Food Engineering
Hassan Sabbaghi
Abstract
Introduction: Doughnuts are fried foods that absorb significant amounts of oil, and commercial doughnuts made from wheat flour typically contain 24 to 26 percent oil. The use of additives, especially hydrocolloid compounds, in reducing the oil uptake in doughnuts is a practical method because there is ...
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Introduction: Doughnuts are fried foods that absorb significant amounts of oil, and commercial doughnuts made from wheat flour typically contain 24 to 26 percent oil. The use of additives, especially hydrocolloid compounds, in reducing the oil uptake in doughnuts is a practical method because there is no need to make changes in the industrial design of the equipment. Usually, the level of these compounds used to achieve the desired properties in food systems is less than 2% dry basis. When these compounds are added to the dough, they form a gel as soon as possible, which has a high water holding capacity in contact with hot oil, and this will be effective in maintaining and controlling the moisture content of the product and preventing oil penetration. Controlling moisture outflow from the product reduces the rate of moisture migration and compounds from the food into the frying oil, and subsequent peroxidation reactions in the oil consumed are reduced, thus increasing the useful life of the oil. Therefore, in this study, the effect of xanthan gum and carboxymethylcellulose on doughnut quality characteristics was studied. Materials and Methods: For this purpose, in the formulation of doughnut dough, xanthan gums, carboxymethylcellulose and a combination of these two gums (50:50) were used at the rate of 0.5, 1 and 1.5 percent based on the weight of the dry matter of the dough. The doughnut dough was prepared and molded. The fermentation was carried out for 30 minutes under similar industrial conditions (40°C and 40% humidity). Then the frying process at 180°C, icing and packaging were performed, respectively. The moisture content of the dough and doughnuts, the oil uptake of doughnut, density, color and the porosity ratio of the samples were measured. Also, the qualitative characteristics of shelf life, including soft tissue assessment, non-absorption of icing and non-growth of mold were investigated within 14 days. Statistical analysis was performed in a completely randomized design (p<0.05). Results and Discussion: The results showed that, the moisture content of the samples always increased with elevating the percentage of hydrocolloid compounds. Oil absorption decreased with increasing gum percentage and the lowest oil content was observed in the samples containing xanthan gum. Unlike xanthan, the density decreased and the volume increased with elevating the percentage of carboxymethylcellulose gum. Therefore, the highest porosity and softness of the tissue were found in samples containing carboxymethylcellulose. The brightness parameter (L*) was higher in samples containing xanthan, which could be due to the higher moisture content. The red parameter (a*) was also higher in the samples containing xanthan, which due to the higher compression of the product, a relatively more red pigment was observed with increasing xanthan level. The yellowness parameter (b*) in xanthan-containing samples was lower due to reduced oil absorption. The intensity of the color changes (dE) decreased as the percentage of gums increased. Non-absorption of icing was less in hard sample containing xanthan, which could be due to xanthan's competition with gluten in suction of surface water. Mold growth was not observed for any treatment. Overall, the best additive ratio of xanthan gum was 0.5% in terms of reduced oil absorption. Also, the use of carboxymethylcellulose at the level of 0.5% alone could improve the softening of the product texture and made the doughnut with larger volume. Simultaneous use of xanthan gum and carboxymethylcellulose showed behavior under effect of xanthan gum.In general, xanthan gum was more effective than carboxymethylcellulose gum in reducing oil uptake and moisture retention at test levels. Xanthan has shown the greatest reduction in oil absorption due to its thermal gelling and thickening properties, as it has a high capacity for water retention when product is in contact with hot oil. Since the xanthan gum is inexpensive, its effects on product volume can be further investigated in research work, along with bulking agents or other hydrocolloid compounds.
Food Technology
Hassan Sabbaghi
Abstract
Introduction: Doughnut is one of the most widely consumed food products in the world due to its suitable organoleptic properties. Today, with the spread of diabetes and lifestyle changes, consumers are paying more attention to the use of sugar-free foods and dietary products. In the bakery products, ...
