Mathematical modelling of the diffusion of ascorbic acid during water blanching of surface frozen potato strips

Partial freezing is a promising technique in producing good quality french fries, especially for potatoes stored for an extended period and containing large amounts of reducing sugars. The cell damage of the potato strip caused by surface freezing facilitates surface leaching of reducing sugars and other soluble solids like ascorbic acid, preventing undesirable dark colour in the finish-fried products. A theoretical study to describe the diffusion of ascorbic acid during the water blanching at 50°C of surface frozen potato strips (10 x 10 x 80 mm) is developed. Ascorbic acid profiles as a function of blanching time, position in the potato strip and thickness of the frozen layer are shown as well as average retention as a function of blanching time, frozen layer thickness, apparent diffusion coefficient and mass transfer coefficient of ascorbic acid. Significant changes in the ascorbic acid concentration during blanching are produced in the frozen layer. Ascorbic acid average retention is mainly influenced by frozen layer thickness and temperature, and the mass transfer coefficient is less important.     

Thermal properties of potatoes and a computer simulation model of a blanching process

Thermal conductivity and specific heat were experimentally determined in Bintje potatoes. The methods used were a transient hot-strip method and differential scanning calorimetry respectively. The results from the experiments were used in a theoretical calculation model using the finite element technique. Temperature profiles and heat transfer coefficients were simulated and the theoretical calculations were compared with experimental data for the blanching process. The correlation between experi-mental data and theoretical calculations was good if the heat transfer coefficient was assumed to be 750 W/m²°C.     

热烫预处理降低鲜切油炸薯片中丙烯酰胺的工艺优化研究

研究热烫预处理对鲜切油炸薯片中丙烯酰胺的影响和薯片油炸工艺优化。在相同条件下,分别对原料进行热烫和非热烫处理,在不同油炸温度和时间条件下进行油炸试验,采用DNS法测还原糖,液质联用法检测薯片中丙烯酰胺含量。结果表明,通过对切片的马铃薯进行热烫处理,即75℃热烫150s,能明显降低还原糖含量,160℃油炸120s,可降低丙烯酰胺含量至67.18μg/kg。     

Quality of French Fried Potatoes as Affected by Surface Freezing and Specific Gravity of Raw Potatoes

Russett Burbank potatoes were stored at 7.2°C for 9 months and processed into French fries. Some of the potato strips were subjected to partial freezing at – 20°C for 25 min before water blanching for surface leaching. The samples treated by partial (surface) freezing generally absorbed more oil during par-flying but less oil during finish-frying compared to the untreated samples (control). French fries made from partially frozen potato strips contained less reducing sugar and ascorbic acid and were lighter in color than the control. Darker fries were produced from potatoes of lower specific gravity because of the higher reducing sugar content. The surface freezing treatment was an effective means of decreasing oil absorption and improving color of French fries.     

STUDIES of WATER TRANSPORT PHENOMENA DURING POTATO-DRYING

Sorption isotherms and drying mechanisms of Bintje potatoes have been investigated experimentally. the product temperature and the moisture content was determined during the drying experiments. A simulation method to estimate the diffusion coefficient and the moisture content profiles for the potatoes was performed by means of a finite-difference computer program. Experimentally determined moisture contents were compared with calculated for different estimated values of the diffusion coefficient. If the diffusion coefficient was assumed to be 0.8.10^?9 m²/s the correlations were acceptable. the simulated moisture content profiles also gave the surface moisture content. The surface mass transfer coefficient has been calculated, the resulting β values were in the range of 0.014–0.029 m/s.     

STUDIES ON THE EFFECT OF SURFACTANT, BLANCHING AND OSMOTIC PRETREATMENTS ON THE CONVECTIVE DRYING OF POTATOES

Moisture removal from solids is an integral part of food processing. To approach the drying process of a given product it is necessary to have a knowledge of the drying kinetics. However, foodstuffs are often subjected to certain chemical and physical treatments, such as surfactant impregnation, osmotic dehydration or blanching, prior to convective drying. The drying behavior of potatoes was monitored within an experimental air tunnel dryer, with the mechanism of internal moisture migration being identified as diffusion. This was subsequently evaluated on the basis of the liquid diffusion theory. Experimental data revealed osmotic concentration as a favorable process. A 29±2% reduction in diffusion coefficient was observed following osmotic dehydration with a salt/sugar solution, however, this provided a 63±3% reduction in drying time. Structural modifications were induced by blanching, with this rendering a 10±1 and 27±2% lowering of the moisture transfer rate during drying at 30 and 60C, respectively.     

