Influence of ultrasound pretreatments on diffusion coefficients,texture and colour of osmodehydrated sweet potato (Ipomea batatas)

The moisture and solute diffusivities, texture and colour of sweet potato pretreated in distilled water with ultrasound (UD), osmotic dehydration without ultrasound (OD) and osmotic dehydration with ultrasound (UOD) were investigated to determine the effects of ultrasound on osmodehydrated sweet potato. The moisture and solute diffusivities of all the pretreatments ranged from 2.89 to 4.95 × 10^?10 and 1.17 to 1.67 × 10^?10 m² s^?1, respectively. The moisture diffusivity of samples treated in UOD was significantly (P < 0.05) higher than that of OD and UD by 30.82% and 41.62%, respectively. Compared to the fresh sample, the firmness of samples treated in OD and UOD was reduced by 83.53% and 92.47%, respectively. UD increased the brightness (L*) and reduced the redness (a*) of sweet potato, while UOD enhanced the yellowness (b*) and chroma (C*) of sweet potato. The sweet potato samples treated in UOD had the highest total colour difference (ΔE).     

Effect of pulsed electric fields on the structure and frying quality of “kumara” sweet potato tubers

The aim of this research was to study the effect of pulsed electric fields (PEF) on the microstructure of “kumara” sweet potato (Ipomoea batatas cv. Owairaka) and its quality after frying. Whole sweet potato tubers were treated at different electric field strengths ranging from 0.3 to 1.2 kV/cm with specific energy levels between 0.5 and 22 kJ/kg. Cell viability was determined using tetrazolium staining to investigate the uniformity of the PEF effect across tubers. Based on the patterns of viable cells it was observed that the effect of PEF was not homogeneous across the tuber. This result was also supported by the pattern of enzymatic browning due to PEF facilitating the reaction of polyphenoloxidase and phenols. PEF treatment resulted in significant softening of the ground tissues, but not on the dermal tissues, as determined by texture analysis. With respect to frying quality, tubers pre-treated with PEF at electric field strength of 1.2 kV/cm and fried at 190 °C had an 18% lower oil content than non-PEF treated samples. The kinetics of browning as a function of frying time could be described by a fractional conversion model. The activation energy (Ea) of the browning rate during frying increased (more temperature sensitive) due to PEF pretreatment at 0.5 kV/cm and 1.2 kV/cm. It implies that PEF pretreatment allows frying the potato chips at lower temperature in order to achieve the same brown colour intensity as the non-PEF treated tubers. This study shows clearly that PEF could reduce the energy required for cutting and frying of kumara.Industrial relevanceThis study provides evidence that the effect of PEF processing on whole kumara tubers is not uniform, demonstrating heterogenous distribution. These findings provide important information for food industry to design appropriate PEF processing conditions for solid materials. More importantly, PEF treatment reduced the energy required for cutting and frying of kumara, and reduced the oil content in the fried kumara chips.     

Acrylamide content and color development in fried potato strips

Acrylamide formation and changes in color of fried potato strips was investigated in relation to frying temperature and three treatments before frying. Potato strips (0.8 × 0.8 × 5 cm) of Bintje variety were fried at 150, 170 and 190 °C until reaching moisture contents of ∼40 g water/100 g (total basis). Prior to frying, potato strips were treated in one of the following ways: (i) immersed in distilled water for 0 min (control), 60 min and 120 min; (ii) blanched in hot water at six different time–temperature combinations (50 °C for 40 and 80 min; 70 °C for 10 and 45 min; 90 °C for 3 and 10 min); (iii) immersed in a citric acid solution of 10 g/L for an hour; (iv) immersed in a sodium pyrophosphate solution of 10 g/L for an hour. Acrylamide content and color was determined in the potato strips after frying. Immersed strips in water for 120 min showed a reduction of acrylamide formation of 33%, 21% and 27% at 150, 170 and 190 °C, respectively, when they were compared against the control. Potato strips blanched at 50 °C for 80 min had the lowest acrylamide content when compared against strips blanched at different conditions and fried at the same temperature (135, 327 and 564 μm acrylamide/kg for 150, 170 and 190 °C, respectively). Potato strip immersion in citric acid solution of 10 g/L reduced much more the acrylamide formation after frying than the strip immersion in sodium pyrophosphate solution of 10 g/L (53% vs. 17%, respectively, average values for the three temperatures tested). Acrylamide formation decreased dramatically as the frying temperature decreased from 190 to 150 °C for all the pre-treatments tested. Color represented by the parameters L* and a* showed high correlations (r² of 0.79 and 0.83, respectively) with French fry acrylamide content.     

