Low temperature and high velocity (LTHV) application in drying: Characteristics and effects on the fish quality

Journal of Food Engineering 91 (2009) 173–182

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Low temperature and high velocity (LTHV) application in drying: Characteristics and effects on the fish quality Aydin Kilic * Biology Department, Faculty of Science, Nigde University, 51200 Nigde, Turkey

a r t i c l e

i n f o

Article history: Received 1 May 2008 Received in revised form 16 August 2008 Accepted 21 August 2008 Available online 28 August 2008 Keywords: Fish drying Cold drying Drying quality Rainbow trout

a b s t r a c t The present study investigates the cold air drying characteristics and their effects on fish quality. For this purpose, a single layer drying with low temperature and high velocity (LTHV) is applied, and 200 g each of the salted rainbow trout (Oncorhynchus mykiss) is used under the following experimental conditions: drying temperature (4,10,15 and 20 °C), velocity (7 m/s) and relative humidity (40–50%). Based on the experimental data for the salted fish samples dried, some key characteristics of such a drying process, namely moisture content, dimensionless mass loss, drying rate, drying time and the best drying temperature are determined and discussed in detail. In addition, some food quality indicators such as mass shrinkage, total volatile nitrogen, thiobarbituric acid, free fatty acids, and microbiological properties are investigated for the salted fish samples. Consequently, an optimum drying temperature is found as 4 °C since the fish samples have the best quality. Therefore, it is suggested that LTHV method be applied to prevent or minimize the microbiological and biochemical decompositions of the fish. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Drying is considered the oldest food preservation technique, and a common unit operation in many chemical and process industries. The removal of moisture prevents the growth and reproduction of microorganisms causing decay and minimizes many of the moisture mediated deteriorative reactions (Dincer, 1996, 1998; Dincer et al., 2002; Akpinar et al., 2003). Drying is also particularly considered an important technique to conserve the perishable foods (Akpinar et al., 2003). Drying processes can be conducted either at high temperature and short time (HTST) or at low temperature and long time (LTLT) (Arason, 2003; Piga et al., 2003; Lewicki, 2006). In this regard, many types of dryer providing cold air or hot air can be used. Actually, in this study, cyclone type cold air dryer is used. In the thermal processes of perishable foods, cooling is employed as one of the preservation techniques to prevent their spoilage and maintain their quality (e.g., self-life) (Abbas et al., 2006). Under these considerations, it can be said that drying is a process of simultaneous heat and moisture transfer, which induces changes in the product undergoing dehydration (Midilli and Kucuk, 2003). The quality of the dried product is greatly influenced by drying conditions. The higher temperature of the material during drying leads to several irreversible biological or chemical reactions as well as structural, physical and mechanical modifications, includ* Tel.: +90 388 2252115;fax: +90 388 2250180. E-mail address: [email protected] 0260-8774/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2008.08.023

ing the coloring, crust formation, decrease of sensory quality, inactivation of bacteria and enzymes, loss of nutrients and aroma, and changes of shape and texture (Abid et al., 1990). In this case, undesired food flavor, color, vitamin degradation and the loss of essential amino acids may be produced. However, these properties of fresh food can be maintained if drying can be performed at low temperature (Kosuke et al., 2006). Low temperature drying process has a positive influence on the quality of biological materials but require longer processing times, which have a detrimental effect on quality and a higher cost of operation. For example, cold air drying of fish minimizes fatty acid oxidation and reduces protein denaturizing because low-temperature drying process results in lower nutrient degradation (Lewicki, 2006). During low temperature drying, lipid oxidation and antioxidant losses can be reduced by short drying times, low temperatures, and low pressures. Thus, this type of drying process should aim at minimizing the chemical degradation reactions since loss of nutrient can be viewed as the decomposition of a particular chemical compound (Van Loey et al., 2005). Under these important explanations, the main objective of the present study is to experimentally investigate the cold air drying characteristics and their effects on the general quality indicators of fish samples. This lack of information is the motivation for this work. Moreover, its originality comes from the application of LTHV single layer drying technique for fish quality. As a result of this work, it can be concluded that, comparing to the microbiological and chemical properties of fresh and dried fishes in the literature, the LTHV single layer drying technique contribute to improve the

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Nomenclature M Mt Mo Me Mt+Dt t W v MSR

weight loss, kg weight of fish at t, kg weight of fish at t = 0, kg weight of fish in equilibrium state, kg moisture content at t + Dt, kg water/kg dry matter, wb time, h moisture content,% the constant of drying velocity, h1 Mass shrinkage ratio

CP

control point

Abbreviations TVB-N Total volatile basic nitrogen TBARS Thiobarbituric acid reactive substances FF Free fatty acids TVC Total viable count TPC Total psychrophilic count TYM Total yeast and mould Water activity aw

Subscript e equilibrium state

quality indicators of dried fish in this work. This important result indicates that LTHV single layer drying technique would be an alternative to drying methods for the perishable food industry or semi-dried foods. Accordingly, in practice, it is expected that this paper will contribute to increase the motivation on the application of LTHV thin layer drying process in perishable food industry.

where dM/d,is the drying rate at any time of drying, Mt+Dt is moisture content at t = t + Dt and Mt is moisture content at t = t.

