Microstructural evaluation of deep-fat fried chicken nugget batter coating using confocal laser scanning microscopy

Porosity and pore size distribution are very important microstructural properties of fried foods needed in process optimization and product development. The objective of this study was to characterize the pore properties and quantify fat distribution in deep-fat fried chicken nuggets batter coating using confocal laser scanning microscopy. Samples were fried at three temperatures namely 170, 180 and 190 °C. Detached batter coatings were stained non-covalently and 2-D images were obtained at fluorescence and reflection modes of the microscope. The images were analyzed for fat and pore distribution. Fat distribution obtained from image analysis was significantly (P < 0.05) affected by the frying temperature and time, and it decreased within the depth of the sample thickness. There was a strong correlation between fat distribution and fat content obtained by the conventional method at two temperatures, 180 and 190 °C. Porosity ranged between 4.97% and 32.7% and was significantly influenced by frying temperature. Pore size varied approximately between 1.20 and 550 μm. Frying process led to the formation of more micropores (pores < 40 μm) and bigger (pore ? 216 μm) pores.     

Effect of microwave pre-thawing of frozen potato strips on acrylamide level and quality of French fries

The objective of this study was to reduce frying time, and hence acrylamide level of French fries by microwave pre-thawing of frozen potato strips. Effect of this pre-treatment on acrylamide content and quality attributes of French fries was determined. Frozen par-fried potato strips (8.5 × 8.5 × 70 mm) were thawed in a microwave oven prior to final frying in sunflower oil at 170, 180, and 190 °C. Potato strips that were final fried without pre-thawing were considered as the control. Acrylamide analysis was performed by liquid chromatography–mass spectrometry (LC–MS) method. Microwave pre-thawing of frozen strips reduced the acrylamide level of French fries by 10% (from 17.7 to 15.9 ng/g), 89% (from 72.1 to 8.0 ng/g), and 64% (from 50.5 to 18.4 ng/g) for frying at 170, 180, and 190 °C, respectively, in comparison to the control samples. Quality attributes (texture, color, and oil content) of pre-treated strips were found to be comparable to those of the control.     

Kinetics of oil uptake during frying of potato slices:: Effect of pre-treatments

Oil uptake in fresh, blanched and, blanched and dried potato slices was studied during frying. Potato slices blanched in hot water (85 °C, 3.5 min) and potato slices blanched (85 °C, 3.5 min) and then dried until to a moisture content of ∼60 g/100 g (wet basis) were deep fried in sunflower oil at 120, 150 and 180 °C. A control treatment consisted of unblanched potato slices without the pre-drying treatment (fresh samples). It was studied applying two empirical kinetic models in order to fit the oil uptake during frying: (i) a first order model; (ii) a proposed model, with a linear time behavior for short times, while time independent for long times. Oil uptake was high even for short frying times at the different temperatures tested suggesting that oil wetting is an important mechanism of oil uptake during frying. For control slices, oil uptake increased approximately by 32% as the frying temperature decreased from 180 to 120 °C at moisture contents ?1 g water/g dry solid. No apparent effect of frying temperature in oil uptake was observed at moisture contents ?0.5 g water/g dry solid in fried slices previously blanched and dried. The two kinetic models studied fitted properly the values of oil uptake during frying, with similar correlation coefficient r².     

Experimental measurement of physical pressure in foods during frying

Two important factors affecting the oil uptake in foods during deep fat frying are water content and pressure development. In the past frying studies, the physical pressure has not been measured experimentally but was calculated using computer models, which has resulted in disagreements about its magnitude. The present study tries to explain the complex mass transfer mechanisms taking place during deep fat frying with respect to real time pressure variations inside potato discs and chicken nuggets. Frying experiments were performed at two temperatures of 175 °C and 190 °C for 200 and 240 s for potato discs and chicken nuggets, respectively. The gage pressure increased rapidly above the atmospheric pressure immediately after the samples were introduced into the hot oil. The rise in pressure was greater in potato discs with greater initial moisture content. This was expected due to sudden moisture flash-off. As frying progressed, the temperature inside the samples increased whereas the gage pressure started decreasing and became negative. The onset of negative pressure was observed during initial stages of frying for chicken nuggets, but in the middle of frying for potato discs. The negative pressure values before the product is taken outside the fryer may cause increased oil uptake during frying itself. During the post frying cooling, the pressure further decreased and reached negative values. The negative pressure is expected to have caused rapid absorption of surface oil during both frying and cooling stages.     