油炸马铃薯片中丙烯酰胺形成的影响因素的研究

丙烯酰胺是富含碳水化合物和氨基酸的食品经高温加热发生美拉德反应而产生的,但有关影响丙烯酰胺形成因素的研究却较少。探讨了油炸温度、原料中还原糖和氨基酸含量、鲜薯切片浸泡液的柠檬酸浓度、油炸前薯片的水分含量及抗氧化剂和油的使用时间对丙烯酰胺形成的影响。结果表明:原料中还原糖和氨基酸含量越高,产品中生成的丙烯酰胺就越多;油炸温度越高,产品中丙烯酰胺含量也相应越高;浸泡液柠檬酸浓度越大,产品中丙烯酰胺含量越低;而随着半成品中含水量的降低,产品中的丙烯酰胺含量也逐渐减少;在油中添加不同浓度的BHT和TBHQ以及采用使用时间不同的油,对加工出来的薯片之间丙烯酰胺含量没有显著的影响。     

Improvement of color and limpness of fried potatoes by in situ pectinesterase activation

 Low-temperature, long-time blanching (LT–LT blanching) was performed on potato strips prior to frying. Application of this treatment actived the pectinesterase enzyme in situ resulting in a decrease in the limpness of the fried potatoes; likewise, an improvement in color was obtained as the result of a decrease in reducing sugar content. LT–LT blanching at 65  °C for 45 min gave the greatest improvement in the color and limpness of french fries. Received: 1 December 1998     

两种干燥方式对马铃薯全粉营养品质和加工特性的影响

以新鲜"云薯304"切片后,采用漂烫冻干、不漂烫冻干、漂烫烘干和不漂烫烘干四种工艺条件制备干片,再经研磨粉碎制成半生的马铃薯全粉,研究不同工艺条件下全粉的营养品质和加工特性。结果表明:采用漂烫冻干工艺,马铃薯全粉的颜色与鲜马铃薯颜色最接近,制备的马铃薯全粉还原糖含量最低,为0.15%;蛋白质含量(9.01%)、总淀粉含量(69.12%)、直链淀粉含量(31.60%)与其他三种工艺处理无显著差异;从加工特性上看马铃薯全粉的持水力(5.27 g水/g全粉)和持油力(1.55 g油/g全粉)、透光率(83.43%)和各个温度下的膨胀度(50℃:9.32 g/g;60℃:11.98 g/g;70℃:13.11 g/g;80℃:15.35 g/g;90℃:12.50 g/g)均最高,都显著高于其他三种工艺(P<0.05);且凝胶流程最短,为10.96 mm。因此,可认为漂烫冻干是制备马铃薯全粉的最优工艺。     

不同马铃薯品种对鲜切油炸薯片中丙烯酰胺的影响

研究11个不同马铃薯品种,在相同的油炸条件下进行鲜切油炸薯片实验,采用液相-质谱联用检测其不同品种鲜切油炸薯片的丙烯酰胺含量,结合品种的还原糖含量,对丙烯酰胺含量进行分析。方法：热烫60s,表面干燥,180℃油炸100s,脱油,样品预处理,待测。结果：在相同油炸实验的基础上,检测出丙烯酰胺含量较低的是D519、陇薯3号、中薯7号、LK99等;丙烯酰胺含量较高的是中薯8号、夏波蒂、中薯3号等,并且丙烯酰胺含量与品种的还原糖含量走向基本相似。     

