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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

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    Deep frying : from mechanisms to product quality
    Koerten, K.N. van - \ 2016
    Wageningen University. Promotor(en): Maarten Schutyser, co-promotor(en): Remko Boom. - Wageningen : Wageningen University - ISBN 9789462576476 - 119
    deep fat frying - quality - chips (french fries) - evaporation - crusts - moisture meters - fried foods - crisps - frituren - kwaliteit - patates frites - evaporatie - korsten - vochtmeters - gebakken voedsel - aardappelchips

    Deep frying is one of the most used methods in the food processing industry. Though practically any food can be fried, French fries are probably the most well-known deep fried products. The popularity of French fries stems from their unique taste and texture, a crispy outside with a mealy soft interior, but also because of the ease and speed of preparation. However, despite being a practical and easy method, the fundamental phenomena that occur during frying are very complex. This thesis aimed at gaining a deeper understanding of the frying of French fries. This was done at the product level, with regards to heat transfer, moisture loss, oil uptake and crust formation, and at the process level, which encompasses the oil movement in a frying unit and the consequent oil-fry interactions.

    Firstly a numerical model was developed to describe the water evaporation during frying (Chapter 2). Though various models exist for describing moisture loss, they all use constant values for the heat transfer coefficient. However, the heat transfer coefficient actually varies greatly due to the varying degrees of turbulence, induced by the vapour bubbles escaping from the fry surface. Therefore, the model in this thesis incorporated an evaporation rate dependent heat transfer coefficient. Other than the varying heat transfer coefficient, the model was heat transfer dependent, with a sharp moving evaporation boundary and Darcy flow describing the flow of water vapour through the crust. The model was successfully validated against experimental results for moisture loss and temperature profiles in the fry.

    For oil uptake during frying, a pore inactivation model from membrane technology was adopted (Chapter 3). In membranes, pores will inactivate when the transmembrane pressure becomes too low. In fries, this can be translated as pores in the crust inactivating when the evaporation rate becomes too low. As pores stop expelling water vapour, oil can migrate into the fry. The model also took into account the lengthening of the pores with increasing crust thickness, allowing for more oil uptake in inactivated pores. The model fitted well with experimental data for oil uptake during frying. Also, the pore inactivation model better described oil uptake during the initial stages of frying, where the evaporation rate is still relatively high, compared to the linear relation between oil uptake and moisture content, which is usually assumed in literature.

    Both the influences of frying temperature and moisture content on crust structure and consequent textural properties were studied (Chapter 4). The crust structure was visualized and quantified using X-ray tomography (XRT), which uses multiple 2D X-ray pictures of a rotated sample to reconstruct a 3D density map. Textural properties, like hardness and crispness, were quantified using force deformation curves from a texture analyser. Moisture loss was shown to greatly increase porosity and pore size in fries. More crispy behaviour was also shown for higher moisture loss, though not significantly at moisture contents close to the initial moisture content. Though increased frying temperatures also showed an increased porosity and pore size, there was no significantly observed increase in crispness. This is most likely because the texture analysis was not sensitive enough to discern any increased crispness for porosities below a certain degree. Strikingly, for frying temperature around 195 °C, a decrease in crispness was observed. These samples visually also showed more plastic behavior. The most likely cause for this is degradation of sucrose, which happens around 186 °C, and consequent caramelization of glucose, thus increasing the glass transition temperature.

    At the process level, oil flow and fry quality distribution were investigated using a pilot scale cross-flow fryer (Chapter 5). Oil circulation velocities were varied to observe the initial fluidization behavior of the fry bed through an observation window. This fluidization behavior was well described by the Ergun equation, modified for non-spherical particles. The distribution in moisture content of the fries was used as an indicator for quality distribution. Though increased oil circulation initially increased the homogeneity of the moisture content, upon fluidization the homogeneity actually decreased. Image analysis of fries before and after frying showed local packing of fries around their fluidization point. This was due to the non-spherical shape of the fries, making them more sensitive to channelling.

