Structural changes and shrinkage of potato during frying

Summary Light microscopy was used to study changes in cell size, blister formation and crust evolution during potato frying. Frying experiments with both French fries and crisps of different thickness (1–5 mm) were performed at temperatures of 140 and 180 °C. Thickness, volume and density changes were also measured. The formation of crust in French fries starts after the potato surface reached approximately 103 °C, and then the crust thickness increased linearly with the square root of frying time, this increase being faster at 180 °C. The potato volume decreased during frying, although in the late stages the volume may increase because of oil uptake and cell separation caused by entrapped water vapour. Shrinkage was adequately described by the Weibull model with a residual value, with shrinkage rate increasing with temperature and decreasing with potato thickness. The residual volume was not affected by temperature (65% for French fries and 59–30 for crisps, depending on thickness). Volume appeared to decrease mainly as the result of water loss, except for very low water content, and thus potato density changes were very small.     

On the Possibility of Nonfat Frying using Molten Glucose

Fried products impose a health concerns due to considerable amount of oil they contain. Production of snack foods with minimal oil content and good management of oil during frying to minimize the production of toxic compounds continue to be challenging aims. This paper aims to investigate the possibility of producing a fat‐free food snack by replacing frying oil with a nonfat medium. Glucose was melted and its temperature was then brought to 185 °C and used to fry potato strips, to obtain a product referred here as glucose fries. The resulting product was compared with French fries prepared conventionally under conditions that resulted in similar final moisture content. The resulting products were also examined for crust formation, texture parameters, color development and glucose content. Stereo microscope images showed that similar crusts were formed in the glucose fries and French fries. Texture parameters were found to be similar for both products at 5 and 2 mm penetration depth. The maximum hardness at 2 mm penetration depth was also similar for both products, but different from cooked potato. The color development that characterized French fries was also observed in glucose fries. The glucose content in glucose fries was found to be twice the content of French fries, which is to be expected because glucose absorbed or adhered to the surface. In conclusion, glucose fries, with similar texture and color characteristics to that of French fries, can be prepared by using a nonfat frying medium.     

Calcium Absorption in Rats Consuming Olive Oil or Sunflower Oil Unused or Used in Frying

Olive and sunflower oils were used in repeated potato‐frying operations without turnover until the oils reached the 25% polar compound limit allowed by law. During a 28‐d period, 5 groups of rats were fed diets containing 8% of unused olive oil, olive oil used in 48 and 69 potato‐frying operations, unused sunflower oil, and sunflower oil used to fry potatoes 48 times. No significant differences in food intake, body weight, and food efficiency was observed. The 2 sunflower oil diets increased calcium absorption efficiency without modifying calcium balance, urinary calcium, blood‐serum calcium, and calcium in the carcasses. Intake of oils used in frying did not induce any significant changes. Sunflower oil enhanced calcium bioavailiability slightly, but oil used for frying did not produce any additional effect.     

Sunflower oil versus olive oil and iron metabolism in rats. Influence of a frying process

Olive oil and sunflower oil were used in repeated potato‐frying operations without turnover until the oils reached the limit of 25% polar compounds allowed by law. Over a 28 day period, five groups of rats were fed diets containing 8% unused olive oil, olive oil used in 48 and 69 potato frying operations, unused sunflower oil and sunflower oil used to fry potatoes 48 times. In the final week, faeces and urine were collected and, on day 28, animals were sacrificed. The type of oil, unused or used in frying, did not modify food intake, body weight, faecal iron and its apparent absorption or retention, but consumption of oils used in frying tended to increase urinary iron excretion. No differences were found in serum iron, total iron‐binding capacity and haemoglobin values. Iron contents and concentrations in liver, spleen and skin did not vary between groups. Iron concentrations in erythrocytes were significantly higher in rats fed sunflower oil, either unused or used in frying. It was concluded that the consumption of sunflower oil compared to olive oil could affect intra‐ and extracellular haematic distribution of iron, probably associated with changes in membrane permeability, and that some alteration products of the oils originated during frying tend to increase urinary iron losses without repercussions on iron metabolism. © 2000 Society of Chemical Industry     

Effect of potato presence on the degradation of extra virgin olive oil during frying