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Introduction: Doughnut is one of the most widely consumed food products in the world due to its suitable organoleptic properties. Today, with the spread of diabetes and lifestyle changes, consumers are paying more attention to the use of sugar-free foods and dietary products. In the bakery products, sweeteners play an important role in providing moisture, sweetening, texture formation and increasing the shelf life of the final product. Sweeteners intended to replace sucrose should be water-soluble, with enough flavor, and cost-effective. Also, these compounds must also comply with national and international law. Stevia, which is composed of steviol and glycosides, is resistant to heat and pH and is not fermentable. Therefore, its amount should be chosen in such a way that it has no effect on cooking. In contrast, sweeteners containing dextrose and maltodextrin are also heat-resistant and can affect cooking and maintain the quality properties of fermentation. In order to use stevia in products as a substitute for sugar to maintain the formulation ratios, the fillers including low-calorie sweeteners such as erythritol and maltodextrin should be used. Therefore, in this study, the feasibility of producing sugar-free doughnuts as a dietary product was studied using stevia, erythritol and maltodextrin as sugar substitutes. Materials and Methods: The low calorie sweetener blend (stevia, erythritol and maltodextrin) was designed in the first step. It was found that if 46.175 g of stevia and 1997.82 g of erythritol were mixed and homogenized, a mixture would be obtained with sweetness four times higher than sucrose. So, the "sweetener blend" was used for one-fourth of the sugar removal mass and the rest was compensated with filler as maltodextrin. Instead of sugar, this mixture was added with zero ratio (control), 50 (low sugar) and 100% (no sugar) in the formulation of doughnuts and icing. Evaluation of qualitative characteristic including moisture content, oil absorption, density, color and porosity ratio and sensory analysis for doughnuts were performedThe properties of shelf life, including soft tissue assessment, non-absorption of icing and non-growth of mold were also investigated within 14 days. Statistical analysis was performed in a completely randomized design (p < 0.05). Results & discussion: The results showed that replacing 100% sugar in doughnuts was successful in terms of maintaining quality characteristics. The quality properties of doughnuts with 50% replacement (low sugar product) were not desirable. Sugar-free doughnuts had good quality properties (density, porosity, color and shell-life) compared to the control. Replacing the type of sweetener from sucrose to alcohol sugar can increase the moisture level of the product. As the amount of erythritol increased (no sugar sample), the texture became more moist and soft. The oil uptake of sugar-free doughnut did not show significant difference from the control sample and its sensory evaluation was so favorable. Maltodextrin could act as a bulking agent, stabilizer, and thickener agent in a sugar free formulation. Sugar free doughnuts had more brightness (L*) and less redness (a*), and its yellowness (b*) did not show significant difference with the control sample. It can be said that because more air has penetrated in sugar free doughnut the brightness parameter (L*) increased. The use of polyols (erythritol) results in a brighter color in the product, which is due to the lack of participation of polyols in the Millard reaction because they lack a reactive aldehyde group. The early development of the Millard reaction led to changes in hardness. Therefore, replacing reducing sugars with non-reducing compounds such as alcohol sugars in the formulation decreased tissue changes in the direction of hardening. Because of Millard reaction will reduce the sugars available as a plasticizer in the formulation. Sugar free doughnut showed suitable soft tissue during 14 days and indicated that sweetener has an effect on texture because it controls hydration and tends to disperse starch and protein molecules, thus preventing the formation of a continuous mass. No sugar doughnuts showed suitable sensory evaluation about color, taste, smell, texture, and overall acceptance.Therefore, a mixture of no-calorie and low-calorie sweeteners in this study can be used in the preparation of dietary doughnuts.
Hassan Sabbaghi; Aman Mohammad Ziaiifar; Mahdi Kashani-Nejad
Abstract
In this research, stepwise cooking and temperature fuzzy controller were designed during the infrared irradiation of apple with intermittent heating method. For this purpose, the dry blanching process and dehydration of apple slices were examined at three temperatures of 70, 75 and 80 °C based on ...
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In this research, stepwise cooking and temperature fuzzy controller were designed during the infrared irradiation of apple with intermittent heating method. For this purpose, the dry blanching process and dehydration of apple slices were examined at three temperatures of 70, 75 and 80 °C based on the blanching speed and vitamin C preservation. The fuzzy controller of the temperature with the feedback loop was designed, simulated, and implemented by comparing two first and second order transfer functions in MATLAB software. Simulation efficiency was examined using the indices of integral squared error (ISE), integral absolute error (IAE), integral time-weighted absolute error (ITAE) and steady state error (ess). The results revealed that the temperature of 80 °C and time of 15 minutes were appropriate for blanching operation and temperature of 70 °C was appropriate for dehydration. The simulation results confirmed that the higher order of the transfer function led into a faster response, but increase in oscillations and reduction in the stability were not appropriate. For the first-order transfer function, the values of efficiency indices, including (ISE), (IAE) and (ITAE) were calculated to be 0.760, 0.821 and 0.589, respectively, of second-order transfer function. The simulation indicated the reliability of the fuzzy control model and showed an acceptable computational efficiency, since the fuzzy rule test during simulation showed a high sensitivity to maintain steady state error (ess) close to zero.
