital scale with 0.01 g accuracy (Ohaus Corporation, USA), until the decrease in weight was negligible. The final moisture content was considered to be the equilibrium moisture content. The moisture ratio (MR) was calculated using the following equation:

(1) $MR=\frac{\left(M_{td}-M_{ed}\right)}{\left(M_{0d}-M_{ed}\right)}$(1)

where M_td is the moisture content at any time for the dehydration study (g water/100 dry matter); M_ed is the equilibrium moisture content for the dehydration study (g water/100 dry matter); and M_0d is the initial moisture content for the dehydration study (g water/100 dry matter). The experimental dehydration data (MR) were fitted to the drying models shown in Table 1.

Table 1. Models used for the fitting of the dehydration and rehydration kinetics of tilapia cubes.

Tabla 1. Modelos usados para el ajuste de las cinéticas de deshidratación y rehidratación de cubos de tilapia

Model name	Equation	Reference
Dehydration Page	$MR=exp\left(-k\ast t^n\right)$	Jain and Pathare (2007)
Wang and Singh	$MR=1+a\ast t+b\ast t^2$	Kadam, Tiwari, and O’Donnell (2015)
Lewis	$MR=exp\left(-k\ast t\right)$	Guan, Wang, Li, and Jiang (2013)
Modified Page	$MR=exp(-k\ast t{)}^n$	Vega-Gálvez et al. (2011)
Rehydration Peleg	$RR=W_0+t/\left(k_1+k_2\ast t\right)$	Chen, Li, Jiao, and Cui (2016)
First Order	$RR=W_e+\left(W_0-W_e\right)exp\left(-k_r\ast t\right)$	Muñoz, Arnau, and Gou (2012)
Weibull	$RR=W_e+\left(W_0-W_e\right)exp\left[-{\left(t/\beta \right)}^{\alpha }\right]$	Chen et al. (2016)
Exponential Association	$RR=W_e\left[1-\mathrm{e}\mathrm{x}\mathrm{p}\left(-k_r\ast t\right)\right]$	Cox, Gupta, and Abu-Ghannam (2012)

2.4. Physicochemical analysis

Moisture, crude protein (N x 6.25), ethereous extract, and ash contents were determined according to the AOAC (2000) methods. Water activity (a_w) was measured at 25°C using an AquaLab 4TEV (Decagon Devices Inc., Pullman, USA) equip. The color was determined with a Minolta CR-300 color meter (Minolta, Tokyo, Japan). The measured values were expressed according to the CIELAB color scale L* (lightness), a* (redness-greenness), and b* (yellowness-blueness). The L*, a*, and b* values of the white standard tile used as reference were 94.44, −0.23, and 3.89, respectively. The whiteness (Wh) was calculated according to the following equation (Li, Wu, & Guan, 2017):

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

2.5. Microbiological analysis

Tilapia samples were prepared and analyzed for aerobic bacterias, yeasts, molds, total coliforms, and fecal coliforms (Downes & Ito, 2001). The counts were expressed as colony-forming units (CFU) per gram or most probable number (MPN) per gram in the case of fecal coliforms. All the culture media used were from BD Difco (Becton, Dickinson and Co., Franklin Lakes, NJ, USA).

2.6. Microstructure analysis

The microstructure of the tilapia samples was observed with a scanning electron microscope (SEM) (SEC, Mini-SEM SNE-3200M, South Korea) at an accelerating voltage of 20 kV. Before using the SEM, the samples were coated with gold using an ion sputter coater (MCM-100, SEC). The SEM images were obtained at 700 x magnification.

2.7. Rehydration characteristics

Samples of ~10 g of tilapia were placed in a 1 L glass jar with 1 L distilled water, which had been previously heated to the required soaking temperature (30°C, 40°C, or 50°C) in a water bath thermostatically controlled at the required temperature (± 1°C). Water absorption was determined using a digital scale (Ohaus Corporation, Parsippany, NJ, USA) as the increase in the sample weights recorded every 10 min until the difference between consecutive weight measurements was insignificant (0.05 ± 0.01 g). This was considered to represent the point of saturation moisture content. After the specified soaking time, the samples were removed from the soaking solution, drained in a kitchen strainer for 0.5 min, blotted with paper tissue, and weighed. The weight gain was measured, and the samples were returned to the soaking solution at the required temperature. All soaking tests recorded as the percentage of moisture (d.b.).

The rehydration ratio (RR) was calculated according to the following equation:

(3) $RR=\frac{W_r}{W_d}$(3)

where W_r is the weight after rehydration (g) and W is the weight of the dehydrated material. The experimental rehydration data (RR) were fitted to the models shown in Table 1.