Effect of Boiling and Frying on the Total Carbohydrate,Vitamin C and Mineral Contents of Irish (Solanun tuberosum) and Sweet (Ipomea batatas) Potato Tubers

In a study aimed at ascertaining whether boiling or frying best conserves nutrients; since the two methods are commonly adapted in Nigeria, the effect of boiling and frying on total carbohydrate, vitamin C and mineral contents of Irish (Solanum tuberosum) and sweet (Ipomea batatas) potato tubers were investigated. The determination of total carbohydrate was carried out using the phenol-sulphuric acid method and the spectrophotometric method was used to determine vitamin C content at 520 nm. The minerals evaluated are iron, zinc, magnesium, sodium, calcium and copper. These were determined by the atomic absorption spectrophotometric method. There was no significant difference (p > 0.05) in the total carbohydrate content of the boiled and fried sweet and Irish potatoes compared with their raw tubers. There was significant difference (p < 0.05) in the vitamin C content of the boiled and fried potato tubers. Boiling and frying of Irish potato resulted in a loss of 37.34 mg/100 ml (63.90%) and 30.44 mg/100 ml (53.90%) vitamin C respectively. Boiled sweet potato lost 51.16 mg/100 ml (72.37%) and fried lost 43.05 mg/100 ml (60.90%) of vitamin C. The mineral compositions of the boiled and fried Irish and sweet potatoes were significantly different (p < 0.05) from their raw tubers. Boiling retained more iron and copper while frying retained more zinc, magnesium, sodium and calcium in both Irish and sweet potato tubers. Boiling retained more carbohydrate while frying retained more vitamin C and minerals.     

Reduction of acrylamide by taurine in aqueous and potato chip model systems

Acrylamide in foods is mainly produced by Maillard reaction. Taurine can participate in the reaction, which has led us to investigate the possibility of reducing acrylamide formation by use of taurine. In an aqueous system, the lower the pH of the solution the greater the inhibition of acrylamide formation within a pH range of 5.0–8.0 was found, and the inhibition of acrylamide formation by taurine was dose-dependent. In a fried potato chip model, prior to frying at 170 °C for 3 min, the potato slices soaked in 0.l% to 2% taurine solution for 30 min showed significant reductions of acrylamide formation; however, these reductions were not dose-dependent. Also, the soaking treatments for 15–60 min significantly reduced acrylamide formation, but the inhibitory effects were not time-dependent. Thus, taurine, when used in a narrow range of reasonably low levels, is a candidate to inhibit acrylamide formation during frying process.     

Effects of osmotic pretreatment and frying conditions on quality and storage stability of vacuum-fried pumpkin chips

The effects of osmotic (OP), ultrasound-assisted osmotic pretreatment (UAOP) and frying conditions on quality and storage stability of vacuum fried pumpkin chips were investigated. The pumpkin samples were pretreated in maltodextrin solution and subsequently fried at different temperatures (90–110 °C) and time periods (10–30 min). The results demonstrated that the moisture content, water activity, lightness, yellowness and carotenoid content of the fried chips decreased, while oil content, hardness and a* (dark brown colour) value increased with increasing frying temperature and time. UAOP reduced about 16.0% of oil absorption and enhanced approximately 70% of carotenoid retention in the fried chips. UAOP samples were also more stable during storage than the untreated ones, indicated by lower degradation kinetics constants of key quality parameters. The proposed pretreatment could be an effective method for food industries to develop vacuum fried pumpkin chips with improved quality and stability.     

Investigation of the formation of (E)-2-butenal in oils and foods during frying

The formation of (E)-2-butenal in oils subjected to temperatures applied in frying as well as in fried foods was evaluated. For this purpose, a sensitive and accurate stable isotope dilution essay was developed and validated using as an internal standard (E)-2-butenal d6 synthesized in the lab. Heating of corn oil, sunflower oil, extra virgin and refined olive oil, ω-3 frying oil and palm oil at 170 °C for a total of 8 h resulted in the formation of (E)-2-butenal at levels between 0.05 and 1 μg g^− 1 and the highest amounts were already formed after 2 h of heating. The highest (E)-2-butenal formation was observed in the oil containing ω-3 unsaturated fatty acids indicating that (E)-2-butenal content is mainly affected by the fatty acid composition. No significant amount of free (E)-2-butenal was detected in fried foods purchased from fast food shops with the exception of one chicken sample and a pork brochette. Further experiments including several foods did not indicate any increase of free (E)-2-butenal content due to frying. It can be suggested that fried foods may not be a significant dietary source of free (E)-2-butenal for humans. However, these results in combination with the available literature indicated that investigation of the contamination of the indoor environment where frying is taking place could be more relevant than the presence of free (E)-2-butenal in oils and foods.     