This work investigates the effect of potato presence on the degradation of extra virgin olive oil (EVOO) during repeated frying. For this reason the performance of EVOO during frying was compared with its performance during heating at frying temperatures. In order to make meaningful comparisons the temperature profiles obtained during the frying experiments were replicated during the heating experiments by means of a high energy exchange rate cooling/heating system. The effects of potato‐to‐oil ratio (1/7 and 1/35 kg_potatoes/L_oil) and number (N) of batches (N = 0, 1, 0, 20, 30, 40) were examined. EVOO was analysed using high performance size exclusion chromatography (HPSEC). It was found that polymerisation products increased linearly during frying and heating and that they were not affected by potatoes presence at low ratio. The effect of potato presence became statistically significant (yet not remarkable) at the high ratio. Decomposition products increased during frying whereas they decreased during heating.     

The influence of oil type and frying temperatures on the texture and oil content of French fries

The purpose of the present study was to determine the effects of the frying medium and temperatures on fat content and texture of French fries. The material taken for the study consisted of seven types of vegetable oil: refined sunflower, rape, soy, olive oil, palm, partially hydrogenated rape oil (modified oil I) and a blend of vegetable oils (modified oil II). The French fries prepared from Asterix potato variety were fried at oils heated to 150, 160, 170, 180 and 190 °C. The length of frying (12, 10, 8, 6.5 and 4.5 min, respectively) depended on oil temperature. Fat content and the texture of French fries were determined. The type of frying medium significantly affects the texture of French fries. Temperature influenced both the fat content and texture of product. The increase of frying temperature decreased fat uptake and hardness of French fries. French fries fried in rape oil exhibited the most delicate texture and the lowest oil absorption when compared with French fries fried in other types of oil under investigation. Copyright © 2005 Society of Chemical Industry     

Deep-Fat Fried Potato Product Oil Uptake as Affected by Crust Physical Properties

Physical properties were determined on the crust region of a restructured potato product undergoing deep-fat frying. Deformability modulus of the raw material, ranging from 37 to 127 kPa markedly affected moisture content and oil uptake of the restructured product, and its crust. It also affected crust thickness at frying time of 5 min. Lower deformability modulus promoted thicker crust ranging from 327 ± 17 to 650 ± 29 μm. Crust porosity decreased throughout frying, and was markedly affected by lower gel-strengths. Crust yield strength was affected by trying time and deformability modulus of the raw material. Crust oil uptake stopped when crust yield strength reached a critical value of 210 to 240 kPa. Oil uptake in the crust ranged from ca. 35 to 38% after 1 min and 60 to 85% after 5 min frying.     

French Fries oleuropein content during the successive deep frying in oils enriched with an olive leaf extract

Olive oil, sunflower oil and soybean oil were enriched with an olive (Olea Europaea) leaf extract rich in oleuropein at a supplementation level of 100 mg total phenolics per kg oil. Supplemented oils were used for deep frying of potatoes for eight successive frying sessions without replenishment under domestic frying conditions. Supplemented oils contained oleuropein while deep frying resulted in a gradual decrease in oils oleuropein content. After the eighth successive frying, oleuropein was still detected in oils, its retention being 3.2–12.4%. Deep frying of potatoes in supplemented oils succeeded in preparing French fries containing oleuropein even after eight successive fryings. Contrary to the oil type, the number of frying sessions affected potatoes oleuropein content. Oleuropein intake by consuming French fries deep‐fried in enriched oils was approximately ten times higher in potatoes prepared in the first frying session as compared to the respective intake after the eighth frying.     