For the estimation of the effective diffusivity (D_eff) was assumed the Fick’s second law for a cube as follows:

(4) $\frac{\mathrm{\partial }x}{\mathrm{\partial }t}=D_{eff}\left(\frac{{\delta }^{2}X}{\delta x^2}+\frac{{\delta }^{2}X}{\delta y^2}+\frac{{\delta }^{2}X}{\delta z^2}\right)$(4)

A solution for cube geometry has been presented by Crank (1975) under the assumption of (1) a uniform initial moisture content, (2) constant effective diffusivity throughout the solid, (3) negligible external resistances and (4) negligible shrinkage (Equation (5)(5) $RR=\frac{W_r}{W_d}={\left(\sum _{n=0}^{\mathrm{\infty }}\frac{8}{\left(2n+1\right){\pi }^2}exp\left(-D_{eff}\frac{{\left(2n+1\right)}^2{\pi }^{2}t}{4a^2}\right)\right)}^3$(5) ).

(5) $RR=\frac{W_r}{W_d}={\left(\sum _{n=0}^{\mathrm{\infty }}\frac{8}{\left(2n+1\right){\pi }^2}exp\left(-D_{eff}\frac{{\left(2n+1\right)}^2{\pi }^{2}t}{4a^2}\right)\right)}^3$(5)

where RR, W_r, and W_d mean as described above, and a the characteristic edge length (m).

For long RR times, Equation (4)(4) $\frac{\mathrm{\partial }x}{\mathrm{\partial }t}=D_{eff}\left(\frac{{\delta }^{2}X}{\delta x^2}+\frac{{\delta }^{2}X}{\delta y^2}+\frac{{\delta }^{2}X}{\delta z^2}\right)$(4) can be simplified to the first term by setting n = 0.

(6) $RR=\frac{8^3}{{\pi }^6}exp\left(D_{eff}\frac{3{\pi }^{2}t}{4a^2}\right)$(6)

Based on this equation, the moisture diffusivity D_eff was determined by the method of slopes. To illustrate the effects of soaking temperature on D_eff, the Arrhenius equation was applied (Başlar, Kılıçlı, & Yalınkılıç, 2015). The Arrhenius law can be represented as follows:

(7) $D_{eff}=D_{0}exp\left(\frac{E_a}{RT}\right)$(7)

where D₀ is the pre-exponential factor (m² s⁻¹), E_a is the activation energy (J mol⁻¹), R is the gas constant (8.31451 J mol⁻¹ K⁻¹), and T is temperature (K). A plot of ln D_eff versus 1/T gives a straight line of the slope of E_a/R.

The water absorption capacity (WAC), dry basis holding capacity (DHC), and rehydration ability (RA) were used as indices to estimate the rehydration characteristics, according to Markowski and Zielinska (2011).

2.8. Statistical analysis

All experiments and analyses were conducted in triplicate, and data are expressed as the mean value ± standard deviation (S.D.). Statistical data analysis was performed with Statgraphics Centurion XVI software (Statistical Graphics Corp., USA). The t-Student test was applied to examine the differences between C-15 and US-15 and C-30 and US-30 at a level of significance of p < .05. Fitting procedure and the dehydration and rehydration constants were obtained using least-squares analysis by Solver in Microsoft Excel 2010 (Microsoft Corporation Inc., New York, NY, USA). The coefficient of determination (R²), reduced chi-square (χ²), and root-mean-square error (RMSE) were the three criteria of statistical analysis used to evaluate the adjustment of the experimental data from the dehydration and rehydration studies to the different models. Lower values of χ² and RMSE, and higher value of R² indicate better fitting.

3. Results and discussion

3.1. Dehydration kinetics

The variation of the MR of tilapia cubes as a function of time and ultrasound treatment during the dehydration is shown in Figure 1. Such a figure shows that MR of the dehydration fish reduced exponentially as the dehydration time increased, which is consistent with the dehydration of most biological material, including fishes as tilapia (Kituu et al., 2010) and pirarucu (Martins & Pena, 2017). Diffusion is a basic physical mechanism for moisture movement. At the initial stage of the dehydration process, the moisture movement is rapid because of the evaporation of moisture at the product’s surface. However, it decreases exponentially while dehydration because of the difficulty of the transport of the evaporated water through material (Başlar et al., 2015), as was observed in this study.

Figure 1. Effect of ultrasound treatment on dehydration kinetics of tilapia cubes: (a) control 15 min (C-15), ultrasound 15 min (US-15), (b) control 30 min (C-30), ultrasound 30 min (US-30).

Figura 1. Efecto del tratamiento del ultrasonido en las cinéticas de deshidratación de cubos de tilapia: (a) control 15 min (C-15), ultrasonido 15 min (US-15), (b) control 30 min, ultrasonido 30 min (US-30 min)

On the other hand, power ultrasound is an emerging technology that can induce chemical, biological and mechanical changes in meat due to cavitation in liquid systems, which contribute to accelerated mass transport (Kang, Gao, Ge, Zhou, & Zhang, 2017). In the present study, the results indicated that the ultrasound treatments reduced the dehydration time of tilapia cubes 19.2% (Figure 1(a)) and 8.7% (Figure 1(b)) for US-15 and US-30, respectively, in comparison with the control treatments. The shortest dehydration time of US-15 may be due to the lower amount of moisture that should be removed during dehydration, since it is possible that US-30 have increased the diffusion of water and salt to the tilapia cubes during the osmotic treatment with ultrasound (Bellary & Rastogi, 2012).