Quality changes in chicken nuggets fried in oils with different degrees of hydrogenatation

Quality changes in chicken nuggets fried in different mixtures of hydrogenated and non-hydrogenated canola oils were studied. Colour, texture, oil and moisture contents of the chicken nugget samples fried at 190 °C for 30, 60, 90, 120, 180, 240 and 300 s were investigated. The proportions of hydrogenated to non-hydrogenated frying oils used in the study were 0%, 20%, 40%, 60%, 80% and 100%.Results indicate that increase in frying time resulted in decreased product lightness (L*) whereas redness (a*) and yellowness (b*) increased. The maximum load to puncture increased with increasing frying time. In addition, oil content increased slowly with frying time, and moisture content decreased as expected. Both frying times and oil types have significant effects on the quality parameters investigated. First-order kinetic equation was used to describe colour changes as well as oil and moisture contents of the samples. Oil and moisture contents had negative correlation relationship. With increasing degree of oil hydrogenation, the surface colour of the fried chicken nuggets samples were lighter, texture increased, oil and moisture contents decreased.     

Effect of microwave pretreatment on mass transfer during deep-fat frying of chicken nugget

The effect of a microwave pretreatment at different time duration on the mass transfer of chicken nuggets during deep-fat frying was studied. Coated chicken nugget samples pretreated in a microwave oven for 1-2 min were fried at 160 °C for times ranging from 0 to 300 s to evaluate the mass transfer as compared to the samples without a microwave pretreatment. Microwave pretreatment had a significant effect on moisture loss and oil uptake of chicken nuggets during deep-fat frying.     

油炸过程中挂糊油炸猪肉片外壳食用品质的变化

本文研究了不同的油炸时间和温度对小酥肉品质的影响。分析了150、160、170、180、190 ℃条件下油炸150、170、190、210 s后小酥肉的含水率、含油率、质构、色泽、硫代巴比妥酸值（TBARs值）,胃蛋白酶消化率的变化情况。结果表明,随着油炸温度的升高和油炸时间的延长,小酥肉整体水分含量显著降低（P<0.05）,而油脂含量显著升高（P<0.05）,硬度、咀嚼性、胶粘性、弹性均显著增加（P<0.05）,TBARs值呈现逐渐增加的趋势,胃蛋白酶消化率随着油炸时间的延长呈现逐渐升高的趋势,随着油炸温度的升高呈逐渐降低的趋势。170 ℃条件下油炸170 s的小酥肉整体的含水率为44.15%,肉芯的含水率为64.19%,整体的含油率为50.77%,肉芯的含油率为28.97%,硬度适中,色泽金黄,外酥里嫩,口感最佳。

     

Frying Oil Temperature: Impact on Physical and Structural Properties of French Fries During the Par and Finish Frying Processes

The physical, mechanical, and structural changes in French fries at different frying temperatures (170, 180, and 190 °C) during par and finish frying were investigated. The experimental frying process includes blanching, par frying, freezing, and finish frying at the point of consumption. Par frying temperature had a greater effect on moisture reduction than on processing time with the greatest moisture loss at 180 °C after 160 s. At 180 °C, fries had the highest total oil uptake (29.56 % wb). Frying caused a significant change in color with the greatest effect at 190 °C. Micro-CT scan images showed structural changes in French fries over the course of the frying process. Frying time increased the volume of the pores relative to the size of the crust, and this was positively correlated with the hardness and shear cutting force. Overall, within the range of frying conditions studied, the higher temperature (190 °C) resulted in the lowest amount of oil uptake and the crispiest texture.     

The formation of potentially harmful compounds in churros, a Spanish fried-dough pastry,as influenced by deep frying conditions

Colour, moisture, hydroxymethylfurfural (HMF) and acrylamide (AA) were investigated in traditional Spanish churros. Samples were deep-fried in sunflower oil at lab-scale temperatures of 180, 190 and 200 °C and for frying times of 2, 3, 5 and 7 min. Fresh made churros were also obtained from local producers. HMF ranged from 1.2 ± 0.02 to 221.4 ± 2.02 mg/kg for lab-scale experiments and an average of 74.3 ± 47.5 mg/kg was recorded in commercial samples. AA ranged from below the limit of quantitation to 90 ± 0.6 μg/kg for lab-scale experiments and an average of 46 ± 24.5 μg/kg was measured in commercial samples. Temperatures between 185 and 200 °C are commonly used to obtain churros with an acceptable palatability and a crispy surface. However, HMF and AA levels increased nearly two-fold from 190 to 200 °C at the same frying times, indicating that a more precise control of frying temperatures is required to minimize their formation.     

