Food Bioprocess Technol (2012) 5:155–165 DOI 10.1007/s11947-009-0236-5

ORIGINAL PAPER

Effect of Freezing on Textural Kinetics in Snacks During Frying Tanushree Maity & P. S. Raju & A. S. Bawa

Received: 1 May 2009 / Accepted: 22 July 2009 / Published online: 7 August 2009 # Springer Science + Business Media, LLC 2009

Abstract Kinetics of texture development during frying of snacks subjected to different initial conditions such as frozen, frozen–thawed, and unfrozen was investigated. The temperature dependency of the changes in the form of reaction constants was explained by Arrhenius equation. The increase in hardness and decrease in cohesiveness followed first-order reaction kinetics (R2 =0.94–0.99) in all the samples. Frozen samples showed induction (phase I) and development/degradation (phase II) periods for textural parameters during frying. The activation energies for hardness were 33.81, 25.63, 19.09, and 20.13 kJ/mole for frozen (phase I and II), frozen–thawed, and unfrozen samples, respectively with the R2 =0.96–0.99. Frozen samples showed high activation energies for textural parameters during frying as compared to the frozen–thawed and unfrozen samples. The increase in chewiness was found to follow the kinetics of zero-order reaction for all the samples. Temperature and time were found to have a significant effect (P<0.01) on the changes in textural profile during frying. Keywords Kinetics . Frozen . Snacks . Frying . Texture Nomenclature Ea k ka n

activation energy rate constant (min−1) Arrhenius equation constant order of the reaction

T. Maity (*) : P. S. Raju : A. S. Bawa Defence Food Research Laboratory, Siddarthanagar, Mysore 570011 Karnataka, India e-mail: [email protected]

Q Qo R T t TPA

measured TPA textural parameter (hardness, chewiness, cohesiveness) at frying time t measured TPA textural parameter at zero time universal gas constant (8.314 kJ/mol) frying temperature (K) frying time (min) texture profile analysis

Introduction A variety of frozen food products have been introduced in the market to furnish the increasing demand for ready-to-eat food items. Frozen ready-to-eat snack foods require minimal processing in the form of thawing and warming for consumption (Rahman 1999). These types of snacks could be frozen without frying which retain their quality unaltered during storage for longer periods. It also eliminates the problem of foams and smokes which usually occurs in industrial frying. They could also be consumed after frozen storage by frying in edible oil with/without subsequent thawing (Creed 2006). Frying is a widely used cooking method to prepare tasty and crispy snacks foods. Sahin et al. (1999) described deepfat frying as a process used for drying and cooking of food products in hot oil. Baumann and Escher (1995) reported that dehydration in hot oil at temperature between 160– 180 °C is critical to ensure desired changes in the fried product. Control on the process of texture development in snacks food products is an important requirement for the manufacture of high quality products. Order of the reaction, reaction rate constants, and temperature dependencies are some of the important parameters which could be used for

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improving the quality and reducing the losses during thermal processing (Nourian and Ramaswamy 2003). Uniform crispy texture is highly essential for a fried snack food. A consistent crispy texture along with the appetizing aroma and flavor of the deep-fat-fried foods increases acceptability largely (Dagerskog 1979). Protein denaturation and starch gelatinization have been described as the two typical phenomena during frying of majority of cereal flour-based snacks products. Moyano et al. (2007) described the textural changes during frying as the result of various physical, chemical, and structural changes involved in the frying process. These include heat and mass transfers together with chemical reactions. Different textural properties such as hardness, firmness, cohesiveness, gumminess, chewiness, and springiness could be determined by analyzing force deformation curves using a texture analyzer (Bourne 1982). Sanz et al. (2004) successfully developed a frozen battered food without a pre-frying step. Velez-Ruiz and Sosa-Morales (2003) analyzed changes in instrumental texture profile of wheat flour-based donuts during deep-fat frying at 180–200 °C. Ateba and Mittal (1994) gave mathematical models describing the kinetics of crust formation and firmness changes during deep-fat frying of meatballs. Nourian and Ramaswamy (2003) explained the kinetics of textural changes during cooking and frying of potatoes and reported increase in textural properties with frying time, which followed a first-order kinetic model. Jayendra Kumar et al. (2006) reported kinetics of color and textural changes in ethnic sweet meats i.e. gulabjamun balls and temperature dependency of the changes was modeled using Arrhenius equation. Various pre-treatments such as blanching, freezing, air drying, and osmotic dehydration were also reported to cause decrease in hardness and chewiness with frying time in sweet potato, while making it springier (Taiwo and Baik 2007). Prediction of changes in texture during frying could be helpful in a better process control and improvement in overall acceptability of a fried snack food from ready-tocook frozen raw snack such as vegetable stuffed patties. Reports exist regarding the importance of controlled crispy texture during frying of potato chips (Smith 1975; Scanlon et al. 1994; Rosen and Hellenas 2002), donuts (Velez-Ruiz and Sosa-Morales 2003), salmon (Kong et al. 2007), and meat balls (Ateba and Mittal 1994). However, to the best of our knowledge, there are no reports available on the changes in texture during frying of snacks subjected to frozen storage or after thawing. Hence, a study was carried out with the objective inclusive of evaluation of textural changes during frying of frozen sample, frozen–thawed sample or unfrozen sample in the form of vegetable stuffed patties as effect of different durations and temperatures as well as to determine the kinetic parameters in terms of reaction rate constants and activation energy.