    The results obtained in this thesis were finally discussed, together with the possibility to also model the process scale of the frying process (Chapter 6). The possibility of modelling the oil flow through a packed bed of fries, and the free-convective heat transfer during frying, using a CFD software package (STARCCM+) was shown. Additionally, the possibility of linking oil flows computed using CFD to the general models developed in this thesis was discussed. Modelling the momentum transfer of the expelled vapour bubbles to the oil, but also the movement of the fries themselves is still a faraway goal. However, a multiphase model that can describe both the entire frying setup as the consequent individual fry parameters would be invaluable.

    Tillage for soil and water conservation in the semi-arid Tropics
    Hoogmoed, W. - \ 1999
    Agricultural University. Promotor(en): U.D. Perdok; L. Stroosnijder. - Wageningen : Wageningen University [etc.] - ISBN 9789058080264 - 184
    grondbewerking - verzegelen - losmaken - korsten - bodembescherming - grondbewerking gericht op bodemconservering - infiltratie - west-afrika - brazilië - tillage - sealing - loosening - crusts - soil conservation - conservation tillage - infiltration - west africa - brazil

    Soil tillage is the manipulation of soil which is generally considered as necessary to obtain optimum growth conditions for a crop. In the same time the resulting modification of soil structure has serious implications for the behaviour of the soil to erosive forces by water and wind. In Chapter 1 an introduction is given to the most important aspects: the objectives of tillage, the conflicting requirements set to tillage, the characteristics of soil and water conservation in the semi-arid tropics, and the nature of tillage research including modelling.

    Chapter 2 treats in detail the characteristics of the soils often found in the semi-arid tropics: the SCH soils (sealing, crusting and hardsetting). Sealing and crusting causes problems with emergence of seedlings and with infiltration. The hardsetting soils are difficult to manage, particularly when tillage has to be performed with limited energy inputs. Physical characteristics and low organic matter contents are primarily responsible for SCH behaviour.

    In case 1, research undertaken in Mali is reported. Sandy soils of the Sahel area, mainly cropped to millet (Pennisetum glaucum (L.) R.Br.) are very sensitive to crust formation. These crusts were found to strongly reduce infiltration capacity. On the typically gently (1-3%) fields runoff is a widespread phenomenon; on the average 25% of the rain (mainly in the form of a few large storms during the rainy season) is lost by runoff. Crust formation and its effect on the infiltration rate was studied in experiments using a rainfall simulator are discussed. On untilled soils the presence of a crust is a permanent feature, and the effect of superficial tillage on crust disturbance disappeared quickly under subsequent rainfall. It was established that rainfall characteristics (aggressiveness, intensity) play a key role in crust formation.

    Research reported in case 2 was carried out in Niger. Here, important processes of soil structural changes under rainfall were assessed, to obtain a basis for a proper development of improved soil management methods. Soil and rainfall characteristics of a millet growing area close to Niamey, were determined. Laboratory tests showed a confirmation of what was observed in the field, namely that the coarse sandy soil of the area shows a mechanical behaviour which is extremely dependent on the moisture content at the time of soil handling. Therefore, the workability range is very narrow. Special tillage under wet conditions, resulting in smearing of the surface layer caused a condition which was more resistant to wind erosion. In an extension of the analyses reported in case 1, it was found the rainfall in this region is aggressive; even small storms may fall with high intensities.

    The major rainfall characteristics of the Sahel differ significantly from those of other semi-arid areas (such as India). The erratic rainfall pattern in combination with the sandy nature of the soil in the region studied, leads to an extremely small number of days available for planting millet, on average around 11 for the season. Therefore, time-efficiency of soil preparation and planting methods is even more important than a positive effect on SWC and crop emergence aspects.

    Chapter 3 gives a review of the various tillage systems as they may be applied for soil and water conservation, based the soil characteristics and on different mechanization levels.