Reduction of acrylamide formation in potato slices during frying

Reduction of acrylamide formation in potato chips was investigated in relation to frying temperature and three treatments before frying. Potato slices (Tivoli variety, diameter: 37 mm, width: 2.2 mm) were fried at 150°C, 170°C and 190°C until reaching moisture contents of ∼1.7 g water/100 g (total basis). Prior to frying, potato slices were treated in one of the following ways: (i) soaked in distilled water for 0 min (control), 40 min and 90 min; (ii) blanched in hot water at six different time-temperature combinations (50°C for 30 and 70 min; 70°C for 8 and 40 min; 90°C for 2 and 9 min); (iii) immersed in citric acid solutions of different concentrations (10 and 20 g/l) for half an hour. Glucose and asparagine concentration was determined in potato slices before frying, whereas acrylamide content was determined in the resultant fried potato chips. Glucose content decreased in ∼32% in potato slices soaked 90 min in distilled water. Soaked slices showed on average a reduction of acrylamide formation of 27%, 38% and 20% at 150°C, 170°C and 190°C, respectively, when they were compared against the control. Blanching reduced on average 76% and 68% of the glucose and asparagine content compared to the control. Potato slices blanched at 50°C for 70 min surprisingly had a very low acrylamide content (28 μm/kg) even when they were fried at 190°C. Potato immersion in citric acid solutions of 10 and 20 g/l reduced acrylamide formation by almost 70% for slices fried at 150°C. For the three pre-treatments studied, acrylamide formation increased dramatically as the frying temperature increased from 150°C to 190°C.     

Frying — Effect of coating on crust microstructure,color, and texture of lean meat portions

Skinless, deboned chicken breast fillets were deep fat fried (190 °C) with and without a batter and breading coating system for up to 6 min (internal temp 70 °C). Non-coated samples show surface muscle fiber splitting as soon as placed in oil. As time progressed, the depth of damage increased. Shear force and work to shear increased significantly as a dense hard crust layer was forming. By the end, fillets lost about 1/3 of their weight and had a dark, dry, brittle crust. Frying with coating protected the meat inside and no major damages to muscle fibers were observed. Shear force also increased but the values were about half of what was observed in non-coated products, meaning that the meat inside was protected. Coated products became progressively golden brown (increased a* and b*, decreased L*) while the crust adhered well to the product. After frying, the products showed a small gain in weight which was the result of no structural shrinkage and absorption of a small amount of oil.     

Osmotic pre-treatment effect on fat intake reduction and eating quality of deep-fried plantain

The relationship between compositional changes during frying and an osmotic dehydration step before frying is described, including and explanation of their effects on eating quality of plantain chips. Three solutions were used to soak plantain slices (glucose, salt (NaCl) and glucose + salt) prior to deep-frying (170 °C). The temperature, moisture and fat changes during the frying were monitored, and the quality attributes and physicochemical properties were also determined. The osmotic pre-treatments had significant effects on most parameters of plantain chips. In general, treatments decreased oil intake, moisture content, and total volume and also reduced frying time, while the colour, rancidity, crispiness and sensory evaluation increased after 5 min of frying. However, a negative effect on high temperature induced colour development was observed for some of the treatments. Rancidity induction times were significantly higher for pre-treated samples, which is probably related to lower oil content, shorter frying times and lower water content.     

Moisture loss and oil uptake during deep fat frying of “Kroštula” dough

The mechanism of moisture transfer by diffusion and uptake of frying oil was studied during deep fat frying of Krotula dough with an initial moisture content of 0.4358 kg/kg (db). The experimental data were found to fit well to a first-order exponential model for moisture transfer (with a regression coefficient of 0.99). The effective moisture diffusivity of Krotula dough was determined for two periods of moisture loss (intensive in first 60 s and constant after 60 s of frying). In the frying range 0–60 s at temperatures of 160, 170, 180 and 190±1 °C, the effective moisture diffusivity values were 5.837, 6.607, 8.472 and 9.728×10^–9 m²/s, from which the activation energy (30 kJ/mol) was calculated by using an Arrhenius-type equation. The effective oil diffusivity values of Krotula dough in the frying time range 0–210 s at temperatures of 160, 170, 180 and 190±1 °C were 0.932, 1.135, 1.094 and 1.054×10^–9 m²/s, from which the activation energy (5.5 kJ/mol) was calculated by using an Arrhenius-type equation.     