The values of R² (>0.9676), of χ² (<0.0032), and of RMSE (<0.2506) presented in Table 2 indicate that, overall, all models tested are able to predict, with good precision, the dehydration kinetics of tilapia cubes in the different conditions studied. However, the Page model had the best fit to all studied treatments (R² = 0.9889–0.9977; χ² = 0.0005–0.0020; RMSE = 0.0700–0.1280), in accordance with the studies of hot air drying of thin-layer fresh tilapia fillets (Ponwiboon & Rojanakorn, 2017), drying of titus fish fillets using an indirect passive hybrid solar-charcoal smoke dryer (Nwakuba, Okafor, Abba, & Nwandikom, 2018), as well as drying salmon and trout fillets using oven drying (Başlar et al., 2015).

Table 2. Kinetic parameters for the models fitted to the dehydration kinetic data of tilapia cubes.

Tabla 2. Parámetros cinéticos para los modelos ajustados a los datos cinéticos de deshidratación de cubos de tilapia

Models	Parameter	Treatments
		C-15	C-30	US-15	US-30
Page	k	0.3305 ± 0.06	0.3361 ± 0.04	0.3421 ± 0.03	0.3337 ± 0.08
	n	0.9314 ± 0.07	0.8849 ± 0.04	0.8928 ± 0.02	1.1708 ± 0.08
	R^2	0.9946	0.9889	0.9977	0.9944
	χ^2	0.0011	0.0020	0.0005	0.0009
	RMSE	0.1215	0.1660	0.0700	0.1280
Wang & Singh	a	−0.1741 ± 0.01	−0.2029 ± 0.02	−0.3756 ± 0.05	−0.2858 ± 0.04
	b	0.0029 ± 0.03	0.0034 ± 0.07	0.0064 ± 0.01	0.0043 ± 0.01
	R^2	0.9885	0.9902	0.9906	0.9676
	χ^2	0.0025	0.0012	0.0016	0.0032
	RMSE	0.1824	0.1321	0.1487	0.2506
Modified Page	k	0.3245 ± 0.03	0.3492 ± 0.04	0.3442 ± 0.05	0.3608 ± 0.03
	n	0.9140 ± 0.07	0.8966 ± 0.05	0.8486 ± 0.05	0.9863 ± 0.07
	R^2	0.9882	0.9945	0.9945	0.9941
	χ^2	0.0025	0.0013	0.0013	0.0009
	RMSE	0.1765	0.0260	0.0260	0.1306
Lewis	k	0.2184 ± 0.02	0.1903 ± 0.03	0.2806 ± 0.03	0.2908 ± 0.03
	R^2	0.9761	0.9739	0.9933	0.9941
	χ^2	0.0025	0.0052	0.0013	0.0009
	RMSE	0.1760	0.2861	0.1211	0.1306

Values of kinetic parameters are mean ± standard deviation of three samples measured in each experiment repeated three times. C-15 = control 15 min, US-15 = ultrasound 15 min, C-30 = control 30 min, and US-30 = ultrasound 30 min.

Los valores de los parámetros cinéticos son la media ± desviación estándar de tres muestras medidas en cada experimento repetido tres veces. C-15 = control 15 min, US-15 = ultrasonido 15 min, C-30 = control 30 min, and US-30 = ultrasonido 30 min.

3.2. Chemical composition

Table 3 shows the chemical composition of the dehydrated tilapia cubes by the effect of the ultrasound treatment. According to the obtained results, US-15 clearly influenced a higher elimination of water on the dehydration of tilapia cubes compared with C-15. The moisture content of US-15 was reduced 8.8 times with respect to the tilapia fresh, while the corresponding to US-30 was only 7.1 times. Under ultrasound treatment, the treated material continues to contract and expand with repeated tension and compression. This behavior results in the breaking of the bonds that link the water molecules and surface molecules of a solid, which activates the solid surface and facilitates the removal of moisture tightly bound to the material (Li et al., 2017), as was observed in this study. Due to the moisture elimination, the protein, fat and ash contents of the dehydrated tilapia cubes were concentrated with respect to the tilapia fresh (Table 3). However, the protein, fat and ash contents resulted significantly different (p < .05) between US-15 and C-15, while the fat and ash contents too resulted significantly different (p < .05) between US-30 and C-30. From all chemical components of the dehydrated tilapia cubes, the ash content had a higher increase due to that the ultrasound treatment was applied in saline solution with the consequent diffusion of salt to the tilapia cubes (Deng et al., 2014).