Heat transfer coefficients during deep fat frying of sweetpotato: Effects of product size and oil temperature

Heat transfer coefficient, h during deep fat frying of sweetpotato was determined from designed experiments. Samples of three different disc sizes were fried at four oil temperatures between 150 and 180 °C. Heat transfer coefficient estimation was based on heat energy balance between the oil and sample during frying. Maximum h was reached at the earlier stage of frying (80–120 s). The range of maximum h reached was 710–850 W/m² °C. h at latter period of frying (200–300 s) was 450–550 W/m² °C. h was found to vary directly with oil temperature but inversely with sample size (p <0.05).     

The Impact of Blanching and High-Pressure Pretreatments on Oil Uptake of Fried Potato Slices

The effect of high-pressure (HP) pretreatment on oil uptake of potato slices is examined in this paper. Potato slices were treated either by HP or thermal blanching, or a combination of thermal blanching followed by HP prior to frying. The effect of HP on starch gelatinization and potato microstructure was assessed by differential scanning calorimeter and environmental scanning electron microscope (ESEM), respectively. After treatments, the slices were fried in sunflower oil at 185 °C for a predetermined time. Frying time was either kept constant (4 min) or varied according to the time needed to reach a desired moisture content of ≈2%. The high pressure applied in this study was found not to be sufficient to cause a significant degree of starch gelatinization. Analysis of the ESEM images showed that blanching had a limited effect on cell wall integrity. HP pretreatment was found to increase the oil uptake marginally. When frying for a fixed time, the highest total oil content was found in slices treated at 200 MPa for 5 min. The oil content was found to increase significantly (p < 0.05) to 41.23 ± 1.82 compared to 29.03 ± 0.21 in the control slices. The same effect of pressure on oil content was found when the time of frying varied. On the other hand, HP pretreatment was found to decrease the frying time required to achieve a given moisture content. Thus, high-pressure pretreatment may be used to reduce the frying time, but not oil uptake.     

Effect of Pre-Drying and Hydrocolloid Type on Colour and Textural Properties of Coated Fried Yam Chips

The effect of pre-drying and coating on colour and texture of yam chips were investigated. The colour parameters studied were lightness index (L*), hue angle (h) and browning index (BI). Peak force (PF) of penetration was observed for texture. Coating pick-up (amount of coating adhering to the samples) prior to frying was determined and moisture content of fried samples was also determined. The type of hydrocolloids used in the coating formulation had significant (p < 0.05) effect on coating pick-up, initial moisture content and texture of the chips; while samples that were not pre-dried had higher pick-ups and moisture contents. Pre-dried samples were darker brown in colour and harder in texture than samples that were not pre-dried as evidenced by their higher browning index and peak force values, respectively, thus enhancing the colour and texture of the chips.     

Genetic Algorithm�CArtificial Neural Network Modeling of Moisture and Oil Content of Pretreated Fried Mushroom

In this research, the effect of different pretreatments (osmotic dehydration and gum coating) on moisture and oil content of fried mushroom was investigated, and artificial neural network and genetic algorithm were applied for modeling of these parameters during frying. Osmotic dehydration was performed in solution of NaCl with concentrations of 5% and 10%, and methyl cellulose was used for gum coating. Either pretreated or control samples were fried at 150, 170, and 190 °C for 0.5, 1, 2, 3, and 4 min. The results showed that osmotic dehydration and gum coating significantly decreased (0–84%, depending upon the processing conditions) oil content of fried mushrooms. However, moisture content of fried samples diminished as result of osmotic pretreatment and increased by gum coating. An artificial neural network was developed to estimate moisture and oil content of fried mushroom, and genetic algorithm was used to optimize network configuration and learning parameters. The developed genetic algorithm–artificial neural network (GA–ANN) which included 17 hidden neurons could predict moisture and oil content with correlation coefficient of 0.93 and 96%, respectively. These results indicating that GA–ANN model provide an accurate prediction method for moisture and oil content of fried mushroom.     

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.     

