Nikolaos G. Stoforos

Thermal process design

Abstract To calculate the necessary processing time at some appropriate temperature needed to achieve a target preserving action for several food products, heat penetration data must be coupled with kinetic data of microbial destruction. Quality degradation calculations must thereafter be performed in order to select the thermal process which results in the highest product quality retention. In this overview, the key elements involved in designing thermal processes are outlined.

Rhodosporidium toruloides cultivated in NaCl-enriched glucose-based media: adaptation dynamics and lipid production

In the present report and for the first time in the international literature, the impact of the addition of NaCl upon growth and lipid production on the oleaginous yeast Rhodosporidium toruloides was studied. Moreover, equally for first time, lipid production by R. toruloides was performed under nonaseptic conditions. Therefore, the potentiality of R. toruloides DSM 4444 to produce lipid in media containing several initial concentrations of NaCl with glucose employed as carbon source was studied. Preliminary batch‐flask trials with increasing amounts of NaCl revealed the tolerance of the strain against NaCl content up to 6.0% w/v. However, 4.0% w/v of NaCl stimulated lipid accumulation for this strain, by enhancing lipid production up to 71.3% w/w per dry cell weight. The same amount of NaCl was employed in pasteurized batch‐flask cultures in order to investigate the role of the salt as bacterial inhibiting agent. The combination of NaCl and high glucose concentrations was found to satisfactorily suppress bacterial contamination of R. toruloides cultures under these conditions. Batch‐bioreactor trials of the yeast in the same media with high glucose content (up to 150 g/L) resulted in satisfactory substrate assimilation, with almost linear kinetic profile for lipid production, regardless of the initial glucose concentration imposed. Finally, fed‐batch bioreactor cultures led to the production of 37.2 g/L of biomass, accompanied by 64.5% w/w of lipid yield. Lipid yield per unit of glucose consumed received the very satisfactory value of 0.21 g/g, a value among the highest ones in the literature. The yeast lipid produced contained mainly oleic acid and to lesser extent palmitic and stearic acids, thus constituting a perfect starting material for “second generation” biodiesel.

Production of added-value metabolites by Yarrowia lipolytica growing in olive mill wastewater-based media under aseptic and non-aseptic conditions

Yarrowia lipolytica ACA‐YC 5033 was grown on glucose‐based media in which high amounts of olive mill wastewaters (OMWs) had been added. Besides shake‐flask aseptic cultures, trials were also performed in previously pasteurized media while batch bioreactor experiments were also done. Significant decolorization (∼58%) and remarkable removal of phenolic compounds (∼51% w/w) occurred, with the latter being amongst the highest ones reported in the international literature, as far as yeasts were concerned during their growth on phenol‐containing media. In nitrogen‐limited flask fermentations the microorganism produced maximum citric acid quantity ≈19.0 g/L [simultaneous yield of citric acid produced per unit of glucose consumed (YCit/Glc)≈0.74 g/g]. Dry cell weight (DCW) values decreased at high phenol‐containing media, but, on the other hand, the addition of OMWs induced reserve lipid accumulation. Maximum citric acid concentration achieved (≈52.0 g/L; YCit/Glc≈0.64 g/g) occurred in OMW‐based high sugar content media (initial glucose added at ≈80.0 g/L). The bioprocess was successfully simulated by a modified logistic growth equation. A satisfactory fitting on the experimental data occurred while the optimized parameter values were found to be similar to those experimentally measured. Finally, a non‐aseptic (previously pasteurized) trial was performed and its comparison with the equivalent aseptic experiment revealed no significant differences. Yarrowia lipolytica hence can be considered as a satisfactory candidate for simultaneous OMWs bioremediation and the production of added‐value compounds useful for the food industry.

Bi-phasic growth of Listeria monocytogenes in chemically defined medium at low temperatures.