2. Mathematical formulation

Experimental setup was constructed in a cold store of Biology Laboratory at Nigde University. The experimental set-up (Fig. 1) for the cold air drying process of the fish samples consists of a cyclone-type dryer, a compressor of the cooling system, a cold air evaporator and a circulatory radial fan. Dryer has three stainless steel trays, which were made of wiremesh-bottomed trays, and a diameter of 60 cm and was insulated. The evaporator has a diameter 60 cm and was connected to a radial fan by a pipe equipped a damper. Also, the radial fan was connected to drying cupboard by a pipe equipped a damper and used when cold-air was enough to cool fresh air. The pipes have a diameter 30 cm. The radial fan was also attached to outlet of circulation fan and inlet of cold air dryer.

It was considered that the weight loss equation form was similar to moisture content equation form, and its development was approximately given in the open literature (Dincer and Dost, 1995). Weight loss was estimated using Eq. (1) (Verma et al., 1985; Midilli, 2001), depending on weight changes of fish samples during the drying experiments

M¼

Mt  Me Mo  Me

ð1Þ

where M is the weight loss; M0 the weight of fish at t = 0; Me the weight of fish in equilibrium state and Mt, the weight of fish at t. Using the experimental results, the curve equation of weight loss can be derived applying Eq. (2) (Ozbalta and Tiris, 1991). It was assumed that the exponential form of weight loss was similar to the exponential form of moisture distribution. The development of this equation was presented in the open literature (Dincer and Dost, 1995).

Mðt; v; M o Þ ¼ M o evt

ð2Þ

where t represents time; v the constant of drying velocity, and e equilibrium state. Moisture content of fish samples was calculated by using the mass changes based on drying time throughout the experiments. Eq. (3) (Tasdemiroglu, 1988) was applied to estimate the moisture content.

W¼

Mt  Me  100 Mo

ð3Þ

where W is the moisture content. Mass shrinkage ratio (MSR) of trout samples during drying was estimated by using Eq. (4) (Midilli et al., 2000, 2002).

MSR ¼

Mt Mo

ð4Þ

where MSR is the mass shrinkage ratio. Drying rate of the trout samples during drying were obtained by using Eq. (5)

dM M tþDt  Mt ¼ dt Dt

ð5Þ

3. Experimental set-up and procedure 3.1. Experimental set-up

3.2. Sample preparation Rainbow trout (Oncorhynchus mykiss), were used as a raw material for drying experiments. They were brought to the laboratory on ice. The mean weight of the trout average 300 g (condition factor = 1.33 ± 0.024) (one-year-old in average) was slaughtered, gutted, trimmed and washed. Five samples of the fresh fish have been taken to determine the fresh fish properties before salting and drying process. The other fresh fishes after sampling were salted (%2) by applying dry salting method for 0.5 h at +4 °C (see Table 2 for salt contents). After dry salting and washing process, all experimental fish groups were analyzed to determine the moisture, crude lipid, protein, ash, pH, salt, water activity (aw), condition factor, total volatile basic nitrogen (TVB-N), thiobarbituric acid reactive substance (TBARS) content, free fatty acid (FF), total viable count (TVC) and total yeast and mould (TYM) (see Table 3) before drying process. After this process, some experiments were further conducted using salted fish samples. Due to salting, osmotic effect is absolutely important. The properties of the fresh, salted and dried fish samples before and after drying process can be seen in Table 2 in detail. 3.3. Experimental procedure In order to investigate drying conditions of fish samples and to find an appropriate solution to drying problems for fish quality, all experiments were performed at 4, 10, 15 and 20 °C for constant drying air velocity (7 m/s). The cold-air evaporator, which is one

A. Kilic / Journal of Food Engineering 91 (2009) 173–182

175

Fig. 1. Cold air drying system: 1. Evaporator, 2. Fan, 3. Cold-air Velocity Balance Valve, 4. Radial Fan, 5. Velocity Balance Valve, 6. Cold Air Inlet, 7. Cold Air Outlet. CP: control point, L: Layer (h (height); h1 = 10 cm, h2 = 20 cm, h3 = 100 cm, h4 = h9 = 30 cm, h5 = h6 = h7 = h8 = 30 cm, h10 = 10 cm, l1 = l2 = 20 cm).

Table 1 Experimental condition of cold air fish drying Cold Air Drying Velocity = 7 m/s ± 0.01 Cold air drying temperature °C

T = 4 °C

T = 10 °C

T = 15 °C

T = 20 °C

Reference Temperatures °C Tray inlet temperatures °C Tray outlet temperatures °C Reference relative humidity (%) Tray inlet relative humidity (%) Tray outlet relative humidity (%) Dryer inlet relative humidity (%) Dryer outlet relative humidity (%)