Table 3. Effect of ultrasound treatment on the chemical composition, color, water activity and microbiological characteristics of dehydrated tilapia cubes.

Tabla 3. Efecto del tratamiento de ultrasonido en la composición química, color, actividad de agua y características microbiológicas de cubos de tilapia deshidratados

		Treatments
Properties	Fresh tilapia	C-15	US-15	C-30	US-30
Chemical composition (g/100 g)
Moisture	78.12 ± 0.14	13.07 ± 0.07^a	8.88 ± 0.32^b	12.23 ± 0.76^a	11.00 ± 0.52^a
Protein (N x 6.25)	18.98 ± 0.44	62.50 ± 0.83^a	72.93 ± 0.91^b	64.21 ± 0.88^a	65.68 ± 0.59^a
Fat	1.86 ± 0.01	11.15 ± 0.75^a	9.21 ± 0.49^b	8.99 ± 0.23^a	10.36 ± 0.31^b
Ash	1.04 ± 0.51	13.28 ± 0.53^a	8.98 ± 0.57^b	14.57 ± 0.10^a	12.96 ± 0.07^b
Color
L*	48.98 ± 0.30	48.36 ± 1.04^a	50.48 ± 0.83^b	48.17 ± 1.03^a	48.42 ± 0.86^a
a*	4.32 ± 0.28	4.60 ± 0.33^a	4.52 ± 0.44^a	4.65 ± 0.28^a	4.56 ± 0.30^a
b*	18.30 ± 0.81	21.70 ± 0.74^a	24.06 ± 0.64^b	20.06 ± 0.88^a	22.96 ± 0.93^b
Wh (%)	45.43 ± 0.35	43.8 ± 0.23^a	44.8 ± 0.32^b	44.2 ± 0.15^a	43.4 ± 0.21^b
aw	0.953 ± 0.003	0.442 ± 0.002^a	0.403 ± 0.003^b	0.551 ± 0.001^a	0.542 ± 0.004^b
Microbiological characteristics
Aerobic microorganisms (CFU/g)	25	<10	<10	<10	<10
Molds (CFU/g)	<10	<10	<10	<10	<10
Yeasts (CFU/g)	<10	<10	<10	<10	<10
Total coliforms (CFU/g)	19	<10	<10	<10	<10
Fecal coliforms (MPN/g)	<3	<3	<3	<3	<3

Values are mean ± standard deviation of three samples measured in each experiment repeated three times. Mean values between C-15 (control 15 min) and US-15 (ultrasound 15 min), and between C-30 (control 30 min) and US-30 (ultrasound 30 min) followed by the same superscript letter are not significantly different (p < 0.05)

Los valores son la media ± desviación estándar de tres muestras medidas en cada experimento repetido tres veces. Los valores de las medias entre C-15 (control 15 min) y US-15 (ultrasonido 15 min), y entre C-30 (control 30 min) y US-30 (ultrasonido 30 min) seguidas del mismo superíndice no son significativamente diferentes (p < 0.05).

3.3. Physical characteristics

Table 3 shows the effect of ultrasound treatment on the color and a_w of the dehydrated tilapia cubes. The color of dehydrated food has a great influence on the acceptability of the product by consumers. It directly reflects the phenomenon of biochemical, microbiological and physiological changes in muscle tissue (Li, Wu, & Guan, 2017). In addition, Wh is an important evaluation indicator in aquatic products, because of during the drying process, the color of aquatic products changes in response to the effect of the amino-carbonyl reaction and the drying time, which directly influence the sensory quality of the products (Li et al., 2017). The values of L* and b* of US-15 and the value of b* of US-30 showed slight increases with respect to the controls, while the US-15 improved 1% of Wh with respect to the C-15 but the US-30 decreased 0.8 of Wh. According to Li et al. (2017), the ultrasound treatment in the range of 200–500 W on tilapia fillets (100 mm × 50 mm × 5 mm) subjected to heat-pump drying at 45°C with a wind speed of 2.5m/s, the Wh varied between 41.52% and 44.16%, corresponding the maximum value for the ultrasound treatment at 400 W which was only 1.42% lower than US-15 (44.8%) of this study. The a_w values observed for the control and ultrasound treatments of dehydrated tilapia cubes were below the limiting level of this parameter to ensure microbial stability, because it is generally accepted that no microbial growth will occur at a_w < 0.66 (Ulloa et al., 2015). However, the US-15 and US-30 had lower values than the control treatments because sound waves passing through the food medium make moisture removal easier (Awad, Moharram, Shaltout, Asker, & Youssef, 2012), which also influenced the reduction in a_w of dehydrated tilapia cubes.