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Materials and Methods Preparation of the Snack Raw materials for the preparation of the vegetable snack such as whole wheat flour, refined wheat flour, rice flour, semolina, vegetables (potato and carrot), and sunflower oil were procured from the local market. All the three flours were roasted for 2 min at 150 °C and stored at 6 °C until further processing. All the vegetables were surface sanitized with chlorinated water (50 ppm) before grating. Grated material was blanched (85 °C; 3 min) and sautéd with fat in 1:10 ratio (w/v) to golden brown color. Formulation of stuffing includes vegetables and cheese (2:1), 2% butter, 1.5% salt, 10% condiment paste (onion, garlic, and ginger), and 1.5 % dried and powdered spice mix (cumin, cinnamon, black pepper, red pepper, coriander, and cardamom). For the preparation of casing of the snack, the levels of different flours were selected based on previous trials conducted in the laboratory to get a crispy texture in the fried product. The standardized formulation for the snack consisted of 60% whole wheat flour, 20% refined wheat flour, 15% semolina, and 5% rice flour. The dough was kneaded with equal quantities of water and milk (60% of the flour weight). Casing was prepared by pressing 25 g dough into a sheet (0.3 mm thickness) and vegetable stuffing (30 g) was filled into it. The edges were sealed in such a way that the vegetable stuffing could not drop. The snacks (6×4×2.5 cm, 55 g) were then packed in polyethylene (PE) pouches of 100 µ. The chemical composition of the freshly prepared unfried snack had been analyzed following AOAC (1990) procedures and minerals using an atomic absorption spectrometer (Varian 280 FS model, Switzerland; Table 1). Freezing and Thawing Processes Polyethylene packs of single un-fried snack sample (55 gm) were placed on metallic grids and frozen at −40 °C for 3 h

Table 1 Chemical composition of freshly prepared un-fried snack (n=3) Parameters

Values

Moisture (%) Protein (%) Fat (%) Ash (%) Carbohydrates (%) Zn, mg/100 g Fe, mg/100 g

59.41±0.19 11.51±0.08 10.10±0.07 2.46±0.02 19.21±0.61 4.29±0.00 60.71±0.02

K, mg/100 g Ca, mg/100 g

155.1±0.05 42.92±0.02

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at an air velocity of 8 m s−1 in a blast freezer (Model, SZCV-425-3, Cryoscientific, Chennai) equipped with freezing rate controller. The freezing rate was −2 °C/min and the final temperature reached was −40 °C. Subsequently the frozen samples were stored in deep freezer at −20 °C for 1 month. Thawing of the samples was done for 30 min at room temperature (28±2 °C) before frying. Frying A laboratory scale deep-fat fryer (Continental Equipment Ltd, Bangalore, India) with temperature controller to control within ±2 °C, was used for deep-fat frying (Fig. 1). The fryer was equipped with a stainless steel (SS) wire basket in which sample was placed. The vessel had an effective volume of 5 l. Prior to frying sunflower oil (3:1, v/w) was preheated in the deep-fat fryer to the desired temperature. The raw snack samples, i.e. frozen, frozen– thawed, and unfrozen were fried at three selected temperatures (130 °C, 150 °C, and 170 °C) and the temperature was monitored using a digital thermometer with J type thermocouple probe of 1.6 mm diameter (Spectrochem Pvt. Ltd, Mumbai, India). Individual samples were removed from the fryer at specified time intervals viz., 0–6 min during frying. After frying, the samples were drained (20 s) and gently wiped with adsorbent paper to remove surface oil. The snacks were then cooled to room temperature and packed in PE bags to prevent moisture loss. The snacks were then tested for instrumental texture and sensory attributes on the same day. The experiment was replicated three times. Earlier, the corresponding total frying times and the sampling intervals for each frying temperature were determined experimentally. Kinetic Parameters Kinetic evaluation is necessary to derive basic kinetic information for a system in order to describe the reaction rate as a function of experimental variables to predict