Hassan Sabbaghi; Aman Mohammad Ziaiifar; Mahdi Kashani-Nejad
Abstract
Introduction: Fruits and their products in the dried form are good sources of vitamins, energy and minerals. However, during the process of drying or dehydration there are changes in quality parameters in dried products. Texture is one of the most important quality attributes of fruits during drying, ...
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Introduction: Fruits and their products in the dried form are good sources of vitamins, energy and minerals. However, during the process of drying or dehydration there are changes in quality parameters in dried products. Texture is one of the most important quality attributes of fruits during drying, reflecting their mechanical and microstructural properties. Apple is perishable fruit. Drying of apple is very important because of High losses are experienced during the seasonal glut. A novel process in food industry is the simultaneous infrared dry blanching and dehydration operation (SIRDBD) with intermittent heating method (radiation at constant temperature) exerted on fruits and vegetables that is known to enhance the quality of the final product. In the food industry, end-products must achieve a compromise between several properties, including sensory, sanitary and technological properties. Prediction of changes in texture during drying could be helpful in a better process control and improvement in overall acceptability of a dried snack food. The change of the elastic or viscoelastic texture of the fresh apples to rigid, fragile and brittle in the apple chips were evaluated by instrumental and sensory methods. Many attempts have been made to describe the viscoelastic behavior of dehydrated fruits and vegetables. Maxwell’s or compression models are limited to homogeneous, isotropic materials. In contrast, texture profile analysis (TPA) is more suitable for heterogeneous biological materials and shows a good correlation with organoleptic evaluation. Typical TPA parameters are including hardness, adhesiveness, springiness, cohesiveness, gumminess, chewiness and resilience. In this research, for the first time, textural analysis of dried apple slices by infrared heating at different temperatures and different moisture levels was performed. Finally, the optimum texture and overall acceptance of the product are described according to the instrumental analysis. Materials and methods: Apples (Golden Delicious variety) were purchased from a local market and kept in 0°C±1°C and relative humidity ranging from 90% to 95%. Before every thermal processing, the apple specimens were picked up from the cold storage and then they were put into use after reaching the ambient temperature. The samples were skinned manually and then cut into slices with different thicknesses of 5mm, 9mm and 13mm, all 20mm in diameter. The sliced apples were immediately subjected to simultaneous blanching and infrared drying. The texture of dehydrated apple slices using infrared radiation at three surface temperatures of 70, 75 and 80 °C were studied. The product in three thicknesses was dried to achieve a moisture level of 15, 20 and 25% wet weight basis. Then, texture profile analysis (TPA) was carried out to 50% compression strain using texture analyzer. The sensory evaluation of dried slices was also considered for desire texture (Good mouth feels texture, lack of hard tissue, no shrinkage) and overall acceptance (The final acceptability of the product in terms of total sensory properties including color, texture, flavor and aroma) by 10 professional panelists. For statistical analysis, a completely randomized design (CRD) was used in a factorial form (33) and Duncan test with 95% confidence level. Result & Discussion: The results showed that drying to studied moisture levels reduced the hardness and adhesiveness and increased springiness, cohesiveness, gumminess, chewiness and resilience in comparison with raw apple tissue. Hardness of samples dried at higher temperature was higher due to rapid removal of moisture which might have caused collapse of capillary voids inside the product. Due to shrinkage samples became denser and thus a larger fracture force was to be expected. As water content increases (i.e., higher RH) water plasticizes the cell walls and the material and product becomes softer and more pliable, thus hardness decreases. The increase of hardness could be because the rapid mass transfer that damaged the membrane and cell structure of the fruits during drying. Another important factor responsible for the increase of hardness of finish-dried samples is the low final moisture content when compared with other samples. High temperature drying method enables samples to reach low moisture content at relatively short duration and therefore the product with harder texture was obtained. The maximum value of adhesiveness was observed for fresh apples, which could be attributed to the high moisture and sugar content. Adhesiveness decreased with moisture loss, indicating the availability of free water on the sample surface. A significant decrease in springiness following high-temperature drying could be attributed to the glass transition phenomenon and changes from elastic to plastic behavior. In the period of softening, cohesiveness increased with moisture loss. Hardening caused a decrease in cohesiveness depending on the drying temperature. Gumminess is the energy required to disintegrate a semisolid food to a state of readiness for swallowing. High values of gumminess revealed “firm” and “crisp” with a cell rupture mode of tissue failure and lowest values of gumminess could be classified as “soft”. At the end of drying and with apple hardening, chewiness increased to values equal or above initial chewiness, indicating that a larger amount of energy is needed to masticate dried apples. Resilience had increasing with moisture loss. By increasing the thickness of the slices, the cohesiveness and springiness decreased and hence chewiness significantly decreased. The overall acceptance and desire texture in dried samples was observed at lower water evaporation rate conditions (lower temperatures, lower thickness and higher moisture content). In these conditions, the hardness of apple slices tissue was equal to 695.177 ± 7.685 grams. During drying of the apple, textural behavior was varied from the viscoelastic (higher initial hardness, with cohesiveness, springiness and lower resilience) to elastic and then to plastic or glassy.