不同热烫处理对马铃薯片中还原糖浓度的影响研究

以还原糖含量为评价指标,还原糖的含量通过DNS法测得,旨在研究不同热烫处理在降低马铃薯片中还原糖含量方面的作用,从而抑制切片型薯片油炸过程中致癌物质丙烯酰胺的生成。在单因素实验的基础上进行正交实验,考察热烫温度和时间两个因素在降低还原糖含量方面的作用。结果表明,热烫处理的最佳工艺条件为：热烫温度75℃、热烫时间135s。     

Technical note: loss of nitrate from carrots during blanching

Loss of nitrate from carrots during blanching was studied using several parameters, such as temperature of water, thickness of carrot slices, and volume ratio of carrots and water. It was found that the kinetics of nitrate diffusion fitted well to the Fick's general diffusion equation. Surface mass transfer coefficient (K) was related to temperature by an Arrhenius type reaction. The values of activation energy ( E _a) and the constant K₀ were calculated as 74 kJ/mole and 2769 × 10³ kg/m²/sec. As expected the initial rates of diffusion were proportional to carrot surface and the efficiency of nitrate removal decreased with increasing blancher load. The proposed model may be used to determine the optimum blanching conditions of carrots in the baby food industry.     

Effect of piece shape and size on leaching of vitamin C during water blanching of potato

A mathematical model based on heat and mass transfer and enzyme destruction was used to predict the effect of shape and size on vitamin C retention during water blanching in forced convection and at 100°C of potato parallelepipeds (0·95 cm × 0·95 cm × variable dimension). The model predicted that incresaing the variable dimension from 0·32 to 1·59 cm² produces a significant increase in vitamin C retention. This behaviour was confirmed by experimental results on water blanching of potatoes.     

香菇脆片真空油炸过程中传质规律

真空油炸过程中传质现象对真空油炸果蔬脆片的品质和安全控制极为重要。研究了油炸温度（80、90、100℃）、真空度（0.075、0.085、0.095 MPa）、预处理方式（漂烫,漂烫+浸渍+涂膜）对香菇脆片真空油炸过程中水分扩散和油脂吸收的影响,结果表明:香菇脆片的含水率随油炸时间的增加而逐渐降低,采用Pabis经验模型能较好模拟香菇脆片真空油炸过程中的水分变化;利用菲克定律建立模型,得到香菇真空油炸的水分有效扩散系数为2.47×10-9~7.70×10-9 m2/s,并且随着油炸温度和真空度的升高而增大;采用浸渍和涂膜处理降低了水分有效扩散系数。香菇片的油脂含量随着油炸时间的增加而逐渐升高,一段时间后达到稳定值;浸渍和涂膜处理降低了真空油炸香菇脆片的含油率。Pabis经验模型能够较好的模拟真空油炸香菇脆片的油脂含量变化,油炸温度和真空度对香菇脆片的平衡含油率（Y_max）影响不大,油脂吸收系数（K_Y）随着油炸温度和真空度的升高而增加。     

Current issues in dietary acrylamide: formation,mitigation and risk assessment

Acrylamide (AA) is known as a neurotoxin in humans and it is classified as a probable human carcinogen by the International Agency of Research on Cancer. AA is produced as by‐product of the Maillard reaction in starchy foods processed at high temperatures (>120 °C). This review includes the investigation of AA precursors, mechanisms of AA formation and AA mitigation technologies in potato, cereal and coffee products. Additionally, most relevant issues of AA risk assessment are discussed. New technologies tested from laboratory to industrial scale face, as a major challenge, the reduction of AA content of browned food, while still maintaining its attractive organoleptic properties. Reducing sugars such as glucose and fructose are the major contributors to AA in potato‐based products. On the other hand, the limiting substrate of AA formation in cereals and coffee is the free amino acid asparagine. For some products the addition of glycine or asparaginase reduces AA formation during baking. Since, for potatoes, the limiting substrate is reducing sugars, increases in sugar content in potatoes during storage then introduce some difficulties and potentially quite large variations in the AA content of the final product. Sugars in potatoes may be reduced by blanching. Levels of AA in different foods show large variations and no general upper limit is easily applicable, since some formation will always occur. Current policy is that practical measures should be taken voluntarily to reduce AA formation in vulnerable foods since AA is considered a health risk at the concentrations found in foods. © 2013 Society of Chemical Industry     