D C

B

E

A

Fig. 1 Schematic diagram of experimental set up for frying of the snack samples. (A) Frying cabinet, (B) temperature controller, (C) wire basket, (D) thermocouple probe, and (E) monitor

changes in a particular food during processing (Van Boekel 1996). The rate of change of a food property during a process could be modeled as (Singh 1994): dQ ¼ kQn dt

ð1Þ

For majority of foods, the time dependence relationships appear to be described by zero- or first-order models (Chen and Ramaswamy 2002). In zero-order reaction the relationship between a quality attribute and time is linear and therefore the integrated form of Eq. 1 by substituting n=0, is Q ¼ Q0  kt

ð2Þ

In first-order reactions, the relationship between quality attribute and time is exponential and therefore substituting n= 1 and integrating Eq. 1, we get kinetic model for first-order reaction: ln Q ¼ ln Q0  kt

ð3Þ

The rate constant k was estimated from a straight line plot between ln Q and frying time. The rate constant is generally temperature-dependent, and the relationship can be modeled by Arrhenius equation. k ¼ ka exp½Ea=RT 

ð4Þ

Instrumental Texture Measurement Texture profile analysis (TPA) was carried out with a TAHDi Texture Analyzer (Stable Micro Systems Ltd, London, UK) using a 5 kg load cell and the application program provided with the apparatus (Texture Expert for Windows™, version 1.22). TPA was performed using a spherical probe of 75 mm diameter at a cross head speed of 1 mm s−1 to a fixed distance of 25 mm, withdrawn to the original height with a rest period of 5 s between cycles. The software automatically calculated the textural parameters (Fig. 2) as follows: hardness (N) is given either as the first force peak when there are two peaks on the chart or by the second peak if there are three peaks (Ranganna 1986). Adhesiveness (Ns) is the negative area between the point at which the first curve reached a zero force value after the first compression and the start of the second curve. Springiness (mm) was calculated as the ratio of the distance or time from the start of the second area up to the second probe reversal vs. the distance or time between the start of the first area and the first probe reversal. Cohesiveness is the extent to which a material can be deformed before it ruptures. It was calculated as the ratio (dimensionless) of the positive force area during the second compression portion to the positive area during the first compression. Chewiness (N) was calculated as the product of hardness, cohesiveness, and springiness.

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Fig. 2 A typical force deformation curve of a fried snack sample

Sensory Evaluation The sensory evaluation of the final fried snack samples was carried out in terms of color/appearance, texture/crispness, taste/flavor, and overall acceptability using a 9-point Hedonic scale according to Larmond (1977) by a panel consisting of 10 members. Panelists were scientific staffs of the laboratory who were trained in attributing rating for the characteristics examined. The scores were assigned from extremely liked (9) to disliked extremely (1). Samples were presented in containers labeled with a random number. The panelists were asked to drink some water to cleanse the palate between tastes. Statistical Analysis All the measurements in this study were made in three replications. Data is interpreted by regression analysis to obtain relation of TPA parameters with frying time and temperature using Microsoft Excel, Washington, USA. Analysis of variance (ANOVA) was carried out to calculate critical difference of the data to statistically predict the effect of different variables (frying temperature and time) as well as their interactions on the textural properties. Significance was established at P<0.01 levels.