Hassan Sabbaghi; Aman Mohammad Ziaiifar; Mahdi Kashani-Nejad
Abstract
Introduction: L-Ascorbic acid (vitamin C) is the most important vitamin in terms of nutrition. Ascorbic acid is a thermolabile (heat-sensitive) compound that can be degenerated aerobically or anaerobically. The degradation rates of ascorbic acid differ with the changes in environmental conditions such ...
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Introduction: L-Ascorbic acid (vitamin C) is the most important vitamin in terms of nutrition. Ascorbic acid is a thermolabile (heat-sensitive) compound that can be degenerated aerobically or anaerobically. The degradation rates of ascorbic acid differ with the changes in environmental conditions such as temperature and water activity. It is ascertained that the other nutrients residing in a food can be preserved in case the Vitamin C content is preserved. Thus, the compound is considered as the nutritional quality index during the food processing. The simultaneous infrared dry-blanching and dehydration (SIRDBD) with intermittent heating method is a novel process in which the temperature is kept constant. Over-blanching causes product quality decline and nutrients, especially vitamins, deterioration. Therefore, the precise process conditions (time and temperature) are specified with the objective of preventing over-processing. To do so, such factors as access to the specific center temperature, access to a certain level of enzymatic inactivation and preservation of a given ratio of Vitamin C should be taken into account. This is subject to the biophysical properties of fruits and slices size and shape. The aim of this study was to determine the appropriate operating conditions for blanching step. For this purpose, the effect of irradiation temperature and thickness of the product on the destruction of polyphenol oxidase (enzymatic browning agent) and vitamin C were investigated.
Materials and methods: Apple slices (Golden Delicious variety) were prepared with thickness of 5, 9 and 13 mm and 20 mm in diameter. Irradiation was carried out at three constant temperatures of 70, 75, and 80 ° C. The central temperature of the product was recorded during processing. To evaluate the enzymatic activity of polyphenol oxidase (PPO) and its effect on the product color, apple slices were removed from the device in 2- minute intervals and the process was continued till the time no sign of color change stemming from catechol reagent addition was observable. Vitamin C content measurement was carried out with 30- minute intervals during drying till apple slice reaches constant weight. It was performed based on titration by the use of 2, 6-Dichlorophenol-Indophenol (DCPIP). To calculate the browning index (BI) due to PPO activity, image acquisition was made with the use of a flatbed scanner. The treated samples were placed on the scanner and then a black box was utilized so as to prevent the interferences of the peripheral lights and light reflections. The images featured a 300 dpi quality and were saved in TIFF-24 bit format. Color analysis of the obtained images was carried out in color spaces L*a*b* by the use of “color space convertor” pelagin in ImageJ software, version 1.6.0. Statistical analyses were carried out in SPSS software, version 19. To do so and in order to assess the time required time for the blanching, there was made use of completely randomized design (CRD) in factorial format (32) considering two factors, namely thickness (in three levels) and temperature (in three levels). The statistical analyses of the vitamin degradation kinetic constant (k), as well, were conducted based on randomized complete block design (RCBD) in the course of which the temperature and thickness were considered as the block and the treatment, respectively. Mean comparisons were undertaken based on Duncan test in a 95% confidence level (P
Najimeh Taghavi; Aman Mohammad Ziaiifar; Habibollah Mirzaee; Alireza Sadeghi Mahoonak; Mohammad Ghorbani; Hassan Sabbaghi
Abstract
Introduction: Frying is a unit operation in which food is heated in oil to alter its eating quality. First reviews of frying performed on the principles of heat and mass transfer and oil uptake in foods, because during the frying process heat and mass transfer occurred between product and frying medium. ...