6.54 ± 0.01

10.38 ± 0.01

16.56 ± 0.01

21.51 ± 0.01

4.70 ± 0.01

10.06 ± 0.01

15.24 ± 0.01

20.32 ± 0.01

4.90 ± 0.01

10.17 ± 0.01

15.48 ± 0.01

20.56 ± 0.01

33.81 ± 0.01

47.96 ± 0.01

40.35 ± 0.01

39.50 ± 0.01

37.80 ± 0.01

48.30 ± 0.01

42.13 ± 0.01

41.79 ± 0.01

38.02 ± 0.01

48.83 ± 0.01

42.32 ± 0.01

41.93 ± 0.01

38.68 ± 0.01

49.58 ± 0.01

42.76 ± 0.01

42.05 ± 0.01

38.30 ± 0.01

48.99 ± 0.01

42.47 ± 0.01

42.02 ± 0.01

of the most important parts of the experimental set up, was used as a cooler in the dryer. Before drying experiments, initial moisture contents of fish samples were determined as follows: 10 g of raw material was put down in the high temperature oven, and kept there at 105 °C for an equilibrium moisture. It was obtained that the fresh fish samples had average 70.7% of initial moisture content. On the other hand, instantaneous equilibrium states are not possible by the effect of cold drying-air flow. The experiments were started and the following procedures were particularly repeated in each experimental study. Fresh fish samples was carefully cleaned, trimmed and placed as 200 g samples into the middle tray as a 2.5 mm-single layer. 1st and 2nd trays were left simultaneously empty so that drying air could be homogeneously diffused inside of dryer. In the experiments, tem-

perature of the evaporator was adjusted by a thermostatic sensor. The cold air was absorbed by a circulatory radial fan. In the cold-air assisted drying, fresh air passing through the cooler evaporator. All temperatures were monitored with a temperature controller. Fig. 2 shows the parameter control diagram of cold air- thin layer drying process. During each drying experiment, the following were measured at every half an hour for each control point (CP). 1. Temperatures: all temperatures were measured with a METTLER model digital infrared thermometer, which are cold store temperature (Ts), evaporator outlet temperature (T1), dryer inlet temperature (T2), tray inlet temperature (T3), tray outlet temperature (T4) and dryer outlet temperature (T6). 2. Humidity: all values of humidity were measured with a METTLER model digital infrared humidity meter at various control points of the system, with reading accuracy of ± 0.01 °C), which are cold store humidity (RHs), evaporator outlet humidity (RH1), dryer inlet humidity (RH2), tray inlet humidity (RH3), tray outlet (RH4) and dryer outlet humidity (RH6). 3. Drying velocity: an AIRFLOW, TA-2 model automatic digital thermo anemometer was used to measure velocity at various control points of the system, with reading accuracy of ± 0.01). The velocities measured are evaporator outlet (V1), dryer inlet (V2), tray inlet (V3), tray outlet (V4), and dryer outlet (V6). 4. Weight loss: this was carefully weighed by using a Sinbo SMX 4507 model sensitive digital, with reading accuracy of ± 0.01 °C. Experimental conditions of fish drying are summarized in Table 1. After drying process, the dried fish samples were analyzed to determine the chemical and microbiological properties as well as quality indicators. In order to determine the dimensionless mass loss and mass shrinkage the weight loses of the samples that are experimentally

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Table 2 Chemical composition of the fish dried at 4, 10, 15, 20 °C Analysisa

Experimental groups Before drying

Moisture (%) (wb) Dry material g g1 Protein g g1 Lipid g g1 Ash g g1 Salt g g1 aw pH a

After drying

Raw fish

Salted fish

Drying at 4 °C

Drying at 10 °C

Drying at 15 °C

Drying at 20 °C

70.68 ± 0.07 0.294 ± 0.02 0.655 ± 0.04 0.243 ± 0.03 0.045 ± 0.003 0.003 ± 0.000 0.99 ± 0.07 6.03 ± 0. 08

68.24 ± 0.02 0.3208 ± 0.04 0.658 ± 0.07 0.2571 ± 0.03 0.0767 ± 0.01 0.0188 ± 0.003 0.98 ± 0.06 6.08 ± 0. 11

27.22 ± 0.08 0.500 ± 0.07 0.604 ± 0.02 0.277 ± 0.07 0.1084 ± 0.05 0.0755 ± 0.001 0.80 ± 0.08 6.11 ± 0.13

27.07 ± 0.01 0.489 ± 0.01 0.608 ± 0.02 0.268 ± 0.03 0.1124 ± 0.00 0.0751 ± 0.000 0.81 ± 0.06 6.13 ± 0.11

26.48 ± 0.21 0.490 ± 0.00 0.600 ± 0.04 0.271 ± 0.01 0.1023 ± 0.00 0.0738 ± 0.000 0.80 ± 0.08 6.11 ± 0.04

27.14 ± 0.16 0.482 ± 0.01 0.605 ± 0.02 0.280 ± 0.03 0.1043 ± 0.00 0.0738 ± 0.002 0.81 ± 0.08 6.11 ± 0.01

Drying at 15 °C

Drying at 20 °C

± mean values of ten samples standard deviation (n = 10).