Protective role of vacuum vs. atmospheric frying on PUFA balance and lipid oxidation

The effect of vacuum frying processing on proximate composition, fatty acid profile, oxidative status and sensorial properties of fish patties, was evaluated as compared with conventional (atmospheric) frying.Vacuum frying procedure was carried out at 80 mmHg (water boiling point: 42 °C) and oil temperature (107 °C) determined to obtain an equivalent thermal driving force (Δ65 °C) of that of the atmospheric frying conditions used (165 °C). Frying times of 2, 4, 6, 8 and 10 min were investigated.Vacuum frying significantly prevented degradation of EPA (Eicosapentaenoic acid) and DHA (Docosahexaenoic acid), reducing the polyene index and maintaining a lower ω6/ω3 fatty acid ratio in samples, while no significant differences with conventional frying in total oil content were observed. The use of vacuum also reduced formation of peroxides and carbonyl derivatives. Tocopherol levels decreased in all samples regardless of the frying conditions used, although vacuum-fried samples maintained higher tocopherol levels after processing. These samples also showed higher lightness and lower a* and b* values, which can be associated to lower non-enzymatic browning levels.These results support the applicability of vacuum frying technology for fish patties, since it prevents colour changes, improves juiciness and reduces oxidation when compared to conventionally (atmospheric) fried counterparts.Industrial relevanceConsumers are increasingly aware of the link between food and health, maintaining a high demand for healthy products. In this regard, consumption of fatty fish, with healthy properties widely known, is lower than recommended by health authorities, especially in children. Novel processing technologies focused on increasing the appealing of products based on fatty fish can help in ameliorating this deficient consumption. Vacuum frying is a promising way of obtaining attractive products, due to some product modification after the deep-fat frying process, and yet retaining natural colour, juiciness and healthy properties to a high extent. Vacuum frying allows reducing ω3 fatty acids and tocopherol degradation, differentiating fish products so obtained, which can be launched into the market and benefit from this technology. Compared to conventional frying, the results are better nutritional and sensory properties in a final product.     

Deterioration of olive,corn and soybean oils due to air,light, heat and deep-frying

To follow the relative rate of oxidative deterioration of edible oils, refined olive, corn and soybean oils were analyzed periodically for their peroxide value (PV), p-anisidine value (p-AV) and iodine value (IV) following exposure to air and air–light for 30 days. Changes in the above values of the oils were also examined and after being used for deep frying of French fries at 180 °C for varying periods of time i.e. 30, 60 and 90 min PV and p-AV values increased in the order, deep frying > air–light exposure > air exposure, while the values with respect to the oils increased as soybean > corn > olive. Decreases in IV followed the same pattern, i.e. deep frying > air–light > air and soybean > corn > olive. %Free fatty acid increased with increase in time of deep-frying. Deep-frying of French fries in corn oil was also carried out in presence of caffeic, ferulic, vanillic acid and crude tea extract as antioxidants. All antioxidants effectively reduced the oxidation rate in the oil as detected by decreases in PVs and p-AVs and relatively low reduction rates in IVs for all the frying times. The order of antioxidative activity was caffeic acid > vanillic acid > ferulic acid > tea extract. Variation in %FFA of corn oil due to variation in nature of fried food was also analyzed. %FFA of the oil used for deep frying of chicken drum sticks were higher than the values of the oil used for deep-frying of French fries.     

Phytosterols in frying oils: evaluation of their absorption in pre-fried potatoes and determination of their destruction kinetics after repeated deep and pan frying

The levels of β-sitosterol and total phytosterols were monitored in pre-fried potatoes during eight successive pan- and deep-frying sessions in five different commercial frying oils (virgin olive oil, sunflower oil, palm oil, cottonseed oil and vegetable shortening oil). The amount of these constituents that were transferred from the frying oils to the potatoes at the end of each frying session was determined prior and after frying by GC/FID. Except for the potato enrichment with phytosterols, the kinetics of the destruction of β-sitosterol and total phytosterols, mainly due to thermal oxidation of the oils, were evaluated for each of the five different types of oil. The experiments showed that in most cases the destruction of both β-sitosterols and total phytosterols followed first order kinetics with quite good correlation coefficients (R>0.92) and the reaction constant k was found to be at the range of 0.004 - 0.028 min^-1. Concerning the absorption of the two types of phytosterols in the fried potatoes during frying, they were found to be at the range (average) of 0.4-27% of their initial content in the oil at the beginning of each frying process (eight frying processes/session), depending on the type of oil and the constituent.     