Influence of the frying temperature on acrylamide formation in French fries

It is well established that high oil temperatures during frying strongly increase acrylamide formation in French fries, but it is less clear, which temperature or rather which part of a temperature profile is relevant and if rules or regulatory measures should be established in respect of frying temperature, on which temperature they should refer. In most fryers, the oil temperature strongly drops on adding the potato sticks and may not fully recover up to the end of the frying process (depending on the amount of potato added in relation to the volume of oil and the heating power of the fryer). Since acrylamide is formed towards the end of frying, the temperature during the second half of the process is more important than that regulated by the thermostat. The profile of the frying temperature was optimized regarding product quality (crispness, flavor) and acrylamide formation. An initial temperature of 170–175 °C dropping to 140–145 °C and a virtually isothermal frying at 160 °C resulted in products of similar quality and acrylamide content. At initial temperatures below 160 °C and with main frying temperatures below 140 °C, crispness and the flavor of the French fries suffered: the sticks dried out and became oily. Isothermal frying at 167–170 °C resulted in approximately doubled acrylamide content compared to conditions that were optimal in respect of culinary quality and low acrylamide formation, showing that rules on the initial temperature alone are inadequate to ensure low acrylamide contents. Optimized fryers should program temperature: allowing an initial temperature drop, but then efficiently heating to prevent the temperature dropping below a given limit; after the end of frying, the initial temperature must be restored before frying the next portion.     

A Comparative Study of the Characteristics of French Fries Produced by Deep Fat Frying and Air Frying

Air frying is being projected as an alternative to deep fat frying for producing snacks such as French fries. In air frying, the raw potato sections are essentially heated in hot air containing fine oil droplets, which dehydrates the potato and attempts to impart the characteristics of traditionally produced French fries, but with a substantially lower level of fat absorbed in the product. The aim of this research is to compare: (1) the process dynamics of air frying with conventional deep fat frying under otherwise similar operating conditions, and (2) the products formed by the 2 processes in terms of color, texture, microstructure, calorimetric properties, and sensory characteristics. Although, air frying produced products with a substantially lower fat content but with similar moisture contents and color characteristics, it required much longer processing times, typically 21 min in relation to 9 min in the case of deep fat frying. The slower evolution of temperature also resulted in lower rates of moisture loss and color development reactions. Differential scanning calorimetry (DSC) studies revealed that the extent of starch gelatinization was also lower in the case of air fried product. In addition, the 2 types of frying also resulted in products having significantly different texture and sensory characteristics.     

The influence of frying medium degradation on fat uptake and texture of French fries

The purpose of the present investigation was to study the influence of the degree of degradation of different kinds of oils used as a frying medium on fat uptake and texture of frozen pre‐fried French fries. As has been found, the degradation degree of the frying medium depends on the kind of oil and processing time. Liquid hydrogenated rapeseed oil exhibited the best thermo‐oxidative stability among the oils under investigation. The kind of oil influenced fat uptake and the texture of French fries. The lowest fat uptake was observed with French fries fried in solid oil. Some correlations between fat uptake and changes in fatty acid content were found in the frying medium during frying. Fat absorption increased with increasing unsaturated fatty acids and decreasing saturated fatty acid content. The texture of French fries fried in hydrogenated oils was harder than that exhibited by French fries fried in liquid rapeseed oil. The hardness of French fries fried in liquid oils decreased during frying while the hardness of French fries fried in solid oil increased. Some correlations between the texture of French fries and iodine value and fatty acids content of frying media were found. The hardness of French fries increased with increasing content of saturated fatty acids and decreasing unsaturated fatty acids and trans isomer fatty acid content. Copyright © 2005 Society of Chemical Industry     

Effect of frying oil quality and TBHQ on the shelf-life of potato crisps

Three different systems were used for the evaluation of the effect of frying oil quality and tertiarybutyl hydroquinone (TBHQ) on the shelf-life of fried potato crisps. Batches of potato slices were fried for 4 min at 30 min intervals over a period of 5 h day^?1 for eight consecutive days in refined, bleached and deodorised palm olein (RBD olein) without antioxidant (system 1); RBD containing 200 μg g^?1 TBHQ at the start of the experiment (system 2) and RBD olein which had TBHQ topped up to a level of 200 μg g^?1 at the start of each frying day (system 3). Sensory evaluation of crisps showed that there is an initial rapid decrease in shelf-life with increase in fry number in all systems. There was only a comparatively smaller effect on shelf-life with increase in fry number in later fryings. When the shelf-life of crisps was related to the quality of oil in the frying medium, it was found that the shelf-life of the product is only slightly affected when the oil had deteriorated beyond the acceptable limit of quality of frying oil, where the acceptable limit was taken as 27-30% polar components and 1–2 mg potassium hydroxide g^?1 oil. TBHQ does have a protective carry-through effect. The protective effect is more pronounced when TBHQ is topped up to a level of 200 μg g^?1 at the beginning of each frying day.     

