Percorrer por autor "Varga, Szabolcs"
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- Determination of the heat transfer coefficient between bulk medium and packed containers in a batch retortPublication . Varga, Szabolcs; Oliveira, Jorge C.The external heat transfer coefficient in steam retort processing was determined experimentally in a pilot scale retort. The heat transfer equations were solved applying finite elements and using the actual retort temperature profile as boundary condition. The instantaneous values of the heat transfer coe cient were determined, to analyse its time-variability along a retort cycle. It was found that reliable results for the external heat transfer coefficient at time t could be obtained from the derivatives of the average heat transfer coefficient calculated between time zero and time t. The results showed a sharp increase of the heat transfer coe cient in the earlier times of heating (up to 4±5 min of processing), followed by a slightly increasing pattern during the remaining heating period. Using average heat transfer coe cients for the heating phase (and for the cooling phase) also resulted in quite accurate estimates of the temperature at the geometric centre of a can. The influence of the two average heat transfer coe cients, heating and cooling, on the lethality was studied by building a response surface. It was concluded that the variability of the coe cient during heating has a greater impact, especially in the range 150±260 W/m2 K.
- Influence of the variability of processing factors on the F-value distribution in batch retortsPublication . Varga, Szabolcs; Oliveira, Jorge C.; Oliveira, Fernanda A.R.The F-value distribution in a batch retort processing of in-pack conduction heating foods resulting from the variability of four processing factors (initial temperature, heating time, headspace and external heat transfer coe cient) was determined by mathematical modelling. Three hundred combinations were generated from normally distributed values for each factor, following the Monte Carlo approach. The retort temperature was assumed the same for all containers and the F-value at the geometrical centre of containers of di erent dimensions was calculated by solving the governing heat transfer equations using ®nite elements. It was found that in the range tested the variability of the heating time and of the external heat transfer coe cient had the biggest impact on the F-value distribution and that the head space variability had a negligible impact. It was also found that although each in¯uencing factor had a normal distribution, the F-value distribution was not normal, showing a signi®cant tailing, being well described by a gamma function
- Modelling temperature variability in batch retorts and its impact on lethality distributionPublication . Varga, Szabolcs; Oliveira, Jorge C; Smout, Chantal; Hendrickx, Mark E.Experimental time±temperature distributions from two di erent industrial scale retort systems were statistically analysed. The retort temperature was modelled as the sum of a trend value and a residual, with the trend temperatures being simple functions of time. The residuals were modelled using time-series. The resulting impact on the lethality distribution was assessed by calculating the F-value distribution in the centre of cans simulated via a conduction-heating ®nite element model for 180 simulated temperature histories. Comparing the distributions obtained with those calculated using the actual experimental temperature histories validated the applicability of this approach. The results indicated that the experimental and the modelled average lethalities were statistically similar at 95% con®dence. The standard deviation was also similar for the F-value up to the end of holding but larger for the modelled distribution when considering the whole cycle, which was attributed to a correlation between the heating and cooling parameters that was not considered in the model.