Effect of green tea extract and microwave pre-cooking on the formation of acrylamide in fried chicken drumsticks and chicken wings

Since 2002, it has been well-known that carbohydrate-rich foods have a high level of acrylamide (AA). Some researchers have drawn attention on the formation of AA in coated and fried chicken meat products in recent years. From this viewpoint, the purpose of the present study was to mitigate acrylamide formation in fried chicken drumsticks (CDs) and chicken wings (CWs) using green tea extract (GTE) alone or combined with microwave pre-cooking (MPC) by decrease frying time. In brief, CDs and CWs marinated for one day were divided into eight groups. Four of those were first covered with batter and breading, including 0%, 0.5%, 1.5% and 3% GTE and deep fried in fat for 8.5 min for CDs and 3.5 min for CWs at 175 °C and 8 psi of pressure. In the second step, the remainder of the marinated CDs and CWs were subjected to MPC for 7 min and 5 min, respectively. Then, they were covered with batter and breading including 0%, 0.5%, 1.5% and 3% GTE and deep fried in fat for 5.5 min for CDs and 2.5 min for CWs at 175 °C and 8 psi for pressure. With increasing levels of GTE, the AA concentration decreased gradually in CDs and CWs. In addition, MPC also decreased AA formation in CWs due to the shorter frying time. Moreover, MPC was more efficient at reducing AA formation in CWs when it was combined with GTE. A lighter and more yellowish color was measured in CDs as a consequence of the shorter frying time. It is crucial to state that the incorporation of GTE into the breading or MPC did not negatively affect the sensory properties of CDs and CWs, while it reduced AA formation. In conclusion, it can be stated that GTE alone or combined with MPC is an effective and practical mitigation strategy for AA formation.     

On the mechanism by which oil uptake decreases crispy/crunchy behaviour of fried products

A large problem for fried snack products with a crispy crust and a soft moist interior is that the crust often loses its crispy character fast, in the order of 3–20 min. As known during cooling of these snacks adherent oil will be sucked into the crust. The presence of oil in a cellular solid crispy material was found to affect the sound emitted on fracture dramatically, while it did not affect mechanical characteristics at least for shorter ageing times. Both the number of sound events and the acoustic energy released during fracture decreased. This could be explained by reflection of emitted sound at the oil–air interface. Both the smaller number of acoustic events and the lower loudness will cause a decrease in crispy perception of the crust and in that way oil uptake during cooling of fried snacks can explain their fast decrease in crispness after frying.     

Effect of Ultrasound-Assisted Osmotic Dehydration as a Pretreatment on Deep Fat Frying of Potatoes

The objective of this study was to investigate the possibility of using ultrasound-assisted osmotic dehydration (UAOD) as a pretreatment prior to frying and to study its effects on the quality of fried potatoes. The quality parameters, moisture content, oil uptake, color, texture, and microstructure of fried potatoes, were chosen. Quality of fried potatoes treated with UAOD was also compared with the ones treated with osmotic dehydration (OD). Potato slabs (40 × 40 × 7 mm) were pretreated with different osmotic solutions (15 % sodium chloride and 15 % sodium chloride/50 % sucrose solutions) at different temperatures (25, 45, and 65 °C) with and without ultrasonic waves for different treatment times. The pretreatment conditions which are OD for 90 min and UAOD for 30 min using 15 % sodium chloride/50 % sucrose solution were applied prior to frying at 170 °C for 2, 4, and 6 min. UAOD reduced the oil content of fried potatoes by 12.5 % (db) as compared to untreated fried potatoes at the end of frying. There was no significant difference between OD and UAOD in reduction of oil uptake in fried potatoes. However, UAOD was found to have the advantage of improving the color of French fries. In addition, it shortened the pretreatment time of OD by about 67 %. Cell structure of fried potato was damaged in the presence of pretreatments of OD and UAOD.     

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.     

Effect of superheated steam prefrying treatment on the quality of potato chips

Superheated steam drying (SSD) was used as a prefrying treatment prior to deep‐fat frying for potato slices. The effect of SSD at 130, 150 or 180 °C and steam velocity of 2.0 m s^?1 on the fat uptake, colour and texture of fried potato chips was evaluated; microstructure and degree of starch gelatinization were also evaluated to help explain the fat uptake results. SSD and frying yielded potato chips with the fat content from 0.263 ± 0.002 to 0.304 ± 0.002 kg kg^?1 (d.b.), while frying without SSD led to chips with the fat content as high as 0.359 ± 0.003 kg kg^?1 (d.b.). SSD did not promote starch gelatinization. Lower fat uptake was correlated to modified surface structure and lower moisture content of potato slices prior to frying. Frying with/without SSD pretreatment yielded potato chips of similar hardness, crispness and lightness. On the other hand, SSD significantly increased redness and yellowness of the fried chips.