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Introduction: Frying is a unit operation in which food is heated in oil to alter its eating quality. First reviews of frying performed on the principles of heat and mass transfer and oil uptake in foods, because during the frying process heat and mass transfer occurred between product and frying medium. In fact, the food loses moisture content and lost water will replace with oil. The moisture loss in the product causes rapid formation of the crust while the core remains soft and moist; therefore higher oil uptake occurred in the crust. There is an argument about the effect of frying on the sensory characteristics of foods, changes of nutritional value and health concerns in fried foods and methods applied to reduce oil content. Consumers prefer eating foods with lower oil content. One of the typical methods for reducing oil uptake in food commodities is using of coating before frying process. Edible coatings are known as a thin layer of edible polymers which is placed on the surface of the food. Application of edible coating decreases some destructive factors like the presence of various gases such as oxygen and carbon dioxide and humidity. Hydrocolloides are referred to a group of polysaccharides and proteins which create many features such as consistency in aqueous solutions, foams stability, emulsions, improve the sense of mouth and create a similar state of fatty and oily mode for products with lower content. In fact hydrocolloid coatings can reduce the excessive oil uptake due to their interesting thermogelling properties and at the same time they are invisible and have no negative influence on the sensory attributes of fried foodstuff. Even more, fried products have low fat content with improved nutritional values, higher crispiness and better palatability. Pishmeh is described as deep-fried dough pastry which is usually prepared by local producers of Turkmen (Iran). This product absorbed high oil content during frying process. This study was carried out to evaluate the effects of hydrocolloid coatings including Aloe Vera, Carboxymethyl cellulose (CMC) and Xanthan on the oil uptake of Pishmeh.
Materials and methods: Hydrocholloid materials such as Aloe Vera, Carboxymethyl cellulose (CMC), Xanthan and complex of Xanthan-CMC were used. The gums were prepared at concentrations of 0.5 and 1% by dissolving in water at 70 ° C by stirring with a homogeneous mixer until a clear suspension was obtained. To extract the Aloe Vera gel, first, a thick epidermis (The shell) was separated from parenchyma using cutter. The parenchyma was immersed in a container of distilled water to lose bitter taste for 5 minand then parenchyma was taken out from water and transferred into a mixer (for 1 minute at 25 °C) and finally converted to a uniform colloidal solution. In the next step, the prepared dough samples were immersed in the colloidal suspensions for 1 min and the coated samples were placed on a mesh tray to remove the excess residuary coats. The samples were fried in sunflower oil at 170 °C for 6 min. The samples were then placed on a metal tray after frying to remove excess oil. After removing the oil and reaching the ambient temperature, physical and chemical tests were carried out on the sweet Pishmeh. The experiments included determination of moisture, fat content, measurement of coating parameters (such as coating ratio and frying efficiency) and color analysis.
Results and Discussion: The result showed that coating by hydrocholloid materials was led to decrease water loss in comparison to non-coated sample (control) during frying. Furthermore, oil uptake of coated samples was lower than control sample (p
Hassan Sabbaghi; Aman Mohammad Ziaiifar; Mahdi Kashani-Nejad
Abstract
Introduction: Color is the most important feature of food appearance, since it strongly affects consumer acceptance. The abnormal color is closely related to degradation of food quality or food spoilage. Various factors affecting color of the product include the composition and surface properties of ...
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Introduction: Color is the most important feature of food appearance, since it strongly affects consumer acceptance. The abnormal color is closely related to degradation of food quality or food spoilage. Various factors affecting color of the product include the composition and surface properties of the foodstuff, process conditions (temperature and time), and the type of pretreatment processes. Therefore, for the design of a new process, kinetic modeling is essential for extracting basic kinetic information in a system to predict changes. Intermittent irradiation is the novel processing method in food industry which the surface temperature of product is kept constant. Darkening process occur in the fruit slices during drying, resulting in undesirable color changes. Undesirable color variations can be attributed to Millard's browning reactions between sugar and amine compounds or ascorbic acid oxidation. Thermal degradation kinetic models define maximum maintenance conditions for qualitative factors to produce a safe food. Color descriptive models are known as zero order, first order, and fractional conversion model. The fractional conversion model represents the response rate required at a given time to complete a phenomenon. This modeling is necessary for designing online quality control systems for thermal processing in the food industry and preserving the apparent quality of the product by optimizing the most important drying parameters. In this study, the kinetic of color changes in apple slices was investigated, in order to maintain appearance quality of product during simultaneous infrared dry-blanching and dehydration with intermittent irradiation.