    A study carried out in Brazil is presented in case 3. In a highly mechanized farming situation, erosion problems on sloping, red soils in the state of Paraná are high. Conventional tillage is based on the use of heavy disk ploughs and repeated passes with disk harrows in order to prepare a seedbed. This system causes severe erosion damage because of reduced infiltration rates and unstable topsoil. The zero-tillage system is a promising and realistic alternative, but is not suited for all farms in the state (small fields, high capital investment for equipment required, lacking knowledge and experience of farmers). The possibilities for the use of chisel ploughs (as an alternative between these two systems) on wheat stubble in a wheat-soybean rotation were investigated.

    Experiments showed that, compared to conventional tillage systems with disc implements, chisel ploughing left more plant residue at the surface. Except for the duckfoot type, the chisels were able to penetrate down to the bottom of a compacted layer at 12-20 cm depth. In addition, fuel consumption was significantly lower than disc ploughing and slightly higher than heavy disc harrowing. The capacities of the chisel ploughs were comparable to the heavy disc harrow. On the other hand, weeds and large amounts of straw may cause considerable practical difficulties and require adequately dimensioned chisel ploughs. Thus when applying the alternative tillage, adapted sowing equipment, able to cope with surface residue is required.

    In case 4, studies on the agronomic effect of tillage systems fit for animal traction in West Africa are reported. Crop establishment is an important yield factor for pearl millet in the Sahel. Therefore, a series of experiments was conducted to determine the effects of seed size, depth and method of planting, millet variety, tillage, and soil fertilization upon seedling emergence, crop establishment, and yield. All experiments were conducted on a sandy Psammentic Paleustalf in Niger. Three millet varieties were studied, and for all of these, out of a range of sowing depths from 1 to 7 cm, a sowing depth of 3-5 cm resulted in the highest percentage emergence, the highest above-ground biomass, and most secondary roots. High soil temperatures are common during establishment, typical maximum temperatures at a depth of 1 cm exceed 46 °C. It was found that the adverse effects of wind erosion and these high temperatures were least when sowing in hills (the traditional hand-method); establishment, crop stand survival, and yield were better under hill planting than drilling seed.

    Tillage of the field before sowing increased initial stands and their survival, the latter also depending on fertility. Thus, improved crop yields result from better stand survival and higher yields per hill. Fertilizer application (17 kg ha -1P and 40 kg ha -1of N) caused a threefold increase in grain yields. Ridging without prior tillage and ploughing increased grain and stover yields two- to three-fold. In combination with fertilizer application, sixfold yield increases were obtained. In view of the time limitations, ridging without prior tillage was preferable to ploughing, as it is a much faster operation giving equally good results in terms of crop establishment and yield.

    Chapter 4 deals with simulation models for soil and water conservation and the difficulties of modelling tillage effects. A review of the most important models currently used in SWC is presented with a brief indication if and how tillage is incorporated in these models. Various options of approaching tillage effects by modelling are given.

    In case 5, the development and application of a model simulating the role of tillage in SWC is presented. The data used are mainly from the situation prevailing in the West African Sahel and Sudan zone, characterized by a low input (particular N and P) rainfed farming system, growing cereal crops such as millet. Two types of soil tillage are distinguished: tillage aimed at water conservation (by increasing infiltration and/or surface roughness) and tillage aimed at weed control. Various scenarios are evaluated by combining simulation models for plant production (WOFOST) and soil water movement (SWATRE), developed and adapted for application in these regions. Based on simulations of 35 years of weather data, it was found that water conserving tillage as such has a very small yield-conserving effect because of the limitations set by the nutrient status. Elimination by tillage of the competition by weeds had a larger effect on the grain yield of a millet crop.

    In case 6, the water balance for millet fields and for permanently crusted natural pastures is described, with special emphasis on the role of the crust in governing infiltration and runoff. This study was based on the same field experiments in Mali as described in case 1. It was tried to quantify the effect of tillage as it destroys the crust and increases the surface storage for rainwater. The crust-breaking effect was found to last for only a few rainshowers, but the increase of surface storage is more permanent. The effect of a tillage system on the water balance of a millet crop was calculated. From this calculation it was concluded that tied ridges, giving a surface storage of 20-30 mm, could completely prevent runoff, compared to about 50% loss under the conventional system. Such a savings would allow earlier sowing and thus prolong the vegetative growth by as much as 20 days, which might increase the average millet yield (500 kg ha -1) by 40%.