Structure Oil-Absorption Relationships During Deep-Fat Frying

ABSTRACT Analysis of the oil-absorption process in deep-fat fried potato cylinders (frying temperatures of 155°C, 170°C, and 185°C) allowed to distinguish 3 oil fractions: structural oil (absorbed during frying), penetrated surface oil (suctioned during cooling), and surface oil. Results showed that a small amount of oil penetrates during frying because most of the oil was picked up at the end of the process, suggesting that oil uptake and water removal are not synchronous phenomena. After cooling, oil was located either on the surface of the chip or suctioned into the porous crust microstructure, with an inverse relationship between them for increasing frying times.     

马铃薯片煎炸条件对棕榈油品质的影响研究

通过一系列的实验及定量分析,探讨了不同的煎炸温度、煎炸棕榈油中杂质(薯渣)含量和单位质量棕榈油的煎炸薯片生产率对煎炸过程中棕榈油酸价变化的影响。实验结果表明:煎炸温度、单位质量棕榈油的煎炸薯片生产率和煎炸油中杂质的含量对于煎炸油的酸价变化均有着显著地影响(α≤0.1)。单位质量棕榈油的煎炸薯片生产率对煎炸油的酸价变化影响最为显著,其次为煎炸温度,煎炸油中杂质(薯渣)的含量影响相对最小。当煎炸温度为170℃,单位质量棕榈油的煎炸薯片生产率为80 g/(kg.h),杂质(薯渣)的含量为0.050%时,相同的煎炸时间内对煎炸棕榈油的酸价变化影响最小。     

Two-stage frying process for high-quality sweet-potato chips

Vacuum frying (1.33 kPa), with the aid of a de-oiling mechanism, was used to produce low-fat sweet potato chips. The kinetics of oil absorption and oil distribution in the chips (total, internal, and surface oil content) was studied so that effectiveness of the de-oiling system could be established. Product quality attributes (PQAs) such as moisture content, oil content, diameter shrinkage, and thickness expansion, as well as, color, texture, bulk density, true density, and porosity of chips fried at different temperatures (120, 130, and 140 °C) was performed to evaluate the effect of process temperature on the product.     

食用植物油煎炸过程中的品质变化北大核心CSCD

利用花生油、大豆油、调和油、棕榈油4种食用植物油作为煎炸油对豆腐、裹粉鸡柳、油条3种食材进行煎炸,研究煎炸时间对煎炸油极性组分含量及羰基值的影响,确定煎炸油时间预警点,同时研究了不同煎炸油品种、不同食材对煎炸油极性组分含量的影响。结果表明:极性组分含量、羰基值均与煎炸时间呈正相关(r_(极性组分)=0.809,r_(羰基值)=0.859,P=0.000);煎炸6~11 h煎炸油极性组分含量均值从初始的11.85%增至20.96%,最大值为34.10%,可作为煎炸油质量不达标的时间预警点;煎炸12 h,极性组分含量从低到高依次为花生油、棕榈油、调和油、大豆油,含量分别为21.1%、24.1%、26.8%、27.8%,豆腐、裹粉鸡柳、油条煎炸油中极性组分含量分别为17.7%、25.0%、32.2%;煎炸18 h,极性组分含量增大程度由低到高为棕榈油/调和油、大豆油、花生油,煎炸食材用油为豆腐煎炸油、裹粉鸡柳煎炸油、油条煎炸油。     