Results and Discussion In the present study, textural changes during frying of frozen and frozen–thawed samples showed varied trend as

compared to unfrozen samples. TPA software provided the parameters such as hardness, chewiness, cohesiveness springiness, adhesiveness, and gumminess. However, according to Szczesniak (1995) and Bourne (1995), determination of gumminess is only valid for semisolid foods. Moreover, chewiness and gumminess cannot be determined in the same food. Since fried snacks were considered as solid samples, gumminess was discarded as a TPA parameter in this study. TPA showed an overall decreasing behavior for springiness and adhesiveness during 0–6 min of frying in all the samples (data not shown). For unfrozen sample, springiness decreased from 0.023±0.002 to 0.017±0.001, 0.032±0.003 to 0.021± 0.001, and 0.041±0.002 to 0.028±0.004 mm whereas adhesiveness decreased from 0.063±0.0003 to −0.005± 0.0004, 0.085±0.0005 to −0.081±0.0003, and 0.114± 0.0005 to −0.137±0.0002 Ns at the respective frying temperatures of 130 °C, 150 °C, and 170 °C. Maximum adhesiveness values of 0.108 and 0.148 were recorded for frozen and frozen–thawed samples, respectively during frying but it did not represent a continuous trend during the process. Minimum values for springiness and adhesiveness observed in case of frozen and frozen–thawed samples were 0.023 and −1.406 mm and 0.019 and −1.761 Ns, respectively. However, the trends for the changes in these characteristics were not very well defined. On the contrary, trends regarding hardness, chewiness, and cohesiveness were consistent and well defined and the kinetics of the same was worked out. Hardness and chewiness showed an increasing pattern, which in turns gave a clear view regarding structural and chemical changes, which might

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have occurred in the snacks samples during frying. The cohesiveness of all the samples decreased in a regular pattern with frying time at different temperatures. Hence, hardness, chewiness, and cohesiveness were taken as the major representative textural parameters for the kinetic studies of the snacks during frying. Hardness Kinetics An increase of almost 68% in the hardness was observed in frozen samples due to freezing process compared to unfrozen samples. As the frozen samples were directly deep-fat fried without thawing, the hardness decreased with frying time resulting in softening of the snack within 3 min of frying at all the test temperatures; however, an increase in hardness was observed afterward until the end of frying. Therefore, kinetic parameters for the hardness changes during frying of frozen samples were calculated for two phases: (I) induction phase, in which the hard frozen sample softened and (II) development phase in which the snack surface toughened resulting in a desirable crispy crust. At the end of 3 min of frying, the initial hardness values of 19.3, 17.28, and 18.40 N decreased to 16.36, 13.51, and 12.32 N at 130 °C, 150 °C, and 170 °C, respectively. The hardness value at the end of frying period increased to 21.42–22.43 N depending on the frying conditions. First-order kinetic model (Eq. 3) was used to fit the experimental curves, and Arrhenius model (Eq. 4) was used to express the temperature dependency of the rate constants for the samples (Fig. 3a and b). Rate constants were found to increase with increase in frying temperature. The rate constants for hardness associated with frozen samples (both phases) were observed more temperature sensitive than that of frozen–thawed and unfrozen samples with higher Ea of 33.81 kJ/mol for phase I and 25.63 kJ/ mol for phase II (Table 2). The higher initial hardness values were most probably due to hardening effect of freezing, because of conversion of all the moisture present in the snacks into ice crystal during freezing. The decrease in hardness with frying time may be attributed to the conversion of ice crystals into water within the snack structure. Furthermore, increase in hardness with frying time could be attributed mostly to the toughening of the snack due to moisture loss during frying (Pedreschi and Moyano 2005). No comparable reports were available related to kinetics of textural changes during frying of frozen food products. The impact of freezing was observed in the initial hardness values of frozen–thawed samples at all frying temperatures. A ‘settling in’ time of 1 min was observed during frying, for which the hardness value decreased from the initial one. After this period, change in hardness followed an increasing trend. However, the hardness