    In Chapter 5, the prospects for development of tillage systems for the difficult SCH soils are discussed. Analysis of the various options shows that no-tillage is not a solution for the semi-arid tropics with hardsetting soils. It also can be argued that the introduction of animal traction in situations with purely handlabour, in many cases is not feasible, and notwithstanding all other problems, tillage by tractors should be investigated as a serious option.

    Effects of crust and cracks on simulated catchment discharge and soil loss
    Stolte, J. ; Ritsema, C.J. ; Roo, A.P.J. de - \ 1997
    Journal of Hydrology 195 (1997)1/4. - ISSN 0022-1694 - p. 279 - 290.
    bodem - korsten - scheurvorming - opzwellen - krimp - soil - crusts - cracking - swelling - shrinkage
    Sealing, crusting and cracking of crusts of the soil surface has been observed in many parts of the world in areas with sandy, silty and loamy soils. Sealing and crust formation occurs under the influence of rain storm and drying weather. With prolonged drying, surface crusts might crack, leading to complex situations with respect to infiltration and runoff generation. Cracking of crusted loamy soils appears to be a general process. This study aims to measure the hydraulic properties of fully crusted and cracked-crusted areas and to evaluate the effects of these measurements on catchment discharge and soil loss in a loess region of the Netherlands, using the LISEM soil erosion model. Samples with minimum infiltration rates (fully crusted) and with maximum infiltration rates (cracked-crusted surfaces) were taken from fields with bare soil or winter wheat and their soil hydraulic functions were measured. The results of these measurements were used as input in the LISEM soil erosion model. Simulations of discharge and soil loss were done for each of these two land-uses and for two rain events. Additionally, simulated discharge and soil loss under actual recorded land-use were calculated. In all cases, soils with no surface cracks produced higher figures for discharge and soil loss than those where 10% of the surface crust was cracked. For a good interpretation of the results for soil loss, the spatial distribution of cracked-crusted areas and fully crusted areas has to be investigated in detail. To deal with cracked-crusted and fully crusted areas in simulation modelling, care has to be taken to accurately measure the soil physical functions representing the maximum and minimum infiltration rates. An assignment of these functions to calculation grids has to be made. As the LISEM model is capable of assigning different soil physical functions to each calculation grid, an improved prediction of the soil physical behaviour of the catchment can be simulated.
    The impact of termites and mulch on the water balance of crusted Sahelian soil.
    Mando, A. - \ 1997
    Soil Technology 11 (1997). - ISSN 0933-3630 - p. 121 - 138.
    bodemwaterbalans - bodem - korsten - bodemfauna - isoptera - sahel - soil water balance - soil - crusts - soil fauna - isoptera - sahel
    Effect of termites and mulch on the physical rehabilitation of structurally crusted soils in the Sahel.
    Mando, A. - \ 1997
    Land Degradation and Development 8 (1997). - ISSN 1085-3278 - p. 269 - 278.
    bodemstructuur - isoptera - mulchen - stromulches - turf - bodem - korsten - bodemwater - erosiebestrijding - waterbescherming - bodembescherming - sahel - soil structure - isoptera - mulching - straw mulches - peat - soil - crusts - soil water - erosion control - water conservation - soil conservation - sahel
    Soil variability and effectiveness of soil and water conservation in the Sahel.
    Hien, F.G. ; Rietkerk, M. ; Stroosnijder, L. - \ 1997
    Arid soil research and rehabilitation : an international journal 11 (1997)1. - ISSN 0890-3069 - p. 1 - 8.
    bodem - korsten - erosie - geostatistiek - soil - crusts - erosion - geostatistics
