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In this study, the potential of mathematical models to describe the evolution of microbial and visual quality during the product life cycle of minimally processed vegetables was evaluated in a case study on cut endive (Cichorium endivia). The study includes (i) measurement of residence time (distribution) and temperature conditions in the chill chain from producer to consumer, (ii) microbial growth kinetics, (iii) kinetics of visual quality evolution, and (iv) analysis of existing distribution practices through simulation.
Time and temperature conditions of minimally processed vegetables in the Belgian chill chain were analysed. Temperature abuses were found during transport and unloading at the supermarket, storage and display in cabinets, and finally, in domestic refrigerators. Residence time and residence time distribution of three categories of minimally processed vegetables in retail display cabinets of two local supermarkets were analysed using a stimulus-response technique. This technique was proposed to calculate mean residence time and standard deviation. It was found that more than 50% of the packages were sold during the first day of display.
The kinetics of microbial growth was first approached by measuring and modelling growth of indicator organisms. Emphasis was on model selection, parameter, estimation and model validation. Growth of Pseudomonas marginalis and Lactobacillus plantarum was modelled as a function of time, temperature and gas composition (CO2). It was possible to predict microbial evolution under dynamic temperature conditions using the model parameters obtained under constant temperature. In contrast to the sigmoidal growth curve of indicator organisms in a liquid synthetic medium, the evolution of the intrinsic microflora of cut endive (pseudomonads; psychrotrofic, Gram-negative population; and Mesophilic aerobic count) was described using a modified model for restricted growth. This explicit model was differentiated with respect to time to Mathematical models for risk analysis and total system analysis in food preservation predict growth under dynamic temperature conditions. Predicting microbial growth on cut endive under dynamic temperature conditions was more difficult. After a second temperature shift, the number of pseudomonads and the Mesophilic aerobic count were underestimated.
It was suggested that this phenomena could be related to the complete breakdown of the vegetable tissue (e.g. Ieading to additional nutrient release and matrix change) so that next to temperature, other factors become growth controlling.
Visual quality deterioration of cut endive was measured in a quantitative descriptive test. An extensive statistical analysis revealed that a linear weighted least squares regression on the logarithmic transformed normalised data was the appropriate way to fit the panel responses. Temperature was found to be the most important rate controlling factor. In contrast to microbial growth, the raw materials used (bleaching) significantly influenced visual quality deterioration rate. Visual shelf-life of cut endive was evaluated by an independent consumer panel. Where a maximum allowable microbial limit unambiguous defines the end of microbial shelf-life, the end of visual shelf-life depends on a proportion of the consumer panel rejecting the sample for consumption. The rejection curve was described using a logistic function. Therefore, the visual shelf-life could be related to a certain drop in mean response of the QDA panel. Predicting quality loss under fluctuating temperature conditions using the model parameters obtained under constant temperature was only limited successful. Differences in predicted and observed mean responses of the QDA panel were attributed to poor model parameter estimates. Predictions of visual shelf-life determined by the consumer panel under dynamic temperature conditions were somewhat better.
Using the models and model parameters described above, the effect of existing time and temperature conditions in the chill chain on the microbial and visual quality evolution was simulated. The modified atmosphere was approached by a controlled atmosphere and no synergistic effect of the gas composition at low storage temperature was considered. It was found Mathematical models for risk analysis and total system analysis in food preservation that the microbial and visual shelf-life of cut endive stored under air was not attained under average time and temperature conditions in the Belgian chill chain. For an hypothetical combination of maximum storage temperatures with maximum residence time in each part of the chill chain; both microbial and visual shelf-life will be exceeded before the products are consumed. Reduction of the initial microbial load and the addition of a controlled gas composition of 19% CO2 (with 2% CO2), resulted in a further extension of the microbial shelf-life.
These observations reinforce the general agreement that good temperature and residence time control are necessary to maintain quality of these minimally processed vegetables.
Open dating of refrigerated products is meaningless and gives nothing but a false sense of safety and quality if temperature abuses have occurred. Time-Temperature Integrators (TTls) are proposed to monitor on-line the time-temperature history of the food product and to display the actual status of the quality or safety attribute of interest. However, for minimally processed vegetables other microbial growth controlling intrinsic and extrinsic factors must be incorporated to exactly mimic the growth of the spoilage microflora. It was concluded that additional research on the kinetics of microbial growth was necessary to develop and select the appropriate Product History Integrator. As an alternative, the visual quality evaluation was proposed as a valuable intrinsic Product History Integrator. Furthermore, when it really comes to consumption, other quality characteristics of the food product, such as aroma, have to be evaluated, leaving the visual evaluation not as the sole decision criterion.
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