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increased at a higher rate in frozen–thawed samples than frozen and unfrozen samples during frying. This phenomenon could be accredited to the fact that the cellular structure of vegetables (stuffing) as well as the gluten structure of casing broke down due to freezing of bound water into ice and the entrapped water easily oozed out of the snack during thawing which made dehydration faster during frying. Fuchigami et al. (1995) reported cellular damage and plasma separation in carrot tissue because of freezing. Regression analysis of the data was carried out in 1–6-min frying time (neglecting the initial ‘settling in’ period). The reaction followed first-order reaction kinetics with R2 ≥0.97 for frozen–thawed sample (Fig. 3c) and the Ea was found to be 19.09 kJ/mole, which were slightly lower than the Ea of unfrozen sample (Fig. 3d). The hardness of unfrozen samples progressively increased with increase in frying time (0–6 min) as well as with increasing frying temperature. The obtained trend represented the structural and chemical changes developed in the samples during frying. After 5 min of the frying at 130 °C, the samples showed more than 70% changes from their original texture while at the same duration 105% change was recorded in the samples subjected to frying at 170 °C. The hardness of the unfrozen sample increased from 10.88 to 20.70 N, 10.87 to 24.58 N, and 10.90 to 28.66 N during frying at 130 °C, 150 °C, and 170°C, respectively. However, Velez-Ruiz and Sosa-Morales (2003) observed a minor reduction in the hardness values of deep-fat-fried donuts. The change in textural characteristics of crust could be attributed to the change in physical properties of cereal proteins due to the reaction of carbohydrates with proteins, which might have resulted in toughening and hardening during frying. Due to frying, the inside vegetable stuffing became mealy. The hardness for unfrozen sample as a function of time at different temperatures also followed a first-order reaction kinetics based on the regression analysis (Fig. 3d). The resultant rate constants, activation energy and R2 values are shown in Table 2. The activation energy (Ea) for hardness in the frying process of unfrozen samples was 20.13 kJ/mol. The observed value of Ea was lower than typical Ea for increased hardness of gulabjamun balls during frying which followed zero-order kinetics with Ea of 77.58 kJ/mol (Jayendra Kumar et al. 2006). Nourian and Ramaswamy (2003) reported Ea of 60.8 kJ/mol for change in hardness of potatoes during deep-fat frying. The differences in the Ea described by these authors might be due to variation in products and reactions. Analysis of variance of the hardness change of the frozen, frozen–thawed, and unfrozen samples during frying as affected by frying temperature and time along with their interactions are presented in Table 3, which demonstrated a significant influence of both variables on hardness (P<0.01).

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Fig. 3 Kinetic plots of changes in hardness of snacks fried at different time–temperature. (open diamond) 130 °C, (open square) 150 °C, (open triangle) 170 °C. a Frozen (phase I), b frozen (phase II), c frozen– thawed, and d unfrozen

0.7

0.05

a

0.00 -0.05

0.5

-0.10 -0.15

0.4

-0.20

0.3

-0.25

0.2

-0.30

0.1

-0.35

0.0

-0.40

ln Q/Qo

b

0.6

-0.1

-0.45 0

1

2

3

0.9

3

4

5

7

6

0.8

c

0.8

2

4

d 0.7

0.7

0.6

0.6

0.5

0.5

0.4 0.4

0.3

0.3

0.2

0.2

0.1

0.1

0.0

0.0 0

2

4

6

8

0

2

4

6

8

Frying time (minutes) Chewiness Kinetics Chewiness of the frozen sample followed an increasing trend during frying with an initial induction phase up to 3 min of frying during which the chewiness decreased. This is probably due to equilibration of heat between the frozen snack and the frying oil. At the initial stages of frying the frozen samples was brought down to the temperature of the frying environment before it starts cooking (texture development). Due to this reason, a minor reduction in oil temperature was also observed. After this period, the chewiness increased till the end of frying. The initial value for chewiness in frozen sample was 7.45 which decreased to 6.67, 6.07, and 5.58 at 130 °C, 150 °C, and 170 °C, respectively, during phase I (induction phase). It again started increasing due to the crust formation and reached to a value ranging 8.35–10.75 at the end of 6 min of frying. The negative slope shown in Fig. 4a represented phase I of chewiness change during frying, which is induction phase, and positive slope in Fig. 4b showed phase II in which the chewiness values increased. Chewiness changes associated with each frying time (both phases) were consistent with zero-order kinetics (R2 ≥0.94). The Ea for frozen samples (both phases) was higher than the Ea of unfrozen sample (Table 2). The higher Ea values in these samples indicated

that change in chewiness was more heat sensitive than the other samples. In the case of frozen–thawed samples, the reaction followed zero-order reaction kinetics with R 2 ≥ 0.98 (Fig. 4c). The rate constants of chewiness associated with frozen–thawed sample were least temperature sensitive than unfrozen and frozen counterparts, with a lower Ea of 19.51 kJ/mole (Table 2). Lower Ea suggested that comparatively lower heat is required to promote chewiness changes in frozen–thawed sample. Damaged cytoplasmic structure of the snacks (vegetable stuffing as well as casing) due to freezing and thawing might be responsible for the lower Ea in frozen–thawed samples than the unfrozen samples. Freezing and thawing of foods could impart detrimental effects on their water holding properties because of physical disruption to cellular components (Jul 1984). Inoue et al. (1994) reported the disappearance of glutenin subunits and oligomers in the dough subjected to freezing and thawing which suggested that the structure of glutenin was altered by freeze–thaw cycles. Frozen–thawed samples evidenced a trend similar to that of unfrozen samples. As the frying time progressed, chewiness of the unfrozen sample increased at all frying temperatures, a trend similar to the hardness change. The initial chewiness values ranged from 4.17 to 4.27 and it