    Sahelian sylvopastoral lands often degrade into bare and crusted areas where regeneration of soil and vegetation is impossible in the short term unless soil and water conservation measures are implemented. Five combinations of tillage with and without mulch on three crust type/soil type combinations were evaluated. The texture and organic matter content of the crusts are determined more by crust type than by soil type. Differences in crust characteristics are linked with the genesis of the crusts. Germination of Cassia tora L. was lower on runoff crusts than on erosion crusts. Combining soil tillage with mulching resulted in a higher average germination than soil tillage or mulching as single measures. We also monitored the mortality of C. tora seedlings 4 weeks after germination. Seedling survival showed a similar, though less pronounced, trend to germination. Average survival was slightly better on erosion crusts than on runoff crusts. Although tillage is necessary to trigger germination of herbaceous species, it may simultaneously inhibit establishment of these species by bringing about severe soil crusting. Soil tillage combined with mulching provided the best conditions for the ultimate establishment (the product of germination and survival) of C. tora. It appears that soil differences have less influence on germination and survival than do differences in surface conditions.
    Soil surface coatings at Costa Rican recently active volcanoes.
    Jongmans, A.G. ; Mulder, J. ; Groenesteijn, K. ; Buurman, P. - \ 1996
    Soil Science Society of America Journal 60 (1996). - ISSN 0361-5995 - p. 1871 - 1880.
    andepts - andosols - costa rica - korsten - pedologie - hellingen - bodem - bodemmicromorfologie - vulkanische gronden - andepts - andosols - costa rica - crusts - pedology - slopes - soil - soil micromorphology - volcanic soils
    Effects of termites on infiltration in crusted soil.
    Mando, A. ; Stroosnijder, L. ; Brussaard, L. - \ 1996
    Geoderma 74 (1996)1-2. - ISSN 0016-7061 - p. 107 - 113.
    burkina faso - korsten - hydraulisch geleidingsvermogen - infiltratie - isoptera - kwel - bodem - burkina faso - crusts - hydraulic conductivity - infiltration - isoptera - seepage - soil
    In northern Burkina Faso (West Africa), a study was undertaken to explore the possibilities of restoring the infiltration capacity of crusted soils through the stimulation of termite activity. Treatments consisted of the application of a mulch of a mixture of wood and straw without insecticides (resulting in ''termite plots'') and the application of the same mulch and an insecticide (Dursban with chloropyrifos as the active ingredient) to prevent termite activity (resulting in ''non-termite plots''). Three rainfall simulations of 60 minutes duration with an intensity of 50 mm/h at an interval of 24 hours between the first and the second and 72 hours between the second and third simulation were applied, to study the effect of consecutive showers on termite-modified soil characteristics. Cumulative infiltration amounts, final infiltration rates, soil water content and porosity were larger and bulk density was smaller on termite plots as compared to non-termite plots. This suggests that termites may be an important agent in soil-crust control and in the improvement of soil physical properties in Sahelian ecosystems.
    Surface sealing and hydraulic conductances under varying-intensity rains
    Giménez, D. ; Dirksen, C. ; Miedema, R. ; Eppink, L.A.A.J. ; Schoonderbeek, D. - \ 1992
    Soil Science Society of America Journal 56 (1992)1. - ISSN 0361-5995 - p. 234 - 242.
    crusts - hydraulic conductivity - infiltration - pedology - rain - relationships - runoff - seepage - simulation - soil - soil micromorphology - korsten - hydraulisch geleidingsvermogen - infiltratie - pedologie - regen - relaties - oppervlakkige afvoer - kwel - simulatie - bodem - bodemmicromorfologie