Materials and Methods: Apple slices (Golden Delicious Variety) were dried in three thicknesses of 5, 9 and 13 mm using three constant surface temperatures of 70, 75 and 80 ° C. Image acquisition of apple slices was made use of a flatbed scanner with an interval of 15 minutes during processing. The treated samples were placed on the scanner and then a black box was utilized so as to prevent the interferences of the peripheral lights. The images featured a 300 dpi quality and were saved in TIFF-24 bit format. The color analysis of the images was carried out in color space of L*a*b* by the use ImageJ software, version 1.6.0. The lightness parameter (L), redness (a), yellowness (b), the intensity of the color changes (∆E), chroma (Cr) and browning index (BI) were described during product processing using fractional conversion model. Equilibrium color parameters (Cf) were also used as an indicator to compare different process conditions. The fitting of the model was done using the curve fitting toolbox in the 2009 version of the MATLAB software with 95% confidence level (P
Hassan Sabbaghi; Aman Mohammad Ziaiifar; Mahdi Kashani-Nejad
Abstract
Introduction: Frying phenomena occur during the immersion of the product in oil at a temperature of 150–200 ºC, where a simultaneous heat and mass transfer take place. This is the most popular thermal processes of potato cooking. This fast drying is critical to improve the mechanical and structural ...
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Introduction: Frying phenomena occur during the immersion of the product in oil at a temperature of 150–200 ºC, where a simultaneous heat and mass transfer take place. This is the most popular thermal processes of potato cooking. This fast drying is critical to improve the mechanical and structural properties of the final product. These conditions lead to high heat transfer rates, rapid cooking, browning, texture and flavor development. The fried potato is easier to transport and provides better texture. Researchers have assumed the existence of two regions for fried product, separated by an interface: the core (unfried) and crust (fried) regions. In general, frying process is very complex for two main reasons: i) due to the simultaneous heat and mass transfer between food material and frying oil, ii) due to the progressive deterioration of the oil and structural changes in foods (crust and core regions). The moving boundary problem may be found in many areas of frying research involving heat and/or mass transfer. In this study, heat and mass transfer is entirely investigated during frying of potato strips. The transport phenomena during frying are including: i) Heat convection from the hot oil to the interface via the crust region, ii) Water evaporation at the moving interface at a temperature of 100 ºC, iii) The unsteady state heat conduction in both regions of crust and core, iv) The oil uptake into food. As a result, high temperature and low moisture conditions develop as frying proceeds. Water vapor bubbles escaping from the surface of the food cause considerable turbulence in the oil. Therefore, Heat and mass transfer are dependent on each other during frying process. In fact, heat and mass transfer during frying can be controlled by heat transfer at the product surface. Evaporation rate depends on the temperature difference between oil and boiling point of water. There is little information on modeling, both empirical and phenomenological, for moisture loss and oil uptake during frying. Knowledge of accurate heat and mass transfer parameters is important for modeling processes. Designing of frying processes is possible through the use of mathematical models. The aim of this study is to develop a more completely and realistic approach for determining of heat and mass transfer parameters and their relation to oil temperatures. The main process parameters influencing oil uptake are frying temperature and duration. Heat transfer coefficients for different oil temperatures determined using simple method. Mass transfer of water was assumed to be governed by Fick's law of diffusion. For more details, empirical models were used to describe the mass transport in forms of moisture and oil.
Materials and Methods: The frying operation of potato strips was performed in the fryer that was equipped by thermo controller system with K type thermocouple at three different oil temperature of 145, 160 and 175 ºC for 60, 120, 180 and 240 seconds. The core temperature changes of product recorded on computer during process using T type thermocouple connected to data logger. The moisture and oil content of samples measured for each process time and temperatures. The heat and mass transfer parameters such as kinetic coefficients of moisture (Km) and oil transfer (Ko), mass transfer coefficient (Kc), effective diffusivity (D) and heat transfer coefficient (h) were evaluated with dimensionless temperature and concentration ratio plots and also empirical equations. Relationship of these parameters to the temperature of the oil investigated using the Arrhenius equation. Thermal conductivity of potato strips during frying determined as a function of moisture content using the Anderson and Spell equations.
Results & Discussion: The results showed that mass transfer Biot number (Bim), mass transfer coefficient (Kc) and effective moisture diffusivity (D) increased significantly with increasing in oil temperature. In regression models, the linear correlation between kinetic constant of water loss and oil uptake was observed that is verification on effect of drying pretreatment on reducing oil uptake. In fact, with increasing of oil temperature the kinetic constant of water loss increased and caused increased in kinetic constant of oil uptake. Kinetic models could correctly confirm determination of mass transfer parameters. The heat transfer Biot number (Bih), convective heat transfer coefficient (h) and product thermal conductivity (k) decreased significantly with an increase in process temperature. With increasing in the rate of evaporation, following greater amount of input energy used for water loss. This would reduce the amount of available energy to increase internal energy of product and thus reduce the convective heat transfer coefficient at high temperatures. Frying process caused remove of water from product and increasing of porosity, thus observed gradually fell in thermal conductivity. Although the minimum thermal conductivity at various temperatures are close together, but two equations of Anderson and Spell showed significant difference for values of thermal conductivity and Spell was more close to published papers. High activation energy is achieved for lower moisture content that is normally due to the strong water-substrate interaction.