3.4. Microbiological characteristics

Fish products are extremely perishable and sensitive to microbial growth. Bacterial growth also causes spoilage, leading to the formation of off-odors and off-flavors. Spoilage is typically detected when microbial counts are above 10⁶–10⁷ CFU/g (Sugawara & Nikaido, 2014). The microbial contamination of fish can occur in the environment or during handling, processing, transport, and storage (Semeano et al., 2018). Table 3 shows the results of the microbial counts of fresh tilapia and the dehydrated tilapia cubes by the effect of ultrasound treatment. According to such results, the ultrasound treatments did not have an effect on the microbiological counts of the dehydrated tilapia cubes (US-15 and US-30) because shown the same values of microbial counts than the control treatments (C-15 and C-30). However, the counts of the aerobic microorganisms and total coliforms were lower in all dehydrated tilapia cubes than the fresh tilapia, maybe for the washing effect of osmotic soaking in the saline solution applied to the tilapia cubes before dehydration. In any case, all the dehydrated tilapia cubes showed an acceptable microbiological quality, which varies widely depending on the type of dehydrated food and microbial group. In the case of dried food-grade gelatin, dehydrated space food, and processed spices, the limits of the aerobic mesophilic bacteria counts are 10³, 10⁴ and 10⁶ CFU/g, respectively (Jay, Loessner, & Golden, 2005a). In contrast, the safety limit for total coliforms in dehydrated egg products and dry milk powder are 10³ and 10² CFU/g, respectively (Jay, Loessner, & Golden, 2005b), while for fishing products as fresh, chilled, frozen and smoked, the fecal coliforms are 230–400 MPN/g according to the Mexican legislation (Secretaria de Salud [SSA], 2010). Therefore, dehydrated tilapia cubes from this study had better microbiological characteristics compared with the above-mentioned foods.

3.5. Microstructure

The microstructures of the dehydrated tilapia cubes are illustrated in Figure 2. Figure 2(b) (US-15) and 2d (US-30) show a porous structure, with large and irregular cavities or pores resulted from water elimination during dehydration, in comparison with control treatments of Figure 2(a) (C-15) and 2c (C-30), which have the most compact and coherent structure. According to Deng et al. (2014), the degree of porosity improved the texture and rehydration ability of dehydrated squid fillets, in comparison with the same product of compact structure, while Rajewska and Mierzwa (2017) demonstrated that ultrasound increased the porosity of plant tissues, as was observed in this study.

Figure 2. Effect of ultrasound treatment on the microstructure of the dehydrated tilapia cubes: (a) control 15 min, (b) ultrasound 15 min, (c) control 30 min, (d) ultrasound 30 min.

Figura 2. Efecto del tratamiento de ultrasonido en la microestructura de los cubos de tilapia deshidratados: (a) control 15 min, (b) ultrasonido 15 min, (c) control 30 min, (d) ultrasonido 30 min

3.6. Rehydration characteristics

Figure 3 shows the effect of ultrasound treatment on the rehydration kinetics of the dehydrated tilapia cubes at 30°C, 40°C and 50°C. In all the cases, the amount of moisture absorbed increases with rehydration time but at a decreasing rate up to saturation level. On the other hand, the time of the rehydration stability was reduced of 160 min to 120 at 40°C and 50°C in comparison with 30°C in both ultrasound and control treatments.

Figure 3. Effect of ultrasound treatment on the rehydration kinetics at 30°C, 40°C and 50°C (a) and Arrhenius-type relationship between effective diffusivity (D_eff) and absolute temperature for estimation of activation energy of the rehydration of dehydrated tilapia cubes (b). Control 15 min (○ C-15), ultrasound 15 min (● US-15), control 30 min (○ C-30), ultrasound 30 min (● US-30).

Figura 3. Efecto del tratamiento de ultrasonido en las cinéticas de rehidratación a 30°C, 40°C y 50°C (a) relación tipo-Arrhenius entre la difusividad efectiva (D_eff) y temperatura absoluta para la estimación de la energía de activación de la rehidratación de cubos de tilapia deshidratada (b). Control 15 min (○ C-15), ultrasonido 15 min (● US-15), control 30 min (○ C-30), ultrasonido 30 min (● US-30)

Increased level of moisture absorption during the beginning of soaking period is attributed to surface adsorption and capillary action (Seremet, Botez, Nistor, Andronoiu, & Mocanu, 2016), while the reduction after is attributed to the drop in driving force required for moisture uptake when the rehydration progresses and approaches the equilibrium (Moreira, Chenlo, Chaguri, & Fernandes, 2008). Observations similar to those obtained in this study were reported in the previous studies for squid fillets subjected to hot air drying, freeze-drying and heat pump drying (Deng et al., 2014), squid fillets by infrared radiation assisted heat pump drying (Wang et al., 2016), salted cod dried at low-temperature assisted by high power ultrasound (Ozuna, Cárcel, Walde, & Garcia-Perez, 2014), and microwave vacuum-dried mackerel (Viji et al., 2019).