Effects of combined radio frequency with hot water blanching on enzyme inactivation,color and texture of sweet potato

Sweet potato is an important food crop with rich nutritional value, and also a commonly used feed and industrial raw material. However, the quality of sweet potatoes may decline during storage and thermal processes because of enzymatic reaction induced by peroxidase (POD). Radio frequency (RF) blanching was thus proposed to reduce POD activities. Effects of electrode gaps and sample thicknesses on the RF heating rate and uniformity in sweet potato were studied. Influences of hot water blanching and combination of RF heating with hot water blanching on enzyme inactivation and physiochemical properties of sweet potatoes were also analyzed. Results showed that the optimum RF heating uniformity was obtained at an electrode gap of 90 mm and a sample thickness of 60 mm. Combined RF with hot water blanching effectively inactivated POD in sweet potatoes. Compared to hot water blanching, combined RF with hot water blanching gave better sample color, texture values, and lower weight loss when achieving the same level of enzyme inactivation (< 10%). Therefore, combined RF with hot water blanching is recommended as an effective, uniform and high-quality blanching technology for sweet potatoes.     

MATHEMATICAL MODELING OF ROUGH RICE DRYING IN A SPOUTED BED

A theoretical model, based on the well-known man and energy transport equations for a continuous media, was rigorously developed to describe the liquid diffusion controlled drying of cereal grain in a batch operated, well-mixed spouted-bed dryer. The equilibrium moisture content equation and the effective diffusion coefficient are required to solve the heat and mass transfer equations. Numerical techniques were used to solve these equations. The model was tested using laboratory-scale experiments where rough rice was dried in a spouted-bed. A good agreement was obtained between the experimental and theoretical model.     

Heat Transfer and Moisture Loss of Spherical Fresh Produce

A computerized procedure was developed for simulating the post harvest cooling and moisture loss of produce by using a reliable mathematical model. This model was derived by assuming temperature and time variable respiration heat generation and temperature variable thermophysical property values. The developed procedure was employed to examine influence of five key dimensionless parameters on the cooling rate and moisture loss. These parameters are related to the rates of changes in density and thermal conductivity with changes in produce temperature, surface heat transfer conductance, transpiration rate, and environmental relative humidity. Theoretical results obtained from our examination were verified through heat transfer and moisture loss experiments by using fresh potatoes and tomatoes.     

How much reducing sugar may potatoes contain to avoid excessive acrylamide formation during roasting and baking?

For many consumers, potatoes fried or roasted in professional or private kitchens are the most important source of exposure to acrylamide. Acrylamide formation can be reduced by appropriate preparation techniques, but suitable potatoes are a prerequisite. The tendency of potato to form acrylamide can be approximated by the content of reducing sugar. Roast potatoes (hash browns, Rösti) and oven-fried potatoes (Bratkartoffeln) were prepared to optimum culinary quality regarding crispiness and then evaluated in terms of browning, roasting flavor and acrylamide content. Preparation procedures were optimized to produce a minimum of acrylamide. It is concluded that potatoes with less than 0.2 g/kg fresh weight fructose and glucose are not suitable for roasting (insufficient browning and flavor), while roasted products of minimum crispiness prepared from potatoes with more than 1 g/kg reducing sugar contain more than 500 µg/kg acrylamide. It is proposed that potatoes which may be used for roasting and frying should contain less than 1 g/kg fresh weight of reducing sugar. This can easily be fulfilled with the most important potato cultivars grown in Switzerland, but presupposes that potatoes are no longer stored at 4 °C.     

Surface heat transfer in thawing by forced air convection

In thawing by forced air the effective surface heat transfer coefficient, h_eff depends not only on convective sensible heat transfer but also on simultaneous mass transfer. It could be useful to distinguish a true convective surface heat transfer coefficient, h_c, from h_eff. Methods of measuring and calculating the surface heat transfer coefficient are discussed. An equation for calculating h_eff is given. Experiments using a metal model have been carried out. Comparison between calculated and measured values of h_c and especially of h_eff shows good agreement.