Hassan Sabbaghi; Aman Mohammad Ziaiifar; Mahdi Kashani-Nejad
Abstract
Introduction: Texture is one of the most important attribute in foods and is always issues for the manufacturing of fried products, because texture plays a crucial role in consumer acceptance and the perceived quality of foods. Prediction of changes in texture during frying can be helpful in process ...
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Introduction: Texture is one of the most important attribute in foods and is always issues for the manufacturing of fried products, because texture plays a crucial role in consumer acceptance and the perceived quality of foods. Prediction of changes in texture during frying can be helpful in process control. Structurally, this quality parameter in fried potato strip made from the formation of a composite structure with two layers as: dry and oily outer layer (crust), and a moist or soft interior layer (core). So, the explanation of texture development during frying is difficult because of the innate heterogeneity of the fried potato tissue. Moreover, both thermal degradation and texture development are functions of variables such as processing time, oil temperature and vaporized moisture of product. Textural changes during frying described as the result of various physical, chemical, and structural changes involved in the frying process. One solution to reduce the complexity of real bio systems in engineering is using various empirical correlations. Application of these relations for prediction of textural changes during frying can help us for understand the proper conditions to achieve desired texture. The purpose of the present investigation was to study the influence of the frying temperatures on texture of potato strips. In fact, textural changes during various stages of frying potatoes including initial heating, surface boiling and falling rate period were investigated. Texture evaluation is done by mechanical measurements, because the stimulus in texture perception is mainly mechanical. Also, textural studies are not clearly performed up to now with focus on cook value as a main factor in potato frying. According to the definition of cook value, this parameter indicates total time of baking in 100 °C. Fractional conversion applied as a technique for analyzing texture degradation kinetics and softening of vegetables upon prolonged heating. A few assumptions were made during study: i) Potato shrinkage is neglected ii) Product is not completely dried iii) The stages of frying were considered separable only based on surface and center temperature iv) Texture of potato strips affected by cook values of each frying stages. Materials and methods: The potato strips with specified size fried at a constant temperature of 145, 160 and 175 °C for 30, 60, 90, 120, 150, 180, 210 and 240 seconds. Then, various stages of the process were separated using surface and center temperature profiles of product that were recorded by data logger and T-type thermocouple in computer. Heating extremity of each stage was determined using cook value parameter. Mechanical properties such as apparent modulus of elasticity (Secant modulus) and toughness were used to show which occurred during frying. The secant modulus (S) variations described using fractional conversion model. The degree of cooking for each sample was expressed in term of cook value and its relationship with the overall acceptance of product was examined. Finally, the suitable temperature was determined by sensory evaluation to achieve the desired texture to determine the proper cook value and to prevent over cooking of product. Results and Discussion: The stages of frying by immersion for experimental conditions can be divided as: I. Initial heating (The first 30 seconds for all oil temperature) II. Surface boiling (30-60 s). III. Falling rate (up to end, longest period). IV. Bubble end point (not considered). Generally, higher oil temperature showed larger center and surface cook values because of the fast temperature increase inside product. During initial heating period (I) because of temperature increase without boiling, the changes in cook values versus time are negligible. Surface cook value increased slowly compared with core during surface boiling. Maximum cook value for core temperature was higher, because of the vapor pressure at the center of the product and thus water evaporation at temperatures above 100 °C (cook value above 1 second). During falling rate period due to gradual reduction of evaporation rates, and thus the vapor pressure drop within product, boiling temperature reduced to 100 ° C. The major part of texture destruction occurred during initial heating period and the changes of textural characteristic were going to be constant at the end of surface boiling. The slope of the regression line for fractional conversion model decreased as temperature increased and therefore product was harder, but equivalent value of secant modulus was independent of process temperature. The consumer texture desired was found for temperature of 160 °C with medium cook value. The kinetics of potato softening followed an exponential decay equation with good correlation on empirical data. The temperature dependence of the degradation rate was reliably modeled by the Arrhenius equation. Activation energy (Ea) for model parameters Se and Ks was 13047.12 and -24949.74 J/mol, respectively. Negative Ea for kinetic constant (Ks) indicated an inverse relationship with oil temperature. In addition, elevated oil temperatures caused less softening of French fries because of higher cook value and thus higher evaporation rate.