Table 3 Some quality properties of the cold air dried fish at 4, 10, 15, 20 °C Analysis

Experimental groups Before drying Fresh fish

TVB-N(mg/100 g) TBARS (mg MA/kg) FFe TPC (log cfu/g) TVC (log cfu/g) TYM (log cfu/g)

After drying Salted

a

15.07 ± 0.22 0.34 ± 0.19a 0.2 ± 0.04a 2.3 ± 0.13a 2.0 ± 0.11a 2.1 ± 0.06a

Drying at 4 °C ab

17.27 ± 0.06 0.32 ± 0.03a 0.8 ± 0.11a 2.2 ± 0.22a 2.7 ± 0.08b 2.3 ± 0.07ab

ab

17.23 ± 0.02 0.42 ± 0.18a 1.2 ± 0.1a 2.3 ± 0.17a 2.6 ± 0.11b 2.6 ± 0.04b

Drying at 10 °C b

18.26 ± 0.58 0.73 ± 0.08ab 1.4 ± 0.3ab 2.6 ± 0.01ab 2.6 ± 0.13ab 2.5 ± 0.04ab

c

19.00 ± 0.19 1.04 ± 0.05b 1.4 ± 0.4ab 2.2 ± 0.15a 2.9 ± 0.11b 3.2 ± 0.14c

19.01 ± 0.18c 1.01 ± 0.13b 2.2 ± 0.7b 2.8 ± 0.17b 3.2 ± 0.08c 3.4 ± 0.16c

a–d e

Mean values within a row with different letter are significantly different (p < 0.05). Free fatty acid.

3.6. pH Cold air outlet

Drying cupboard

Cooling Unit

Layer 1

Control Points (CP) Temperature (T), Humidity (RH) Velocity (V)

Layer 2

Layer 3

Cold air inlet

Radial Fan

Cold air

Ten grams of each fish samples were blended with 90 ml distilled water. After filtering, pH measurements were carried out after cold air drying process an Ino lab ba 12217e model digital pH meter (AOAC, 1995). 3.7. Total volatile basic nitrogen (TVB-N) 10 grams of minced fish and 1 g magnesium oxide was added into a flask. Samples were boiled and distilled into 10 ml of HCl solution with an indicator in a 500 ml conical flask. After the distillation, the content of conical flask was titrated with 0.1 NaOH and defined as mg TVB-N per 100 g fish (Antonacopoulos and Vyncke, 1989). 3.8. Thiobarbituric acid reactive substances (TBARS)

Fig. 2. Parameter control diagram of cold air-thin layer drying process.

measured are taken in to the consideration. Therefore, there is no need to apply any method to determine these parameters. So, Eqs. (1) and (4) are employed used for calculation of the values of these parameters. 3.4. Chemical analysis The fish samples were analyzed for fresh, salted and dried fish. The moisture, protein, ash, fat and salt contents were determined by the standard methods of AOAC (1995). 3.5. Weight loss and aw Weight loss was measured in percentage. aw values were measured using an Aqua Lab Model cx2 (Sensitivity = ±0.003).

The TBARS is an index for lipid oxidation. TBARS was determined after cold air drying process for all groups, brined and cold-smoked trout to evaluate the oxidation stability during the process and storage of the trout samples (Tarladgis et al., 1960). 3.9. Free fatty acids (FF) American Oil Chemists’ Society AOCS, 1989 were used to determine the free fatty acids (FF) (Method Ca 5a-40). 3.10. Microbiological analysis Each series of experiments were examined microbiologically. Twenty-five grams of trout were homogenized in 225 ml of 0.1% (w/v) of sterile peptone solution (0.1% peptone) in a Stomacher 400 Lab Blender for 2 min. The homogenized sample was diluted and spread onto plate count agar for total viable count (TVC)

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(PCA, Merck, Germany) and incubated at 38 °C for 48 h Psychrophyl were determined on Plate Count Agar (PCA, Merck, Germany) with an incubation temperature of 7 °C for 10 days, following the pour plate method (ICMSF, 1978). The yeasts and moulds were counted on potato dextrose agar (PDA, Merck, Germany) after five days of incubation at 25 °C (FDA, 1998). 3.11. Statistical analysis of data The experimental data were analyzed and processed using SPSS-11.0 software (Lead technologies Inc., USA) statistically, and further analyzed with One-Way ANOVA among the different cold drying temperatures. The differences between mean values were analyzed and presented using the Duncan test for comparison and effects were considered to be significant at p < 0.05.

in exponential form are obtained through Eq.()()()()(6)–(9) for each cold air drying temperature (4, 10, 15, 20 °C). Thus, the single layer cold air drying curve equations for fish samples have been obtained to be:

M ðtÞ ¼ 0:97 expð0:11tÞ for 4 C

ð6Þ

M ðtÞ ¼ 0:93 expð0:11tÞ for 10 C

ð7Þ

M ðtÞ ¼ 0:99 expð0:16tÞ for 15 C

ð8Þ

M ðtÞ ¼ 0:97 expð0:23tÞ for 20 C

ð9Þ

Actually, these curve equations indicate the drying behaviour of fish samples during the single layer cold air drying in the cyclone type dryer.

