Adeline Goullieux

Application of image analysis to measure screw speed influence on physical properties of corn and wheat extrudates

Extruded products are usually studied with the respect to their mechanical and sensorial properties. Computer image analysis has been used to provide information on internal structure of a variety of products. In the current study a digital image technique was used to determine structural properties of two types of extruded flour products as screw speed changed. The extrusion cooking of corn and wheat flour was carried out in a twin-screw corotating extruder. Sectional and longitudinal expansion were measured and water solubility and absorption were also determined. The increase in screw speed resulted in products possessing greater longitudinal expansion, higher water solubility and smaller structural patterns. However, differences between the corn and wheat extrudates were also evident at the various screw speeds examined.

Effect of rubber aggregates on the physico-mechanical behaviour of cement-rubber composites-influence of the alveolar texture of rubber aggregates

Abstract The study presented herein has been undertaken in order to examine the physico-mechanical properties of cement–rubber composites by use of two types of rubber aggregates, in the aim of developing a highly deformable material. The results obtained highlight the importance of the alveolar feature and the elasticity of the rubber aggregates in helping improve the flexural strength and deformability of the material. An optical analysis reveals the best level of bonding between the expanded rubber aggregates and the cement matrix.

18 – Ohmic Heating

Publisher Summary Ohmic heating is of growing interest for the treatment of viscous products and products containing large particles within a liquid phase. Internal heat generation, volumetric heating, higher temperature in particles than in liquid and reduced fouling are the main advantages of ohmic heating, which make it possible to apply an HTST process to solid/liquid food mixtures. Thus, it is possible to obtain safe ready meals with a high retention of nutrients and vitamins. Depending on the product to be treated, the choice of the configuration is the first to consider in using ohmic heating. Current ohmic heating modeling is very rewarding because the flow behavior of the products and the invaluable assistance of the control of treatment homogeneity can be better identified. The models can show the influence of the inhomogeneous electrical conductivities on the heating history of the products and their strong impact on the final temperature. On liquid products, modeling also shows that volumetric heaters must be equipped with wall cooling, owing to the low velocities, and therefore, long heating times near the wall to reduce fouling sufficiently. Furthermore, the modeling indicates that free convection effect increases at the bottom of heater but decreases at the top.

Passage time distributions of cubes and spherical particles in an ohmic heating pilot plant

Ohmic or electrical resistance heating involves the passage of electrical current, heating the food by internal generation. The time spent in the heating and holding parts is critical for product sterility and quality. The term passage time (rather than classical residence time) was defined as being more suited to the characteristics of this process. The objective of this study is to determine the passage time distribution (PTD) of real and model foods in the heating column and holding sections of a 10 kW ohmic heating pilot plant. Experiments have been conducted using spherical sodium alginate particles and potato cubes. Experimental variables include particle concentration and volumetric flow rate. PTDs observed in the holding section have small standard deviations. Mean normalised passage times decrease with increasing flow rate and the same pattern is observed for the minimum and maximum normalised passage times. The mean normalised passage times remain constant with increasing solid concentration. PTDs in the heating section were found to have large standard deviations, the ratio between maximum and minimum passage times being as high as 5 in some cases. For an equal thermal treatment all particles are required to spend the same amount of time in the heating and holding parts of the installation. In the holding section this appears to be approximately the case, however, the wide range of passage times measured in the heating section could cause problems of overcooking some particles to ensure commercial sterility.

Processing of cauliflower by ohmic heating: influence of precooking on firmness

The feasibility of processing cauliflower by ohmic heating was investigated. Firstly, cauliflower florets were precooked in tap water at low temperatures (40-70 degrees C) for 0 to 60 min. A control sample was cooked at 95 degrees C for 5 min. No significant textural differences were found between samples treated at 40 or 50 degrees C and fresh samples, but the firmness of samples cooked above 60 degrees C decreased. The effect of precooking time was not found to be significant. Secondly, low temperature precooking was performed in salted water for 30 min and followed by ohmic heating (holding time 30 s at 135 degrees C). After ohmic heating, florets pretreated at low temperatures were firmer than control samples. The firmness of florets precooked at 40 degrees C or 50 degrees C was considerably increased (> 300%) compared to those precooked at 95 degrees C. Low-temperature precooking increased the firmness of cauliflower subjected to ohmic heating. The experimental results show that ohmic heating combined with low-temperature pre-cooking in saline solutions offers a viable solution to high temperature/short time sterilisation of cauliflower florets.

Chapter 22 – Ohmic Heating

Heating is an important step in food processing for the conservation, cooking, and enzymatic inactivation of raw biomaterials. Conventional processes essentially use heat transfer by conduction, convection, and radiation. However, internal resistance is often the limiting factor compared with external resistance by convection, which results in a heterogeneous treatment and a notable loss of product quality. Ohmic heating technology is considered a major advance in the continuous processing of particulate food products. This chapter describes the basic principles and physical modeling of ohmic devices designed to heat Newtonian and non-Newtonian fluids and food mixtures. Technical aspects are discussed and various industrial applications are presented. Potential future applications are also introduced. For the successful application of ohmic heating, careful formulation of the food mixture in terms of electrical and rheological parameters should be carried out to prevent particle/liquid slip for the same electrical conductivity.

Elaboration and kinetic modelling of the formation of a biochemical marker to quantify HTST processing

This study is aimed at developing a new Time-Temperature Integrator (TTI) in order to quantify the effects of High Temperature Short Time (HTST) processing on food quality. A product of non-enzymatic browning was selected as a potential marker and its formation was studied in a glucose/serine system, by means of absorbance measurements at 285 nmn. The formation kinetics of the marker were modelled as a function of both temperature and time of the applied thermal treatment. Three models adequately defined the experimental data. A laboratory-scale ohmic heater served to validate these models. One model resulted in an average prediction error of less than 15%. The marker could be used as a TTI in order to quantity the thermal treatment imposed upon food products in HTST processing.