    In the past, investigations on surface seals developing under simulated rains usually were performed with uniform rainfall intensities. Recent studies, however, showed that varying-intensity rains affect erosion and volumes of runoff. We conducted a study on surface sealing under varying-intensity rains using physical and morphological methods. Four 1-h-duration rainfall patterns were used: S1, S2, S3 and S4, with mean rainfall intensities of 83, 69, 65, and 51 mm h-1, respectively, and highest intensities of 148, 120, 111, and 100 mm h-1, respectively. Two experiments were performed in triplicate: (i) in packed soil columns, pressure heads at 1-cm depth were measured, and runoff (Rt) and wash sediment (St) collected at short time intervals during S1, S3, and S4 patterns; and (ii) packed shallow soil flumes were successively exposed to 10, 15, 30, 40, and 60 min of S2 and S4 patterns, and the surface was then characterized by micromorphological analysis of thin sections (S2) and top-view photographs (S2 and S4). The largest differences between rainfall patterns occurred during the intensity peaks (10–30 min): at the end of these periods, measured surface seal hydraulic conductances (β) attained values of 0.80, 0.25, and 0.51 h-1 for S1, S3, and S4, respectively. These relative β values are thought to be due to partially open surfaces as a result of microrill erosion (S1), complete sealing without erosion (S3), and incomplete sealing (S4). Presented Rt, St, and morphological data support these conclusions. After 5 min of the intensity peaks of S2, surface seals attained maximum observed thickness and minimum macroporosity (>30 µm). Planar voids (>500 µm) were eliminated at this point. Under continuing rain, erosion reduced the seal thickness and increased macroporosity, mainly as planar voids
    Korstbreken bij de opkomst van suikerbieten.
    Bosma, J.L. ; Kouwenhoven, J.K. - \ 1989
    Landbouwmechanisatie 40 (1989)4. - ISSN 0023-7795 - p. 58 - 61.
    beta vulgaris - korsten - cultivators - uitrusting - prestatieniveau - kwaliteit - zaaibedbereiding - zaaibedden - zaailingcultuur - bodem - suikerbieten - grondbewerking - gereedschappen - machines - beta vulgaris - crusts - cultivators - equipment - performance - quality - seedbed preparation - seedbeds - seedling culture - soil - sugarbeet - tillage - tools - machines
    The effect of tillage practices on crust properties, infiltration and crop response under semi-arid conditions.
    Rawitz, E. ; Hoogmoed, W.B. ; Morin, J. - \ 1986
    In: Assessment of soil surface sealing and crusting : proceedings of the symposium held in Ghent, Belgium, 1985 / Callebaut, F., Gabriels, D., de Boodt, M., - p. 278 - 284.
    korsten - hydraulisch geleidingsvermogen - infiltratie - zaaibedbereiding - kwel - bodem - grondbewerking - crusts - hydraulic conductivity - infiltration - seedbed preparation - seepage - soil - tillage
    Crust formation on sandy soils in the Sahel: I. Rainfall and infiltration
    Hoogmoed, W.B. ; Stroosnijder, L. - \ 1984
    Soil & Tillage Research 4 (1984). - ISSN 0167-1987 - p. 5 - 23.
    korsten - sahel - zandgronden - bodem - crusts - sahel - sandy soils - soil
    Sandy soils of the Sahel area in West Africa, mainly cropped to millet (Pennisetum typhoides) are very sensitive to crust formation. Crusts strongly reduce infiltration capacity. In this area most fields are gently sloping (1–3%) and hence runoff is a widespread phenomenon; on the average 25% of the rain (mainly in the form of a few large storms during the rainy season) is lost by runoff. The causes of crust formation and its effect on the infiltration rate are discussed. On untilled soils the presence of a crust is a permanent feature. Rainfall characteristics play a key role in crust formation. Major rainfall characteristics of the Sahel differ significantly from those of other semi-arid areas.
    Een onderzoek naar de verslemping van zeekleigronden
    Albers, H.T.M.P. - \ 1980
    Wageningen : Stichting voor Bodemkartering (Rapport / Stichting voor Bodemkartering no. 1484) - ISBN 9789032700515 - 66
    zware kleigronden - korsten - nederland - poriënvolume - porositeit - bodem - bodemdichtheid - clay soils - crusts - netherlands - pore volume - porosity - soil - soil density
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