4. Results and discussion

4.1. Cold air drying characteristics

In order to determine the effects of single layer cold air drying on fish quality the experiments were carried out by using the experimental set-up in the biology laboratory of Nigde University. In this regard, before the properties indicating the fish quality were determined and discussed in detail, the parameters such dimensionless mass loss, moisture content, mass shrinkage and drying rate of fish were calculated using Eqs. (2)–(5) based on the experimental data, respectively. Moreover, the variations of these parameters as a function of drying time are presented in Figs. 3–8. Using cyclone type cold air dryer, the values of dimensionless mass loss have been estimated using Eq. (1) for each drying temperature. Based on these data, drying curve equations of fish fillets

After the single layer cold air drying process of the fish samples, the following cold air drying characteristics such as moisture content, drying air temperature, relative humidity and drying time should be taken into consideration to describe the cold air drying behavior of the fish samples in the cyclone type dryer. During the cold air drying experiments in the cyclone type dryer, it was determined that the weight of fish samples decreased from 200 g to 111 g at temperature of 4 °C in 26 h, 112 g at temperature of 10 °C in 23.5 h, 110 g at temperature of 15 °C in 21.5 h and to 112 g at temperature of 20 °C in 13.5 h, respectively. During drying experiments, drying air velocity was kept constant at 7 m/s because of the low temperatures in the cyclone type dryer. Using

100 90

4ºC

10ºC

15ºC

20ºC

80

% RH

70 60 50 40 30 20 10 0 0

5

10

15

20

25

30

Drying time (h) Fig. 3a. Variation of drier inlet % RH of cold air versus drying time during drying at 4, 10, 15, and 20 °C.

100 90

4ºC

10ºC

15ºC

20ºC

80

% RH

70 60 50 40 30 20 10 0 0

5

10

15

20

25

30

Drying time (h) Fig. 3b. Variation of drier outlet % RH of cold air versus drying time during drying at 4, 10, 15, and 20 °C.

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75

V= 7 m/s

70

T= 4ºC

Moisture content

65

T= 10ºC

60 55

T= 15ºC

50

T= 20ºC

45 40 35 30 25 0

5

10

15

20

25

30

Drying time (h) Fig. 4. Variation of moisture content of fish samples as a function of drying time at 4, 10, 15, and 20 °C of cold air drying temperatures for fish samples at 7 m/s.

1.00 0.95

V= 7 m/s

T= 4ºC

Mass shrinkage

0.90

T= 10ºC

0.85 0.80

T= 15ºC

0.75

T= 20ºC

0.70 0.65 0.60 0.55 0.50 0

5

10

15

20

25

30

Drying time (h) Fig. 5. Variation of mass shrinkage of fish samples versus drying time at 4, 10, 15, and 20 °C of cold air drying temperatures for fish samples at 7 m/s.

1.0

V= 7 m/s

Dimensionless mass loss

0.9

T= 4ºC

0.8 T= 10ºC

0.7 0.6

T= 15ºC

0.5 T= 20ºC

0.4 0.3 0.2 0.1 0.0 0

5

10

15

20

25

30

Drying time (h) Fig. 6. Variation of dimensionless mass loss at 4, 10, 15, and 20 °C of cold air drying temperatures for fish samples at 7 m/s.

the experimental data, the moisture contents of the fish samples were first estimated using Eq. (3). In this regard, Fig. 3 illustrates the variation of inlet relative humidities of drying air versus drying time while Fig. 4 presents the variation of moisture content of the fish samples as a function of cold air drying time. As shown in Fig. 3a, the initial relative humidity of drying air at the inlet of the dryer range from 25.8% to 46% for drying at 4 °C, 40% to 60% for drying at 10 °C, 35% to 46% for drying at 15 °C, and 40% to 47% for drying at 20 °C. As understood from these values, the

fluctuations of the relative humidity of drying air at the inlet of the dryer may be noticed. The main reason of this is that the experiments are conducted in a cold storing laboratory. Before the cold air drying experiments, it was determined that maximum equilibrium moisture content of the fresh fish samples was approximately 70.7% (wb) while that of the salted fish samples was 68.2% (wb). Under constant drying velocity, it was estimated the salted fish samples contain almost 27% (wb.) at 4 °C in 26 h, 27% (wb) at 10 °C in 23.5 h, 26% (wb) at 15 °C in 21.5 h, and 27% (wb) at

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0.030 V= 7 m/s

.

Drying rate (kg water/h)

0.025

T= 4ºC T= 10ºC

0.020

T= 15ºC 0.015

T= 20ºC

0.010 0.005 0.000

0

5

10

15

20

25

30

Drying Time (h) Fig. 7. Variation of drying rate as a function of drying time at 4, 10, 15, 20 °C of cold air drying temperatures for fish samples at 7 m/s.

0.030 V= 7 m/s

Dryingrate (kgwater/h)

0.025 T= 4 °C T= 10 °C

0.020

T= 15 °C T= 20 °C 0.015

0.010

0.005

0.000 0.0

0.2

0.4

0.6

0.8

1.0

Dimensioneless mass loss Fig. 8. Variation of drying rate as a function of dimensionless mass loss at 4, 10, 15, and 20 °C of cold air drying temperatures for fish samples at 7 m/s.