Evaporation front compared with crust thickness in potato deep-fat frying

The various theoretical approaches that have been proposed for modeling heat and mass transport during deep-fat frying of potatoes provide a rather ambiguous view of the relation between the propagation of the evaporation front inside the food and the evolution of crust thickness. This can be partly attributed to the unavailability of detailed experimental information concerning the temperature field inside the developing crust to validate the models. The objective of the present work is to experimentally study the relation between crust thickness evolution and evaporation front propagation and how this varies with frying conditions. To achieve this goal, a special device has been constructed that permits (1) only 1 side of a potato stick to be exposed to hot oil, and (2) accurate and stable placement of miniature thermocouples in prescribed positions under but very close to the potato surface. Temperature recordings inside the developing crust allowed identification of different heating regimes during frying and a rough estimation of the evaporation front propagation. In addition, crust thickness was determined at intermittent time intervals by 2 independent methods (1) microphotography and (2) a micrometer. Comparison of the evaporation front propagation with crust thickness evolution indicates an interrelationship roughly up to the end of the boiling regime (bubble-end point). After this moment, the propagation of the evaporation front is faster than the evolution of crust thickness. PRACTICAL APPLICATION: Understanding the role of parameters that determine crust formation is of paramount importance since crust characteristics such as thickness and texture dictate the sensory perception of fried foods. This study aims to quantify the relationship between such parameters (that is, crust evolution and the propagation of the evaporation front inside the food) and to examine how frying conditions (oil temperature and frying duration) affect it. In addition, the present findings may be of particular value to deterministic modeling efforts on the coupled heat and mass transfer phenomena during deep-fat frying.     

Microstructural Characterization of Fried Potato Disks Using X‐Ray Micro Computed Tomography

Microstructural properties play a key role to affect oil uptake and product quality during frying of foods. The objective of this study was to observe the complex microstructural changes and mass transfer mechanisms in potato disks during frying. The potato disks of 1.65 mm thickness were fried at 190 °C for 0, 20, 40, 60, and 80 s. X‐ray micro‐computed tomography (CT) was used for 3‐dimensional (3D) imaging of microstructure of porous potato disks. Total porosity, pore size distribution, oil content, and air content of potato disks were calculated from resulting 3D data sets. Oil and air content measured by analysis of micro‐CT images followed trends similar to Soxtec and gas pycnometry methods, respectively. Image analysis showed a significant change in pore size distribution as a function of frying time. Frying time was also observed to have an effect on tortuosity, which is an important microstructural fluid transport property. Tortuosity was measured by path length ratio method from 3D data sets obtained from image analysis. A linear inverse relationship was observed between porosity and tortuosity where tortuosity decreased with the increase of porosity. It was also observed that during frying, oil content increased with the decrease of tortuosity. This phenomenon indicated that the lower tortuosity created a less complicated and sinuous path, thus resulting in less resistance to oil penetration. Micro‐CT technique can serve as an effective tool for elucidating microstructure of fried foods, and can provide complementary information to conventional lab techniques.     

基于主成分分析法评价两种餐饮用油的煎炸性能

目的 分析两种常见餐饮用油(餐饮调和油A和餐饮调和油B)煎炸性能的差异,并对餐饮用油煎炸品质进行综合评估。方法 在模拟西式快餐实际油炸条件下进行煎炸薯条实验,考察了餐饮用油在油炸过程中十项品质指标,并利用主成分分析法(PCA)对两种餐饮用油在油炸过程中的煎炸性能进行综合评价。结果 餐饮用油品质指标间存在不同程度的相关性,PCA将10个品质指标缩减成2个综合性评价指标,累计贡献率达87.920%,反映了餐饮用油煎炸品质的大部分信息,并进一步提取出评估餐饮用油煎炸性能的5个关键指标,即过氧化值、极性组分、碘价、羰基值以及亚油酸/棕榈酸比值。主成分分析综合得分表明,在油炸过程中,餐饮调和油A得分较高,具有较好的煎炸性能。结论 本研究建立的餐饮用油煎炸性能评价体系为煎炸油的筛选和质量控制提供借鉴。     

Effect of the Addition of Basil Essential Oil on the Degradation of Palm Olein during Repeated Deep Frying of French Fries

The potential antioxidant power of basil essential oil under frying conditions was explored. Two concentrations (200 or 500 ppm) were added to palm olein (PO) to evaluate their effect on fat oxidation/degradation during repeated frying of French fries at 180 °C. A higher oxidative stability index was detected for PO with basil essential oil at 200 ppm. Both concentrations showed lower p‐anisidine values than PO without basil essential oil after 5 d of frying. Addition at 500 ppm resulted in the lowest total polar compounds and free fatty acids contents. Thus, the addition of basil essential oil improved the performance of PO during repeated frying of French fries.