Thermal Sterilization by OH and Retort

Although some data from the literature suggest that sterilization by OH could result in an additional non-thermal inactivation of bacterial spores (33–35), data are still scarce on this topic. In our pretrials (data not shown), no additional inactivation by OH in carrot puree was observed. Therefore, in this study, the sole thermal sterilization effect was considered to ensure a safe and comparable OH sterilization process. For purposes of comparison, OH and the conventional retort sterilization processes were performed at the same F₀ values. The common C value was determined as an additional indicator for the thermal load affecting the quality characteristics.

For the conventional retort sterilization, the carrot–oil model purees were preheated to 80°C and filled into glass jars (230 ml), which were then sterilized in a static retort system (Type YRX 900-0 BV, dft technology GmbH, Neumünster, Germany) by spraying hot water (121°C) on the jars. Retorted CRP samples (benchmark), provided by the industrial partner for further analysis, were also sterilized in glass jars (250 ml).

For the OH sterilization runs, the vegetable puree was deposited into the inner treatment chamber (Figure 2) at room temperature. The outer chamber was filled with salt water; the conductivity matched to the product. During the heating process, the salt water in the outer chamber and the product in the inner chamber were heated simultaneously to prevent cold spots close to the wall and on the contact surfaces with the electrode. As similarly reported by Zell et al. (36), a reduction in cold spots by external heating was observed, in this case by the simultaneous heating of salt water in the outer chamber.

To achieve a uniform overall thermal load, the process had to be designed in such a way that slightly higher heating rates resulted in spots close to the wall of the inner chamber. This was necessary, as the subsequent cooling step (flushing the outer chamber with cold water) cooled the volume elements of the sample close to the cylinder wall more quickly due to conventional heat transfer. An increased heating rate in the outer chamber also increased the heating rates in those areas of the inner chamber, thus achieving similar lethality values in the center (T₁) and close to the cylinder wall and the electrode in the lower corner (T₂) (Figure 2). The increased heating rate of the outer chamber was achieved by increasing the conductivity of the surrounding water.

The carrot puree (Table 1) was sterilized by OH and retorted at four different F₀ values (F₀ = 3, 7, 14, and 21 min), covering a range of F₀ values used in industrial sterilization treatments.

To ensure the comparability of results, the CRP was sterilized by OH at the same F₀ value (7 min) as the industrial benchmark retort sample. For the ohmic treatments, the maximum temperature varied while maintaining a constant F₀ value. Thus, CRP was sterilized at maximum temperatures of 115, 121, 125, and 130°C. In the tests, the rapid heating rates resulting from OH were used to quickly reach the desired temperature with the intention of reducing the thermal load. Finally, it was tested which parameters would lead to better preservation of valuable ingredients or to a minimization of processing contaminant formation: a lengthy treatment at reduced temperatures or an abbreviated treatment at higher temperatures. All runs were performed in duplicates.