20 °C in 13.5 h, respectively. As understood from these data, the following important factors affect the drying time of the salted fish samples in a cyclone type dryer: (i) salt content of the samples, (ii) drying temperature, (iii) type of dryer, (iv) the sample thickness, (v) relative humidity of drying air. Before cold air drying experiments, it was determined that the salt content of the fresh fish samples was 0.34% while that of the salted fish samples was 5.18%. Meanwhile, it is important to emphasize that the salted fish samples have a homogeneous salt content. At the end of the drying experiments, it was determined that the samples used in the experiments had almost 7% of final salt content. Here, it can be said that, particularly, drying air temperature affects the final moisture content of the salted fish samples because drying velocity is kept in constant. In this regard, it is obtained that the moisture loss of the salted fish samples dried at 4 °C is more slowly than the others at 10, 15, and 20 °C. Therefore, one can say that the increase of drying air temperature goes up the amount of the evaporated water from the samples. In such a way that, if the salted samples are subject to higher temperature, then they loss moisture very quickly even if the samples have the same salt content. Thus, drying air temperature decreases the drying time of the salted samples, which have the same salt content out of the above values. However, the increase of drying air temperature decreases the fish quality because it accelerates the biochemical and microbiological decomposition

of the salted fish samples. In order to prevent this negative effect of drying air temperature, particularly low temperatures should be selected for drying of the perishable food products. Nevertheless, drying air and drying air temperature should have homogeneous distribution in the dryer in order to decrease drying time of the samples. In this regard, cyclone type dryer is preferred to conduct the cold air drying of the salted fish samples because of the following important advantages: (i) the samples with the same salt content are more effectively subject to cold drying air because the cyclone type dryer creates homogeneous distribution of drying air on the salted fish samples, (ii) the salted fish samples are more homogeneously dried because the cyclone type dryer decreases the fluctuation of drying air temperature, accelerates the moisture loss due to the homogeneous distribution of drying air on the samples, (iii) the cyclone type dryer reduces the energy losses transported by drying air through the salted fish samples. As can be understood from these explanations, the stability and homogeneity of drying air temperature in the dryer and the energy transported by drying air through the samples are quite important to decrease the drying time of the samples. Actually, the drying performance of the cyclone type dryer to decrease the drying time is also based on the sample thickness settled on the layers. Particularly, single layer drying is very important for perishable food products to increase the quality because it affects the drying time of the samples. For

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example, if deep layer drying is applied for drying the salted fish samples at the same conditions, the drying behavior of the samples can be different. Meanwhile, one can emphasize that deep layer drying is not a focus of this study. The single layer cold air drying provides the decrease of drying time because of the homogeneous distribution of drying air through the samples based on the drying air temperature. For the salted fish samples, single layer cold air drying is particularly important because moisture loss will be high, based on the above results. If so, another important parameter to decrease the drying time is the relative humidity of the drying air. If drying air has high relative humidity then the moisture loss from the samples will be low because of the lower sensible heat of the drying air. In particular, the cyclone type dryer decreases this negative effect of drying air. Actually, the use of cyclone type dryer, the application of the single layer cold air-drying, and the drying air with low relative humidity decrease the drying time as, well. Under these considerations, at the end of the cold air drying experiments of the salted fish samples, based on the above important parameters, it can be accordingly said that the moisture loss of the samples with the same salt content goes down with the decrease of the drying air temperature at the same drying conditions. However, in terms of fish quality, the salted fish samples dried at 4 °C should be particularly taken into consideration for quality control because of their low microbiological and biochemical decompositions. It was particularly significant to determine the mass shrinkage in order to define the mass changes taken place on fish samples through drying. Fig. 5 presents variations of mass shrinkage ratios versus drying time. Mass shrinkage ratio of fish samples at each drying temperature was calculated by using Eq. (4) and regularly decreased from 100% to almost 55%. As shown in this figure, it was determined that, at the end of the experiments, the final mass shrinkage of the fish samples dried at temperature of 4 °C was at least 0.555% in the drying time period of 26 h while the samples dried at temperature of 10, 15, and 20 °C were almost 0.563%, 0.550%, 0.560% in the drying time period of 23.5, 21.5 and 13.5 h, respectively. Accordingly, any deformation was not observed on the surfaces and the shapes of the fish samples at drying temperature of 4 °C in cold air dryer. Fig. 6 shows the variation of dimensionless mass loss depending on drying time at 4, 10, 15, and 20 °C of cold air drying temperatures for fish samples at 7 m/s. Dimensionless mass loss of fish samples at each temperature was calculated using Eq. (1). As shown in Fig. 5, the values from the corresponding exponential equation showed a good harmony with the experimental results in the cold air dryer. Consequently, it can be said that the drying curves follow the same trend and show a decrease with the increasing of drying time. Fig. 7 presents the variations of drying rate as a function of drying time. In addition, Fig. 8 presents the variations of drying rate as a function of dimensionless mass loss. Drying rate of fish samples at each temperature was calculated using Eq. (5). As shown in these figures, drying rate decreased with the increase of drying time. Drying rate of fish samples was stable between 15.5 and 22.5 h at drying temperature of 4 °C. Drying rate of fish samples at drying temperature of 4 °C decreased to zero after drying time of 26 h The final drying rates of the fish samples were equal to zero for 10 °C, 15 °C and 20 °C. It can be said that the fluctuation of drying rate of fish samples is based mainly on the moisture content of the samples, and drying air temperature. According to the effect of the homogeneity of drying air on product, the moisture content of products and drying air velocity a temperature. 4.2. Chemical composition of the raw and dried fish samples Table 2 presents the proximate chemical composition of fresh, salted and four fish samples dried.