In addition, the ultrasound treatment increased the amount of moisture absorbed of the dehydrated tilapia cubes, with respect to the control treatments at each rehydration temperature. However, the difference of the water absorbed between the control treatments and ultrasound treatments of dehydrated tilapia cubes was variable depending on the ultrasound time and temperature. For example, the RR for US-15 and C-15 at 30°C were 2.24 and 1.59 (29% of difference), respectively, while the RR for US-30 and C-30 at 50°C were 2.14 and 2.07 (3.4% of difference), respectively, which are the extreme cases. According to Ozuna et al. (2014), the application of ultrasound during drying affected the rehydration ability of dried-salted cod samples, exhibiting a higher final weight gain than control treatments, such as was observed in this study; the higher water gain in dehydrated tilapia cubes could be linked to changes produced by ultrasound in the microstructure (Figure 2).

The rehydration data of dehydrated tilapia cubes obtained from the experiments were fitted to the mathematical models mentioned in Table 1, whose kinetic parameters are summarized in Table 4. The best model describing the rehydration at different conditions of dehydrated tilapia cubes (taking in count the highest R² value and the lowest χ² and RMSE values) was the named First Order model. Some foods where the First Order model properly described the rehydration kinetics are dry blueberries (Zielinska & Markowski, 2016) and instant whole bean (Rezendiz Vazquez, Ulloa, Rosas Ulloa, & Ramírez Ramírez, 2015), in congruency with the results obtained in this study.

Table 4. Effect of temperature and ultrasound treatment on the kinetic parameters for the models fitted to the rehydration data of dehydrated tilapia cubes.

Tabla 4. Efecto de la temperatura y tratamiento de ultrasonido en los parámetros cinéticos para los modelos ajustados a los datos de rehidratación de cubos de tilapia deshidratados

			Treatment
Model	Temperature	Parameter	C-15	US-15	C-30	US-30
Peleg	30°C	k1	0.88 ± 0.03	0.64 ± 0.02	0.84 ± 0.07	0.60 ± 0.02
		k2	6.25 ± 0.50	8.22 ± 0.04	7.02 ± 0.04	7.56 ± 0.10
		R^2	0.9671	0.9584	0.9672	0.9567
		χ^2	0.0156	0.0470	0.0193	0.0445
		RMSE	0.1043	0.1704	0.0942	0.1541
	40°C	k1	0.85 ± 0.01	0.49 ± 0.04	0.82 ± 0.02	0.64 ± 0.01
		k2	7.21 ± 0.21	8.43 ± 0.19	7.49 ± 0.38	8.42 ± 0.25
		R^2	0.9650	0.9527	0.9606	0.9509
		χ^2	0.0481	0.0808	0.6693	0.0894
		RMSE	0.1708	0.2123	0.1942	0.2303
	50°C	k1	0.83 ± 0.01	0.43 ± 0.02	0.78 ± 0.01	0.66 ± 0.03
		k2	7.79 ± 0.22	8.84 ± 0.09	8.53 ± 0.08	8.52 ± 0.04
		R^2	0.9706	0.9529	0.9688	0.9536
		χ^2	0.0217	0.0919	0.0362	0.0648
		RMSE	0.1013	0.2207	0.1407	0.1812
First order	30°C	kr	0.36±0.01	0.48±0.06	0.38±0.01	0.41±0.01
		R^2	0.9956	0.9954	0.9959	0.9933
		χ^2	0.0024	0.0039	0.0024	0.0048
		RMSE	0.1725	0.1729	0.1492	0.2503
	40°C	kr	0.39±0.02	0.51±0.01	0.43±0.01	0.49±0.01
		R^2	0.9917	0.9904	0.9979	0.9941
		χ^2	0.0062	0.2654	0.0013	0.0044
		RMSE	0.2152	0.0099	0.1068	0.1713
	50°C	kr	0.41±0.02	0.58±0.05	0.49±0.05	0.54±0.02
		R^2	0.9926	0.9927	0.9936	0.9708
		χ^2	0.0048	0.0057	0.0046	0.0096
		RMSE	0.1906	0.1066	0.1371	0.2550
Weibull	30°C	α	0.64±0.05	0.57±0.02	0.69±0.08	0.69±0.02
		β	27.74±0.06	19.63±0.53	25.42±0.06	25.20±0.77
		R^2	0.7019	0.8764	0.7193	0.8124
		χ^2	0.1720	0.1114	0.1417	0.1258
		RMSE	1.2473	0.7869	1.0368	1.0261
	40°C	α	0.52±0.01	0.43±0.01	0.54±0.13	0.41±0.01
		β	25.27±0.89	16.97±0.16	21.92±0.45	18.45±0.78
		R^2	0.8737	0.8833	0.7980	0.8302
		χ^2	0.0950	0.1014	0.1252	0.0805
		RMSE	0.5684	0.5263	0.7165	0.6568
	50°C	α	0.42±0.01	0.40±0.01	0.49±0.01	0.44±0.06
		β	20.39±0.84	14.63±0.32	21.58±0.51	20.11±0.95
		R^2	0.6977	0.8007	0.9090	0.8434
		χ^2	0.1628	0.1758	0.0998	0.1371
		RMSE	1.2565	0.9544	0.6491	0.8225
Exponential	30°C	kr	0.34±0.01	0.47±0.01	0.37±0.01	0.34±0.01
		R^2	0.9941	0.9948	0.9940	0.9919
		χ^2	0.4800	0.0045	0.0033	0.0055
		RMSE	0.1748	0.1840	0.1607	0.2459
	40°C	kr	0.37±0.02	0.54±0.01	0.43±0.01	0.49±0.01
		R^2	0.9847	0.9857	0.9976	0.9928
		χ^2	0.0062	0.2654	0.0013	0.0044
		RMSE	0.2152	0.0099	0.1068	0.1713
	50°C	kr	0.47±0.02	0.60±0.05	0.50±0.05	0.56±0.02
		R^2	0.8360	0.9945	0.9926	0.9922
		χ^2	0.1088	0.0048	0.0046	0.0096
		RMSE	0.1906	0.1066	0.1371	0.2550

