Modeling colour degradation and microbial inactivation kinetics of tomato paste during in-container pasteurization process

Introduction: In Iran, tomato paste is the most important tomato product because of its widespread use for the preparation of various food menus. Tomato paste is the concentrated product obtained by evaporating tomatoes from which the seeds and skins have been removed (Singh and Headman, 2014). Lactic acid bacteria and spore-forming bacteria are the primary groups of microorganisms involved in the spoilage of tomato products. Among these microorganisms, B. coagulans is considered to be the most important spoilage bacteria, not only because it is more frequently encountered but it has a higher heat resistance than the other spore formers involved in the spoilage of tomato products (Zimmermann, Schaffner, & Aragão, 2013). It is responsible for causing “flat sour” spoilage of canned foods due to the production of lactic acid without gas formation (Mallidis, et. al., 1990). Tomato paste is pasteurized with a mild heating process to eliminate or reduce the number of spoilage microorganisms to an acceptable level and provide conditions that limit the growth of other spoilage microorganisms. However, when food is heated to destroy microorganisms, several types of chemical and physicochemical reactions also occur, some desirable such as enzyme destruction, cooking, texture softening, others less desirable but often inevitable to some degree (e.g., nutrient destruction and loss of organoleptic quality, color, texture, and flavor). These reactions are essentially chemical and temperature-dependent.
The art of canning depends on being able to choose processes that are microbiologically safe but result in the least loss of quality (Holdsworth and Simpson, 2016). Color as a very important quality factor in tomato concentrates which can influence consumer acceptability may be damaged during heat processing. Discoloration and browning of tomato paste are the results of various reactions. Among them, the most common are pigment degradation, especially carotenoids (lycopene, xanthophyll, etc.) and chlorophyll, and browning reactions such as the Maillard reaction and oxidation of ascorbic acid (Barreiro, Milano, & Sandoval, 1997).
Kinetics of pigment and color degradation of vegetables during thermal processing has been studied. The major finding of these studies is that both pigment and color degradation during thermal processing follows the first-order reaction kinetics and the effect of temperature on the rate of reaction can be modeled by the Arrhenius equation (Chutintrasri and Noomhorm, 2007).
Our previous study also indicated that the cold point location is at the radial center between the middle and top of the can at a height of 60% of the can height from the bottom. To follow-up on our previous research, the following objectives will be pursued in the current study. To develop a mathematical kinetic model describing the color loss in tomato paste as a function of temperature and in the second step, by using results obtained from heat transfer modeling, we develop a model that can measure and quantify temperature at any point, the amount of microbial load in the product, and changes in color indices at different points in the can. Modeling kinetics of microbial inactivation and color loss during pasteurization is a good means of further optimizing pasteurization conditions of tomato paste on an industrial level.
Material and methods: Experiments were run with batches of 400 g of tomato paste (pH = 4.1 and 28°Brix) in cylindrical cans (211×400) and hot water was used as the heating medium. The chemical analysis of the tomato paste sample was performed in the first step and the thermal properties of the tomato paste product were determined based on the sample chemical composition. Temperature changes at various positions in the container were checked with a data logger (Testo, Germany) coupled with computer and thermocouples type-K (at 5 min intervals). The data were used to validate the developed model. A 2D heat transfer model was developed in a cylindrical can by using the numerical solution of the Fourier second law. The computer simulation was done using COMSOL Multiphysics, Ver. 4.0. The microbial inactivation model and the color loss of tomato paste model were considered as a first order reactions and coupled with heat transfer module.
Results and discussion: Firstly, the cold and hot points in the can were determined and validated against experimental data. Results showed that the cold point is located at a height of 60% of the can height from the bottom and the hot point is located at the surface of the can. Since microbial inactivation and chemical reactions which deteriorate the quality of food materials are essentially chemical and temperature-dependent, the changes in microbial load and color loss were monitored in two mentioned points.
The effect of pasteurization temperature on the color degradation and microbial inactivation of B. coagulans in a can were followed first-order reactions and temperature dependence of color loss and microbial inactivation were expressed by the Arrhenius equation and thermal resistance constant (Z_value) respectively. As we expected, results indicated that the very high temperature will cause severe color degradation of the food near the surface long before the food at the center of the container has risen in temperature. The surface color change of tomato paste was more rapid at higher temperatures. The color parameters L, a, and b values changed at different rates with different heating regimes. At the following heating regime as a most severe heat regime (heating temperature = 95 oC, heating time=30 min and initial temperature =70 oC) the L, a and b color parameters at the near surface of the can change from 22.9 to 13.0, from 22.6 to 7.9 and from 10.9 to 9.1 respectively. Another point that can be drawn is that the thermal degradation in parameter (a) was higher than other color parameters, so that about 60% decrease in this index is observed in the above thermal regime.
Although it has been established that the minimum safe heat process given to food with low pH should be at least 5D for pathogenic microorganisms and 8D for spoilage microorganisms (Toledo, 2018), our results showed that the pasteurization tunnel in the most severe heating regime can only provide a 3D reduction of B. coagulans in the thermal center of the cans. This was due to slow heat penetration to the center of the can and high heat resistance of a B.coagulans.
Based on these facts we can conclude that the thermal processing used in the tomato paste industry is inadequate and the first stage of the product's thermal process (tubular pasteurization) plays a more important role in reducing microbial load than in the pasteurized tunnel.
Conclusion: A two-dimensional finite element model was developed to calculate temperature histories at two selected points within container, coupled with microbial inactivation of B.coagulans and color degradation kinetics. Results showed that by applying more intense time- temperature in the pasteurizer, the color of the product was more damaged but the heat processing was not sufficient to reduce the number of microorganisms to a certain degree (5D) in the thermal center. However, it is necessary to point out that, to establish a more accurate thermal process, it is necessary to find the load of B. coagulans in the under-processed product.