Occurrence of heterocyclic amines in cooked meat products

Heterocyclic amines (HCAs), potent mutagens and a risk factor for human cancers, are produced in meats cooked at high temperature. The aim of this study was to determine the HCA content in cooked meat products (beef, chicken, pork, fish) prepared by various cooking methods (pan frying, oven broiling, and oven baking at 170 to 230 °C) that are preferred by U.S. meat consumers. The primary HCAs in these samples were PhIP (2-amino-1-methyl-6-phenylimidazo [4,5-b]pyridine) (1.49-10.89 ng/g), MeIQx (2-amino-3,8-dimethylimidazo [4,5-f]quinoxaline) (not detected-4.0 ng/g), and DiMeIQx (2-amino-3,4,8-trimethyl-imidazo [4,5-f]quinoxaline) (not detected-3.57 ng/g). Type and content of HCAs in cooked meat samples were highly dependent on cooking conditions. The total HCA content in well-done meat was 3.5 times higher than that of medium-rare meat. Fried pork (13.91 ng/g) had higher levels of total HCAs than fried beef (8.92 ng/g) and fried chicken (7.00 ng/g). Among the samples, fried bacon contained the highest total HCA content (17.59 ng/g).     

Determination of thermal conductivity and convective heat transfer coefficient during deep fat frying of “Kroštula” dough

In this study, the influence of oil temperature, water migration and surface temperature of Krotula dough on convective heat transfer coefficient was investigated. The convective heat transfer coefficient during deep fat frying was determined at temperatures 160, 170, 180 and 190±1 °C. Heat transfer coefficient was the highest at the start of deep fat frying process; 579.12±2.46, 583.88±1.81, 597.05±1.10 and 657.91±0.95 W/m² K for 160, 170, 180 and 190 °C of oil temperature, respectively. The smallest heat transfer coefficient was in the case of setting up a uniform period of water migration from sample, which corresponded to surface temperatures slightly higher than 100 °C; 26.53±0.63, 14.42±0.56, 56.78±0.49 and 37.52±0.54 W/m² K for 160, 170, 180 and 190 °C of oil temperature, respectively. Higher oil temperature for deep fat frying increased values of heat transfer coefficient. A steady-state method was used to determine thermal conductivity of Krotula dough in temperature range of 40–70±1 °C. The thermal conductivity first increased with temperature and then after reaching maximum values decreased. The maximum value 0.5985±0.0196 W/(m K) was determined at 47.5 °C. The minimal value 0.4723±0.0192 W/(m K) was determined at 65 °C.     

Effect of hydrocolloid coatings on the heat transfer and oil uptake during frying of potato strips

The effect of the hydrocolloid coatings (gellan gum and guar gum) of a food matrix on the heat transfer during a frying process was investigated and correlated with the oil uptake. While the potato strips coated with the hydrocolloid solutions were fried at 170 °C, the hydrocolloid coatings significantly reduced the heat transfer coefficients as well as oil uptake which became more apparent at higher concentrations. Thus, the oil uptake was found to have a good polynomial correlation (R² > 0.99) with heat transfer coefficients, suggesting that a rapid heat transfer led to an increase in the oil content of the fried samples.     

Porosity determination of deep‐fat‐fried coatings using pycnometer (Fried batter porosity determination by pycnometer)

The effect of processing conditions such as frying time and temperature, and batter formulation on pore development in deep‐fat fried chicken nuggets coatings were studied using helium pycnometer method. Chicken nuggets with preformed and laboratory prepared batter coatings were fried at temperatures between 170 and 190 °C for a time range between 0 and 240 s. There was significant (P < 0.05) effect of frying temperature and batter formulation on porosity. Porosity increased with frying time and temperature, and ranged between 2.15 and 47.92% for the preformed batter and 9.96 and 54.76% for the formulated batters. Apparent and bulk densities of the preformed batters increased and decreased with frying time, respectively, but both declined gradually with increasing frying temperature. As the level of rice flour in the formulation increased, apparent and bulk densities also increased. Batter formulation and frying temperature significantly (P < 0.05) influenced the variation in moisture and fat content of the fried batter. Porosity demonstrated positive and negative correlation with fat uptake and moisture loss, respectively, for all the batter coatings.     