Food Bioprocess Technol (2012) 5:155–165 Table 2 Kinetics of textural parameters in snacks during deep-fat frying at different temperatures

161 Temperature (°C)

k (min−1)

130 150 170 130 150 170 130 150 170 130 150 170

0.0542 0.0916 0.1346 0.0995 0.1550 0.1980 0.0950 0.1340 0.1580 0.0623 0.0873 0.1070

0.98 0.97 0.98 0.99 0.96 0.98 0.98 0.98 0.97 0.96 0.97 0.97

33.81

0.99

25.63

0.98

19.09

0.96

20.13

0.98

130

0.0345

0.99

32.75

0.97

150 170 130 150 170 130 150 170 130 150 170

0.0611 0.0831 0.0864 0.1964 0.3073 0.3682 0.5434 0.6207 0.1244 0.1827 0.2484

0.98 0.96 0.95 0.94 0.99 0.98 0.96 0.98 0.99 0.99 0.98

47.27

0.98

19.51

0.93

25.69

0.99

130 150 170 130 150 170

0.0953 0.1226 0.1714 0.1078 0.1340 0.1571

0.96 0.97 0.97 0.98 0.99 0.99

21.72

0.98

14.00

0.99

130 150 170

0.1460 0.1909 0.2373

0.96 0.95 0.96

18.04

0.99

Hardness Frozen (phase I)

Frozen (phase II)

Frozen–thawed

Unfrozen

Chewiness Frozen (phase I)

Frozen (phase II)

Frozen–thawed

Unfrozen

Cohesiveness Frozen

Frozen–thawed

Unfrozen

increased to the final chewiness ranging from 8.48 to 14.77, depending upon the frying conditions. During frying, crust formation took place, making the snack crispier and harder. Therefore, chewing ability of the snack increased. The chewiness of the unfrozen sample was also fitted to zeroorder reaction kinetics (R2 ≥0.98) that related the changes in chewiness, with respect to process time (Fig. 4d). Ea of 25.69 kJ/mol was calculated for the process. Results agree with those of Velez-Ruiz and Sosa-Morales (2003) who reported increasing trend in chewiness of deep-fat-fried donuts. However, Taiwo and Baik (2007) reported decrease in chewiness with frying time in sweet potato. They

R2

Ea (kJ/mole)

R2

reported that the decreasing trend in chewiness was influenced by decreased hardness during frying, a relationship opposite to our observations for hardness and chewiness, which increased with respect to frying time. This could be due to different product nature and frying conditions. ANOVA of chewiness change of the frozen, frozen–thawed, and unfrozen samples during frying due to frying temperature and time along with their interactions are presented in Table 3. It is observed that temperature and time, as well as their interactions are highly significant (P< 0.01) parameters to control frying process for the snacks to retain maximum quality parameters.

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Table 3 Analysis of variance of factors affecting kinetics for textural parameters of the snacks (n=3) Frying variables

Hardness Independent Time (t) Temperature (T) Interaction t×T Chewiness Independent Time (t) Temperature (T) Interaction t×T Cohesiveness Independent Time (t) Temperature (T) Interaction t×T

df

Frozen

Frozen–thawed

Unfrozen

SS

MS

P>F

SS

MS

P>F

SS

MS

P>F

6 2

109.65 376.19

18.27 188.09

** **

1,372.5 71.92

228.75 35.96

** **

2,295.08 255.21

382.51 127.63

** **

12

48.25

4.02

**

18.36

1.53

**

48.22

4.02

**

6 2

300.29 88.63

60.05 44.31

** **

421.53 17.28

70.25 8.64

** **

196.34 69.55

32.72 34.77

** **

12

33.47

3.34

**

4.99

0.41

**

45.34

3.77

**

6 2

94.78 33.85

15.79 16.92

** **

410.79 9.99

82.15 4.99

** **

485.28 55.52

80.88 27.76

** **

12

7.71

0.64

**

6.27

0.62

**

4.89

0.41

**

**P<0.01 Fig. 4 Kinetic plots of changes in chewiness of snacks fried at specific time–temperature. (open diamond) 130 °C, (open square) 150 °C, (open triangle) 170 °C. a Frozen (phase I), b frozen (phase II), c frozen–thawed, and d unfrozen

