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.
Variation in acrylamide concentration in French fries : effects of control measures in food service establishments
Sanny, M.A.I. - \ 2012
Wageningen University. Promotor(en): Tiny van Boekel, co-promotor(en): Pieternel Luning. - S.l. : s.n. - ISBN 9789461731432 - 199
patates frites - acrylamiden - frituren - kwaliteitscontroles - kwaliteitszorg - voedselbereiding - chips (french fries) - acrylamides - deep fat frying - quality controls - quality management - food preparation
The aim of this thesis was first to identify the major technological and managerial factors and to investigate their contribution to variation in acrylamide concentrations. The second aim was to investigate the effect of technological and managerial control measures on the concentration and variation of acrylamide in the preparation of French fries in food service establishments (FSE). The variation ininitial concentration of reducing sugars, variable frying conditions and food handler’s inadequate control of these factors in their daily practice could lead to the large variation and high acrylamide concentrations. The least variation in acrylamide was found in French fries prepared in chain fast-food services (CFS) compared to institutional caterers(IC) and restaurants, although the mean concentration of acrylamide among the three FSE types was not significantly different. The variation in frying temperature contributed most to the variation in acrylamide, followed by the variation in frying time; no obvious effect of reducing sugars was found. The lack of standardised control of frying temperature and time (due to inadequate frying equipment) and variable frying practices of food handlers seem to contribute most to the large variation and high acrylamide concentrations in French fries prepared in restaurants. Lowering the concentration of reducing sugars in par-fried potato strips can be an effective control measure to reduce acrylamide concentrations in French fries, but only if prepared under standardised frying conditions in CFS and IC. Frying instructions were shown to support food handlers’ decisions to start frying when the oil temperature reached 175oC, although an inconsistent effect of the instructions on the food handlers’ decisions to end frying was observed. The mean concentration of acrylamide for the restaurants as a group was not significantly different, but data analysis for each restaurant showed that if the food handlers properly followed the instructions, the mean concentration of acrylamide was significantly lower compared to before instructions.
|Blancheren frites goed begin : reductie acrylamide [thema : omgaan met risico's]
Zondervan, C. ; Mul, A. de - \ 2007
Voedingsmiddelentechnologie 40 (2007)7. - ISSN 0042-7934 - p. 15 - 16.
frituren - voedingsmiddelen - aardappelproducten - acrylamiden - toxische stoffen - maillard-reactie - simulatiemodellen - kookmethoden - deep fat frying - foods - potato products - acrylamides - toxic substances - maillard reaction - simulation models - cooking methods
Acrylamide helemaal uitbannen gaat niet. De Maillard-reactie waarbij deze genotoxische stof zich vormt, is immers ook verantwoordelijk voor het lekkere bruine korstje op verhitte voedingsproducten. In het kader van het Europese Heatox-project onderzocht AFSG hoe acrylamide in door de consument afgebakken diepvriesfrites valt te reduceren. Blancheren is daarbij een eerste goede stap. Ook werd een simulatiemodel ontwikkeld waarmee acrylamidegehaltes in frites kunnen worden bepaald
Effect of frying conditions on the fat content of French fries : final report
Schuten, H.J. ; Gijssel, J. van; Slotboom, E. - \ 2004
onbekend : Agrotechnology & Food Sciences Group (Report / Agrotechnology and Food Innovations 023) - 16
patates frites - frituren - bakoliën - bakvetten - fysische eigenschappen - temperatuur - vochtgehalte - vetgehalte - chips (French fries) - deep fat frying - cooking oils - cooking fats - physical properties - temperature - moisture content - fat content