The present work reports a novel observation regarding the growth of L. monocytogenes in modified Welshimers broth (MWB) at low temperatures. Specifically, the direct monitoring of the growth of L. monocytogenes Scott A using plate count data revealed that the pathogen displays a bi-phasic growth pattern in MWB at 7 °C. This bi-phasic growth pattern is masked (not observed) when optical density (OD) measurements are used to monitor growth due to the inability of OD readings to detect L. monocytogenes population density increases up to 10(7) CFU/mL. This bi-phasic growth phenomenon was further investigated as a function of growth temperature (4 °C, 7 °C, 10 °C, 14 °C and 18 °C), medium composition (by altering the MWB composition by ten-fold increases in different sets of medium constituents), inoculum level (10(2), 10(3), 10(4), 10(5), 10(6), and 10(7) CFU/mL) and L. monocytogenes strain (10 strains). The growth of L. monocytogenes Scott A in MWB at 7 °C, 10 °C and 14 °C was consistently bi-phasic and independent of growth rate; at 18 °C, growth was consistently mono-phasic (single-phase, typical sigmoid growth curves), whereas no growth was observed at 4 °C. The tested modifications in the composition of MWB did not influence the bi-phasic nature of L. monocytogenes Scott A growth at 7 °C, and, overall, we could not point out any strain-, or serotype-specific effects. On the other hand, the initial inoculum level appears to affect the form of the growth curve, as there was a shift towards mono-phasic growth in trials with increasing initial inocula. A mathematical model, based on a stepwise response and described through two sequential sigmoid curves, was used to describe bi-phasic growth and estimate the kinetic parameters of L. monocytogenes growth. An alternative hypothesis, based on the assumption of the existence of two subpopulations, possessing different growth kinetics, materialized under the stress imposed on L. monocytogenes cells due to the combined effect of three factors (defined medium, low temperature and low initial inoculum) was also proposed and formulated.

Effect of Particle Orientation during Thermal Processing of Canned Peach Halves: A CFD Simulation

The objective of this work was to apply Computational Fluid Dynamics (CFD) to study the effect of particle orientation on fluid flow, temperature evolution, as well as microbial destruction, during thermal processing of still cans filled with peach halves in sugar syrup. A still metal can with four peach halves in 20% sugar syrup was heated at 100 °C for 20 min and thereafter cooled at 20 °C. Infinite heat transfer coefficient between heating medium and external can wall was considered. Peach halves were orderly placed inside the can with the empty space originally occupied by the kernel facing, in all peaches, either towards the top or the bottom of the can. In a third situation, the can was placed horizontally. Simulations revealed differences on particle temperature profiles, as well as process F values and critical point location, based on their orientation. At their critical points, peach halves with the kernel space facing towards the top of the can heated considerably slower and cooled faster than the peaches having their kernel space facing towards the bottom of the can. The horizontal can case exhibited intermediate cooling but the fastest heating rates and the highest F process values among the three cases examined. The results of this study could be used in designing of thermal processes with optimal product quality.

An Overview of Aseptic Processing of Particulate Foods

ABSTRACT Recently developed procedures in designing aseptic processes for foods containing discrete particles are presented. The use of liquid crystals as temperature sensors for particle surface temperature measurements is discussed. Liquid-particle heat transfer coefficients during tubular flow heating in a holding tube simulating system are presented. An experimental methodology for particle residence time distribution measurements is outlined. A mathematical model for microbial destruction and quality factors retention calculations is presented. The effects of various product and processing parameters on process optimization, based on maximum thiamine retention, are briefly discussed.

Recent Studies on Aseptic Processing of Particulate Foods

Recent studies on mathematical modeling, liquid to particle heat transfer coefficient calculations, residence time distribution, and microbiological validation of aseptic processes of particulate foods are briefly presented. The equations coupling particle residence time distribution with the heat transfer problem are also outlined.

A Theoretical Analysis for Assessing the Variability of Secondary Model Thermal Inactivation Kinetic Parameters

Traditionally, for the determination of the kinetic parameters of thermal inactivation of a heat labile substance, an appropriate index is selected and its change is measured over time at a series of constant temperatures. The rate of this change is described through an appropriate primary model and a secondary model is applied to assess the impact of temperature. By this approach, the confidence intervals of the estimates of the rate constants are not taken into account. Consequently, the calculated variability of the secondary model parameters can be significantly lower than the actual variability. The aim of this study was to demonstrate the influence of the variability of the primary model parameters in establishing the confidence intervals of the secondary model parameters. Using a Monte Carlo technique and assuming normally distributed DT values (parameter associated with a primary inactivation model), the error propagating on the DTref and z-values (secondary model parameters) was assessed. When DT confidence intervals were broad, the secondary model’s parameter variability was appreciably high and could not be adequately estimated through the traditional deterministic approach that does not take into account the variation on the DT values. In such cases, the proposed methodology was essential for realistic estimations.

Aseptic Processing Of Liquid/particulate Foods

A generalized analytical solution to the set of coupled energy balance equations governing heat transfer in a system comprised of uniform radius spherical particles, with nonuniform initial temperature, heated in a well mixed liquid with time varying temperature, is initially derived. Based on this solution, analytical equations describing the temperature evolution for both fluid and particles, during processing of liquid/particulate food systems in a typical aseptic processing line, consisting of a heating, a holding, and a cooling section, are explicitly given. Sample calculations, illustrating the effect of the convective heat transfer coefficient at the particle surface, as well as the overall heat transfer coefficient between the liquid and its surroundings, are included.