2.5

1.5

b

a 2.0 1.0

1.5 1.0 0.5

Q/Qo

0.5

0.0

0.0 0

1

2

3

6.0

4

2

3

4

5

6

7

3.0

c

d

5.0

2.5

4.0

2.0

3.0

1.5

2.0

1.0

1.0

0.5 0.0

0.0 0

2

4

6

8

0

Frying time (minutes)

2

4

6

8

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Cohesiveness Kinetics The cohesiveness of the frozen–thawed and unfrozen samples decreased with frying time. However, cohesiveness of the frozen samples showed initially an induction period of 1 min during which there was no change in cohesiveness and then it decreased with increase in the frying time (Fig. 5a). The initial value of cohesiveness under TPA for unfrozen sample was 14.71 N. However, frozen–thawed sample showed higher cohesiveness with value 14.92. This 0.2

a

0.0 -0.2 -0.4 -0.6 -0.8 -1.0 0.2

b

0.0

ln Q/Qo

-0.2 -0.4 -0.6 -0.8 -1.0 -1.2

0.2

c

0.0 -0.2

phenomenon could be accredited to the fact that during freezing and thawing, the structural strength of the vegetables decreased resulting in tissue softening due to disruption of the cellular components. Increased cohesiveness in frozen–thawed could also be due to the changes in gluten structure of casing. Llorca et al. (2007) described leaching of starch components out of the granules in final fried frozen battered products. Berglund et al. (1991) also reported that the ice crystal formation in non-fermented dough during storage was found to cause rupture of the gluten network. The semi-logarithmic plots of rate constants showing first-order reaction kinetics are shown in Fig. 5b and c for frozen–thawed and unfrozen samples, respectively. The logarithm of relative cohesiveness value for frozen samples decreased linearly with time at any frying temperature following first-order reaction kinetics. An Ea of 14 and 18.04 kJ/mole was calculated for frozen– thawed and unfrozen respectively, while frozen samples showed a higher Ea of 21.72 kJ/mole for change in cohesiveness. Higher Ea of frozen may be due to higher heat requirement to overcome the freezing effect in frozen samples during frying. Figures 6 (a, b, and c) showed the plots of logarithm of rate constants and reciprocal of the temperature by which the Ea of different samples were calculated. The influence of frying temperature, time, and their interaction on change in cohesiveness of the frozen, frozen–thawed, and unfrozen samples during frying was analyzed and presented in Table 3. Analysis of variance confirmed that there was a significant (P<0.01) effect of temperature and time, independently, and their interaction on the frying process. Alvarez et al. (2000) reported the effect of frying time more significant than frying temperature on the quality attributes of potato; however, Pinthus et al. (1995) found frying temperature and duration as significant factors due to their importance in oil uptake during deep-fat frying. Therefore, it could be drawn that the relative importance of frying time and duration could be comparable or different depending on the nature of product and frying conditions.

-0.4

Sensory Evaluation

-0.6 -0.8 -1.0 -1.2 -1.4 -1.6 -1.8 0

2

4

6

8

Frying time (minutes) Fig. 5 Kinetic plot of changes in cohesiveness of snacks fried at different time–temperature. (open diamond) 130 °C, (open square) 150 °C, (open triangle) 170 °C. a Frozen, b frozen–thawed, and c unfrozen

Different samples such as frozen, frozen–thawed, and unfrozen were deep-fat fried at 150 °C for 6 min which was found to be suitable condition for cooking and obtained snacks were subjected to sensory evaluation. Table 4 shows the sensory scores of the prepared snacks in terms of color/ appearance, texture/crispness, taste/flavor, and overall acceptability. Appearance of the fried snacks from unfrozen sample scored high on hedonic scale. On the contrary, the panelists liked appearance of fried snacks from frozen sample moderately as it was found to have bumpy surface, which was developed during frying. The reason of bubbled