The chemical composition of fish samples affecting the fish quality and drying performance changes by depending on the age, species, sex, environment and season. Before cold air drying experiments, the moisture (wb), dry material (wb.), protein, ash, fat and salt aw, pH contents of the fresh and salted fish samples were determined. All chemical analyses have been done fort he fresh and salted fish samples. Actually, the changes in chemical composition of the experimental groups have been investigated before and after the cold air drying process in a cyclone type dryer. After drying experiments, the chemical composition of the salted fish samples dried at 4, 10, 15, and 20 °C were determined for each sample. Moreover, the effect of the dryer used in the experiments on the chemical composition of the salted fish samples is essentially discussed by taking into consideration their drying characteristics. As shown in Table 2, it is noticed that the lowest moisture content (50.07% wb), the highest value of dry material (50.0 gg1) have been obtained fort he salted fish sample dried at 4 °C. However, the protein content of the salted fish sample dried at 4 °C has been found as 0.58 gg1 by using analytical method based on the nitrogen quantity. All other values of the parameters can be observed in Table 2. Accordingly, it is necessary to note the fat content, however, was not affected by cold air-drying. The protein, lipid, ash, salt contents are not affected by the different temperatures during cold air drying process. aw (water activity) was found average 0.80 in experimental groups. It is known that the intermediate moisture food contains between moisture 15–50% with aw of 0.60–0.85 (Jay, 2000). 4.3. Some quality properties of the raw, salted and dried fish samples In this section, the following aspects are studied and discussed in detail, namely total volatile basic nitrogen (TVB-N), thiobarbituric acid reactive substances (TBARS), free fatty acid (FF), total viable counts (TVC), total psychrophilic count (TPC) and total yeast and mould (TYM) (Table 3). Before the drying experiments, the following quality parameters are taken into consideration for the fresh and salted fish samples: Total volatile basic nitrogen (TVB-N), Thiobarbituric acid reactive substances (TBARS), free fatty acid (FF), Total viable counts (TVC), Total psychrophilic count (TPC) and Total yeast and mould (TYM) as in Table 3. After drying experiments, the same quality parameters were investigated for the salted fish samples dried in the system. In this regard, the food quality parameters are discussed based on the cold air drying characteristics of the salted fish samples dried at 4, 10, 15, and 20 °C. Moreover, the effect of the cyclone type dryer on the food quality parameters are discussed by taking into consideration the drying characteristic of the salted samples. Table 3 presents some quality properties of the fish samples fresh, salted and dried at temperatures of 4, 10, 15, 20 °C. 4.4. Total volatile basic nitrogen (TVB-N) TVB-N values as quantitative quality indicator of the fish samples dried were also determined for each experimental group. Table 3 also includes TVB-N values of the fish samples as a fresh, salted and dried at temperatures of 4, 10, 15, 20 °C. Under the constant drying velocity, the TVB-N value of the fresh fish samples was 15.07 mg/100 g while that of the salted fish was 17.27 mg/100 g. After drying by LTHV method based on the experimental data, it was determined that the TVB-N values showed significant differences for all groups statistically (Table 3) (p < 0.05). Moreover, it was noticed that the fish samples dried at 4 °C by LTHV method had the lowest TVB-N value as 17,23 mg/100 g while the TVB-N values of other groups dried at 10, 15, 20 °C were higher as 17.23, 18.26, 19.00, 19.01 mg/100 g respectively. Accordingly, it is said that the cold air drying process by LTHV method at 4 °C