Hassan Sabbaghi; Aman Mohammad Ziaiifar; Alireza Sadeghi Mahoonak; Mahdi Kashani-Nejad; Habibollah Mirzaee
Abstract
Introduction: Heat transfer processes are important for almost all aspects of food preparation and play a key role in determining food safety. Temperature difference between the source of heat and the receiver of heat is the driving force in heat transfer. Convection is the transfer of heat by the movement ...
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Introduction: Heat transfer processes are important for almost all aspects of food preparation and play a key role in determining food safety. Temperature difference between the source of heat and the receiver of heat is the driving force in heat transfer. Convection is the transfer of heat by the movement of groups of molecules in a fluid. The groups of molecules may be moved by either density changes or forced motion of the fluid. In a typical convective heat transfer a hot surface heats the surrounding fluid, which is then carried away by fluid movement. The convection heat transfer coefficient or h value is defined as the rate of heat that will be convected at the product surface–fluid interaction through a unit surface area of the material if a unit temperature gradient exists between the product surface and the surrounding fluid and is very important in modeling and design of frying systems for foods. Frying is a very turbulent process with random and dynamic movement of small bubble particles over the boundary layer of the product surface. Deep fat frying is a very fast method of food processing among conventional heat transfer methods. The frying by immersion can be divided into four stages: (1) initial heating (the temperature of the internal part is slowly increased to boiling point); (2) surfaces boiling (crust formation and higher oil turbulence); (3) falling rate (long period, vapor transfer at the surface decreases) and (4) bubble end point (dried product). These four states during frying can be generalized as non-boiling phases (stages 1 and 4) and boiling phases (stages 2 and 3). So, Convection heat transfer during immersion frying may be divided into two stages: 1) free convection during initial heating of the material and 2) forced convection during the boiling phase. In this study, the convective heat transfer coefficient investigated as a function of the water loss rate during frying process due to the effects of evaporation and boiling on this thermal parameter. Understanding of changes in heat transfer coefficient for thermal control of the frying process is very important to achieve optimum quality of product. Materials and Methods: The potato stripes with specified size are fried at temperature of 145, 160 and 175 °C for 60, 120, 180 and 240 seconds using sunflower oil. The center and surface temperatures of potato stripes were recorded with two-second intervals using T type thermocouple and data logger. Moisture content of French fries was measured by drying them within an oven instrument. Mean moisture and oil content of potato stripes was measured. The h value was estimated and its changes studied during process using heat energy balance between the sample and oil by assuming that total heat transferred by convection from oil to potato is equal to the sum of energy spent on heating potato and energy spent on water evaporation. Moreover, changes in the fraction of total heat used for evaporation at different temperatures were expressed as an empirical model. Results and Discussion: The results showed that the temperature at the center increased up to the evaporation temperature (“A” zone: about 60 seconds after process beginning). Then, it remained constant at this temperature for a while due to evaporation (“B” zone: about from 60 s till 160 s). This constant temperature period decreased as temperature increased. After this period, the center temperature approached to that of oil (“C” zone: increasing product surface temperature to oil temperature). These various stages of the process were separated. The moisture loss rate was high at the beginning of frying and oil uptake increased as the moisture content diminished. As oil temperature increases, the sample moisture content for the same frying period decreases since an increase in temperature results in a higher kinetic energy for water molecules leading to a more rapid moisture loss in form of vapor. The heat transfer coefficient is increased as the oil temperature increased and at the first time of process observed higher h value. It was due to more water loss rate and so the higher turbulence within the oil before crust formation and for elevated temperatures. The maximum heat transfer coefficient for the temperatures of 175, 160 and 145 °C is estimated 943.68, 847.81 and 682.64 W/m2°C, respectively. Estimated h value also shows a linear increase with water loss rate. Since most of the energy used is associated with the evaporation of water present in the potato, for lower temperatures, the fraction of total heat needed to complete the evaporation is higher and it also represents lower h values by reducing the oil temperature. Also, the variation in the fraction of total heat used for water evaporation showed exponential rise to maximum behavior. Conclusion: The proposed model for studying evaporation heat fraction fitted experimental data properly, with standard error values range of 0.01-0.03. Evaporation heat fraction could have been affected by oil temperature changes, water loss rate and h value (linear depend with the water loss rate) during process.