Values of kinetic parameters are mean ± SD of three samples measured in each experiment repeated three times. C-15 = control 15 min, US-15 = ultrasound 15 min, C-30 = control 30 min, and US-30 = ultrasound 30 min.

Los valores de los parámetros cinéticos son la media ± desviación estándar de tres muestras medidas en cada experimento repetido tres veces. C-15 = control 15 min, US-15 = ultrasonido 15 min, C-30 = control 30 min, and US-30 = ultrasonido 30 min.

Most dehydrated products are rehydrated before use. Rehydration can be considered as an indirect measure of the damage to the material caused by dehydration and treatment preceding dehydration (Omolola, Jideani, & Kapila, 2017). Table 5 shows the effect of ultrasound treatment on the DHC, WAC and RA of dehydrated tilapia cubes. The values of WAC and RA ranged from 0.82 to 0.90 for the dehydrated tilapia cubes exposed to ultrasound treatment different, in comparison with the control treatments, while the DHC value was 1.0 in all cases. The ultrasound treatment improved 7.3%, 4.7%, and 7.1% the WAC of US-15 at 40°C, US-30 at 50°C and 7.1% at 60°C, respectively, in contrast with the control treatments, while the RA values were increased by 7.3%, 4.7% and 7.1% in US-15 at 40°C, US-30 at 50°C and US-15 at 60°C, respectively. According to Omolola et al. (2017), the DHC, WAC and RA values for squid fillets subjected to heat-pump drying, freeze-drying and hot-air drying were 0.856–0.970, 0.348–0.408 and 0.299–0.396, respectively, which were lower than the obtained for the dehydrated tilapia cubes of this study. The higher water gain of dehydrated tilapia cubes with ultrasound treatments in contrast with control treatments after rehydration, it could be linked to changes produced by ultrasound in the microstructure during dehydration as has been previously reported (Ozuna et al., 2014), which improve the WAC and RA.

Table 5. Effect of ultrasound treatment on water absorption capacity (WAC), dry matter holding capacity (DHC), rehydration ability (RA), and effective diffusivity (D_eff) of dehydrated tilapia cubes rehydrated at 30°C, 40°C and 50°C.

Tabla 5. Efecto del tratamiento del ultrasonido en la capacidad de absorción de agua (WAC), capacidad de retención de materia seca (DHC), capacidad de rehidratación (RA) y difusividad efectiva (D_eff) de cubos de tilapia deshidratados, rehidratados a 30°C, 40°C y 50°C

Treatments	Soakingtemperature (°C)	WAC	DHC	RA	Deff (m^2/s)
C-15	30	0.82 ± 0.02^a	1.00 ± 0.01^a	0.82 ± 0.02^a	6.30 x 10^−09 (±1.23 x 10^−09)^a
US-15		0.88 ± 0.01^b	1.00 ± 0.01^a	0.88 ± 0.01^b	7.17 x 10^−09 (±9.44 x 10^−10)^a
C-30		0.84 ± 0.02^a	1.00 ± 0.01^a	0.84 ± 0.02^a	6.35 x 10^−09 (±7.22 x 10^−10)^a
US-30		0.85 ± 0.02^a	1.00 ± 0.01^a	0.85 ± 0.02^a	6.80 x 10^−09 (±3.91 x 10^−10)^a
C-15	40	0.87 ± 0.02^a	1.00 ± 0.01^a	0.87 ± 0.02^a	7.81 x 10^−09 (±1.00 x 10^−09)^a
US-15		0.88 ± 0.02^a	1.00 ± 0.01^a	0.88 ± 0.02^a	8.80 x 10^−09 (±3.28 x 10^−10)^a
C-30		0.84 ± 0.02^a	1.00 ± 0.01^a	0.84 ± 0.02^a	7.74 x 10^−09 (±8.60 x 10^−10)^a
US-30		0.88 ± 0.01^b	1.00 ± 0.01^a	0.88 ± 0.01^b	8.14 x 10^−09 (±1.05 x 10^−09)^a
C-15	50	0.84 ± 0.01^a	1.00 ± 0.01^a	0.84 ± 0.01^a	9.26 x 10^−09 (±1.05 x 10^−10)^a
US-15		0.90 ± 0.01^b	1.00 ± 0.01^a	0.90 ± 0.01^b	9.80 x 10^−09 (±1.14 x 10^−09)^b
C-30		0.87 ± 0.02^a	1.00 ± 0.01^a	0.87 ± 0.02^a	8.82 x 10^−09 (±9.12 x 10^−10)^a
US-30		0.90 ± 0.02^a	1.00 ± 0.01^a	0.90 ± 0.02^a	9.74 x 10^−09 (±2.58 x 10^−10)^a