164

Food Bioprocess Technol (2012) 5:155–165 -1.40

-1.60

Table 4 Organoleptic scores of final fried snacks (n=10)

a

y = -3.1464x + 5.5161 R2 = 0.9874

Frozen Color/appearance Texture/crispness Taste/flavor Overall acceptability

-1.80

-2.00 -2.20

Unfrozen

7.0±0.26c 8.1±0.21b 8.3±0.17a 7.9±0.27b

8.5±0.25a 8.7±0.32a 8.4±0.28a 8.5±0.19a

Values with different letters in the row differ significantly (P≤0.05)

-2.40

y = -2.4701x + 3.3629 R2 = 0.992

y = -2.3454x + 3.4801 R2 = 0.9775

-2.60

-2.80

y = -4.1425x + 7.367 R2 = 0.9987

-3.00

0.00 y = -2.4026x + 4.9909 R2 = 0.9503

-0.50

b

y = -5.8029x + 11.981 R2 = 0.9866

-1.00

ln k (min-1)

7.9±0.19b 8.5±0.23a 8.4±0.21a 8.3±0.15a

Frozen–thawed

-1.50

-2.00 y = -3.1453x + 5.7183 R2 = 0.9999

-2.50 y = -4.0211x + 6.6334 R2 = 0.9858

-3.00

-3.50 -4.00

-1.30

c -1.50 y = -2.2087x + 3.5544 R2 = 0.9999 -1.70

-1.90

crust in the product obtained from frozen sample may be due to the evaporation of moisture during frying which was present in the form of ice held in the ice pockets of the outer casing. Scores for appearance of fried snacks from frozen–thawed sample was found to be the lowest because of the little deformation in the shape during handling and frying which may be due to the loosening of the snack caused by subsequent freezing and thawing. It also showed slight bubbling on the crust. Freezing and subsequent thawing did not impart significant effect on the taste significantly (P≤0.05) as the taste of fried snacks from frozen, frozen–thawed, and unfrozen samples scored similar values by the panelists. Fried snacks from frozen– thawed sample were observed to have hard-textured crust. It might be due to excessive loss of moisture during frying which has migrated from inside to the outer surface of the casing during the thawing process. Difference between fried snacks from frozen and unfrozen samples in terms of texture and overall acceptability was non-significant (P≥ 0.05). Overall acceptability of the fried frozen–thawed samples was found to be lower than the frozen and unfrozen samples. The quality attributes of frozen products has been reported to enhance by the addition of certain cryoprotectants (Giannou and Tzia 2008; Alvarez et al. 2008).

Conclusions y = -1.7152x + 2.0298 R2 = 0.9986

-2.10

-2.30

y = -2.6493x + 4.1989 R2 = 0.9821

-2.50 2.2

2.3

2.4

2.5

1/T X 10-3 (K-1) Fig. 6 Arrhenius plots for textural changes in snacks. a Hardness, b chewiness, c cohesiveness. (open triangle) Frozen (phase I), (closed triangle) frozen (phase II), (open square) frozen–thawed, and (open diamond) unfrozen

In the present study, the kinetics of textural changes in frozen and frozen–thawed snacks in comparison to unfrozen snacks during frying at different temperatures was established. Freezing showed an increase in activation energies in the snacks for textural parameters. Higher frying temperature (170 °C) was also found to accelerate the textural changes. Results from sensory evaluation indicated that acceptability of fried snacks from frozen samples were closer to unfrozen (control) samples. On the other hand, fried snacks from frozen–thawed samples were observed with deformed and loosened texture. Significant difference was not observed in activation energies of frozen–thawed and unfrozen samples for textural changes during frying. The hardness and cohesiveness in all the

Food Bioprocess Technol (2012) 5:155–165

samples, as a function of time at different temperatures followed first-order reaction kinetics (R2 ≥0.96) while the increase in chewiness followed kinetics of zero-order reaction with R2 ≥0.94. These kinetic parameters could be used to make valid predictions about textural changes during frying of the frozen, frozen–thawed, and unfrozen samples such as stuffed vegetable patties prepared without a pre-frying step. Moreover, information regarding behavior of tissue during certain thermal treatments can be useful in practical purposes particularly in the snack processing or catering industry. Acknowledgment The help rendered in data analysis and manuscript preparation by Mr. Alok Saxena is gratefully acknowledged.

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