Modeling water loss and oil uptake during vacuum frying of pre-treated potato slices

The objective of this research was to determine the kinetics of water loss and oil uptake during frying of pre-treated potato slices under vacuum and atmospheric pressure. Potato slices (diameter: 30 mm; width: 3 mm) were pre-treated in the following ways: (i) raw potato slices “control”; (ii) control slices were blanched in hot water at 85 °C for 3.5 min; (iii) blanched slices were dried in hot air until reaching a moisture content of ∼0.6 g water/g dry basis. The slices were fried under vacuum (5.37 kPa, absolute pressure, at 120, 130 and 140 °C) and atmospheric conditions (at 180 °C). Two models based on the Fick's law were used to describe water loss: (i) with a constant effective diffusive coefficient; and (ii) with a variable effective diffusive coefficient. Oil uptake data were fitted to an empirical model, with a linear behavior for short times whereas the model was time independent for long times. The variable diffusivity model better fitted experimental water loss, giving values of effective diffusivity between 4.73 × 10⁻⁹ and 1.80 × 10⁻⁸ m²/s. The proposed model for the study of the kinetics of oil uptake fitted the experimental data properly. Control and blanched vacuum fried potato chips increased their final oil contents to 57.1% and 75.4% respectively, when compared with those fried at atmospheric pressure. However, the oil absorption of dried vacuum fried potato chips diminished by ∼30%.     

Effect of Cooking Method (Baking Compared with Frying) on Acrylamide Level of Potato Chips

ABSTRACT:  The effect of cooking method (baking compared with frying) on acrylamide level of potato chips was investigated in this study. Baking and frying experiments were conducted at 170, 180, and 190 °C using potato slices with a thickness of 1.4 mm. Raw potatoes were analyzed for reducing sugars and asparagine. Surface and internal temperatures of potato slices were monitored during the experiments to better explain the results. Fried and baked chips were analyzed for acrylamide content using an LC-MS method. The results showed that acrylamide level of potato chips prepared by frying increased with frying temperature (19.6 ng/g at 170 °C, 39 ng/g at 180 °C, and 95 ng/g at 190 °C). In baking, however, the highest acrylamide level was observed in potato chips prepared at 170 °C (47.8 ng/g at 170 °C, 19.3 ng/g at 180 °C, and 29.7 ng/g at 190 °C). The results showed that baking at 170 °C more than doubled the acrylamide amount that formed upon frying at the same temperature, whereas at 180 and 190 °C, the acrylamide levels of chips prepared by baking were lower than their fried counterparts.     

Formation of trans fatty acids in edible oils during the frying and heating process

To assess an impact of heated edible oils on intake of trans fat, the formations of trans fatty acids (TFAs) in cooking conditions was estimated by a frying and heating model system. For the frying model, sliced raw potatoes (10% of the frying oil (w/w)) were fried in commercially available canola oil at 160, 180 and 200 °C, and the 10 frying cycles were performed. The TFAs contained both in fried potatoes and in frying oils were measured by gas chromatography (GC). Lipids content of raw potatoes was about 0.1% (w/w) and TFAs in the raw potatoes were negligible. On the other hand, fried potatoes contained lipids at the level of 8.8%–9.2% and their fatty acid composition was mostly in correspondence with that of the frying oil. The TFAs amount of potatoes fried by the tenth frying operation was at the level of 0.99–1.05 g/100 g lipids. When 100 g potatoes fried in this process were consumed, the TFAs intake was estimated at less than 0.1 g. After 10 frying operations, TFAs content, acid values and peroxide values of the frying oils were measured and compared with those of corresponding heated canola oils without food. The amounts of trans 18:1 FAs contained both in the frying oil and in heated oil were less than the quantitative limit (0.047 g/100 g oil). The increases of trans 18:2 FAs and trans 18:3 FAs of the used frying oil were 0.02 g/100 and 0.05 g/100 g, respectively, compared with those of the fresh oil. trans 18:2 FAs accumulation in the heated oil was slightly less than that in the frying oil. To elucidate TFAs accumulation in various edible oils during cooking, six kinds of commercially available edible vegetable oils were heated to 180 °C in glass test tubes. Small changes in TFAs amounts were observed after four hours heating. These results suggested that an ordinary frying process using unhydrogenated edible oils has little impact on TFAs intake from edible oils.