Values are mean ± standard deviation of three samples measured in each experiment repeated three times. Mean values between each control and ultrasound treatment with a different superscript at each temperature indicate a significant difference (p < 0.05). C-15 = control 15 min, US-15 = ultrasound 15 min, C-30 = control 30 min, and US-30 = ultrasound 30 min.

Los valores son la media ± desviación estándar de tres muestras medidas en cada experimento repetido tres veces. Los valores de las medias entre cada tratamiento control y ultrasonido con diferente superíndice a cada temperatura son significativamente distintos (p < 0.05). C-15 = control 15 min, US-15 = ultrasonido 15 min, C-30 = control 30 min, y US-30 = ultrasonido 30 min.

The influence of ultrasound treatment on the D_eff for rehydration of dehydrated tilapia cubes is shown in Table 5, whose values were within of general range of 10⁻⁸ and 10⁻¹⁰ m.s⁻² for biological materials (Başlar et al., 2015). D_eff values of US-15 at 30°C and 40°C increased significantly (p < .05) 13.8% and 12.7%, respectively, while D_eff values of US-30 at 40°C and 50°C increased significantly (p < .05) 5.2% and 10.4%, respectively, in comparison with the control treatments. According to Santacatalina, Guerrero, Garcia-Perez, Mulet, and Cárcel (2016), the D_eff for rehydration of desalted cod dried at 0°C and 10°C increased by effect of ultrasound treatment (20.5 kW/m³, 21 kHz) 23.8% and 13.56%, respectively, in agreement with the results above of this study. The values of ln D_eff plotted versus the reciprocal of the temperature (1/(T + 273.15)) for dehydrated tilapia cubes by the effect of ultrasound treatment for rehydration are shown in Figure 3. The slope of the line is -E_a/R and the intercept equals ln (D₀). The results show a linear relationship due to Arrhenius-type dependence, which is confirmed by the high value of R². The E_a values were found to be 15.88 kJ/mol and 12.67 kJ/mol for C-15 and US-15, respectively, and 13.38 kJ/mol and 14.49 kJ/mol for C-30 and US-30, respectively. According to Tekin and Balsar (2018), there is a direct correlation between E_a and the energy needed to facilitate diffusion, that is, when less E_a is required, less energy is needed to facilitate diffusion, which clearly demonstrated that ultrasound improved the rehydration of US-15 in contrast with the control treatment. The E_a values reported for rehydration of some foods are 3.17 kJ/mol (Demiray & Tulek, 2017), 0.85–20.50 kJ/mol (Zura et al., 2013), and 36.04 kJ/mol (Pramiu, Rizzi, Do Prado, Coelho, & Bassinello, 2015), for sun-dried red pepper, Chilean papaya, and chickpea, respectively, whose range of E_a comprises the values obtained for the dehydrated tilapia cubes of this study.

4. Conclusions

The ultrasound treatments for 15 and 30 min reduced the dehydration times at 35°C of tilapia cubes l8.7% and 19.2%, respectively, and the dehydration kinetics were well described by Page’s Model. The ultrasound favored beneficial changes as higher values of protein and lower values of moisture and a_w of the dehydrated tilapia cubes, than dehydrated tilapia cubes that did not receive such treatment. In addition, ultrasound caused porous structures with large and irregular cavities on the surface of dehydrated tilapia cubes, which improved some rehydration characteristics depending on the rehydration temperature and ultrasound time. These results may be important to promote the use of ultrasound as a treatment for the production of dehydrated tilapia cubes as a food of easy and practical preparation, as demanded by the current lifestyle of the population.

Acknowledgments

The authors wish to thank CONACYT-Mexico for the scholarship granted to I.Q. Yesenia Anahí Olvera Ríos during her master’s studies.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by Patronage to Administer the Special Tax Destined to the Autonomous University of Nayarit (PUAN-CP-003/2019).