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affected the quality of dried fish samples positively. But there are not any significant difference between 10 and 4 °C for TVB-N. Note that LTHV method has a preservative effect on TVB-N. TVB-N is one of the traditional chemical quality indicators that are widely used for evaluation of the degree of spoilage in fish (Antonacopoulos and Vyncke, 1989) 4.5. Thiobarbituric acid reactive substances (TBARS) and free fatty (FF) acids TBARS value is a widely used as a quality indicator for the assessment of degree of lipid oxidation. Table 3 also includes the variation of TBARS values of the fish samples which are fresh, salted and dried fish samples at 4, 10, 15, 20 °C. At the beginning of process, under constant drying velocity TBARS value of the fresh fish was found as 0.34 mg MA/kg. After salting process, TBARS was 0.32 mg MA/kg. (The differences between fresh and salted fish were not significant statistically (p < 0.05)). Moreover, it was determined that the group dried at 4 °C had the least TBARS value as 0.42 mg MA/kg. TBARS was higher for other groups dried at temperatures of 10, 15, 20 °C significant statistically (p > 0.05). As a consequent, it can be said that cold air drying process affected the lipid quality of dried at 4 °C fish positively in case of cyclone type cold air dryer. It can be said that LTHV method have a preservative effect on fish lipids. Under these considerations, based on the experimental data, it was noticed that TBARS values showed significant differences among all dried fish groups statistically (p < 0.05). The highly unsaturated lipids in fat-rich fish samples can be easily oxidized, and, as a result of this, a rancid smell and taste as well as alterations in texture, color and nutritional value can be observed (Ólafsdóttir et al., 1997). As another lipid quality indicator, FF is a primer lipid metabolite. It was found that FF of the fresh fish samples was 0.2 and that of the salted fish was 0.8. It was determined that the group dried at 4 °C had the least FF value as 1.2. FF values were higher than the other groups dried at temperatures of 10, 15, 20 °C as 1.2, 1.4, 1.4, 2.2 respectively that the differences significant statistically (p > 0.05). The open literature suggest that a maximum TBARS value, indicating the good quality of the fish, is 5 mg malonaldehyde/kg, while fish may be consumed up to a TBARS value of 8 mg malonal} ller, 1969). dehyde (MA)/ kg (Schormu 4.6. Total psychrophilic count (TPC) Table 3 also includes the variations of TPC values of the fish samples as a fresh, salted and dried at temperatures of 4, 10, 15, 20 °C. It is known that psychrophilic bacterias can also increase at chill temperature. It was found that TPC value was 2.3 log cfu/ g for the fresh fish samples and 2.2 log cfu/g for the salted fish samples. This difference was significant (p < 0.05). After cold air drying process, TPC values should be determined because TPC is an important quality indicator referring to the fish quality, as well. The TPC values of all groups (raw fish, and fish samples dried at 4, 10, 15, 20 °C) were at the lowest levels with an average of 2.3, 2.3, 2.6, 2.2, 2.8 log cfu/g, respectively. TPC value is not significant between 4 and 10 °C (p > 0.05). The group dried at 20 °C had the highest value of TPC (p > 0.05) and the TPC value reached at 2.8 log cfu/g for this group, indicating a decrease of the microbiological quality (Table 3). All these results indicate that LTHV method can obtain a protection against to the psychrophilic bacteria during the drying process of fish in case of cyclon type cold air dryer. 4.7. Total viable counts (TVC) Table 3 also includes the variations of TVC values of the fish samples as a fresh, salted and dried at temperatures of 4, 10, 15,

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20 °C. After cold air drying process by LTHV, total viable count should be determined because TVC is an main quality indicator referring to the dried fish quality. Under constant velocity, TVC value was found as 2.0 cfu/g for the fresh fish samples 2.3 cfu/g for the salted fish samples. This difference was significant statically (p < 0.05). TVC value of all groups (raw fish, and fish samples dried at 4, 10, 15, 20 °C) were at lowest levels with an average of 2.0, 2.6, 2.6, 2.9, 3.2 log cfu/g, respectively. However, it was observed that there was no difference between the raw fish and group at 4 °C by depending on the TVC values (Table 3) (p > 0.05). The microbiological flora remained stable in all samples dried at 4 °C in drying time of 26 h. On the other hand, there were some significant differences in the other groups (p < 0.05). Similarly, the same initial TVC values were also observed by Lyhs et al. (2001). However, the TVC value was found as 3.2 log cfu/g for the group at 20 °C, indicating decrease in dried fish quality as microbiologically. These results indicate that LTHV method usable to perishable food protection. 4.8. Total yeast and mould (TYM) Table 3 include the variations of TYM values of the fish samples as a fresh, salted and dried at temperatures of 4, 10, 15, 20 °C. TYM is another important quality parameter for dried food products. Therefore, TYM values should be estimated. Under constant drying velocity, TYM value was found as 2.1 log cfu/g for the fresh fish samples and 2.7 log cfu/g for the salted fish samples. This difference was significant statically (p < 0.05). Based on the experimental data, it was found that TYM values were also the lowest for the samples dried at 4 °C. However, it was noticed that TYM values were higher for the groups dried at 10, 15 and 20 °C. Accordingly, it can be said that, based on the TYM values, the fish samples dried at 4oC have better quality than groups dried at 10, 15, 20 °C as 2.6, 2.5, 3.2, 3.4 log cfu/g respectively (Table 3). All these results indicate that LTHV method can decrease psychrophilic bacteria production during dried to fish.

5. Conclusions In terms of low temperature and high velocity application (LTHV) in drying, the experiments of the rainbow trout (O. mykiss) were conducted to determine the cold air drying characteristics and fish quality properties by taking into consideration the general food quality indicators. These conditions were confirmed in the 2nd shelf. According to the results of the observations and quality analyses, it was deduced that the decreasing drying air temperature show a correlation with quality values.  Final moisture content of fish samples is average 27% (wb) at 4, 10, 15 and 20 °C in 26 h, 23.5 h, 21.5 h and 13.5 h for constant drying velocity of 7 m/s during cold air drying process, respectively.  Although the values of MSR, TVB-N, TBARS, FF, TVC, TPC and TYM of the fish samples dried at 10, 15, 20 °C are higher, fish samples at 4 °C have the lowest values of these total quality indicators. Accordingly, it is expected that, in terms of MSR, TVB-N, TBARS, FF, TVC, TPC and TYM, cold air drying process will contribute to improve the total quality of fish.  The best drying air temperature was selected as 4 °C because fish samples dried at this temperature had the lower values of the total quality indicators. Consequently, it is suggested that cold air drying process be applied to increase total quality of fish samples. Moreover, cold air drying conditions of fish samples dried at 4 °C can be taken into consideration for the practical applications because of the lower

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