A novel approach for calculating shelf life of minimally processed vegetables

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Abstract

Shelf life of minimally processed vegetables is often calculated by using the kinetic parameters of Gompertz equation as modified by Zwietering et al. [Zwietering, M.H., Jongenburger, F.M., Roumbouts, M., van't Riet, K., 1990. Modelling of the bacterial growth curve. Applied and Environmental Microbiology 56, 1875–1881.] taking 5 × 107 CFU/g as the maximum acceptable contamination value consistent with acceptable quality of these products. As this method does not allow estimation of the standard errors of the shelf life, in this paper the modified Gompertz equation was re-parameterized to directly include the shelf life as a fitting parameter among the Gompertz parameters. Being the shelf life a fitting parameter is possible to determine its confidence interval by fitting the proposed equation to the experimental data. The goodness-of-fit of this new equation was tested by using mesophilic bacteria cell loads from different minimally processed vegetables (packaged fresh-cut lettuce, fennel and shredded carrots) that differed for some process operations or for package atmosphere. The new equation was able to describe the data well and to estimate the shelf life. The results obtained emphasize the importance of using the standard errors for the shelf life value to show significant differences among the samples.

Introduction

Fresh fruits and vegetables have both an important nutrition-health and an economic value. Recently, the market demand for minimally processed fruits and vegetables has undergone an important rise because of busy lifestyles, increasing purchasing power and increasingly health-conscious consumers (Baldwin et al., 1995). Minimally processed vegetables, due to processing operations that alter the physical integrity of these products, are more perishable than the original raw materials. The understanding of the processes that result in quality degradation after processing is essential to develop technologies to extend shelf life and to maintain quality during processing and distribution.

Predictive microbiology is a useful tool to determine shelf life of food products. Several attempts have been made toward predictive modelling of the growth of microorganisms inside, or on the surface of foods as a function of time during refrigerated storage. These models are analytical expressions, such as the Gompertz or the logistic curve (e.g. Zwietering et al., 1991) which exhibit the typical sigmoidal appearance of the bacterial growth curve, or are sets of ordinary differential equations (Baranyi and Roberts, 1995).

The empirical sigmoid-like analytical expressions used in predictive food microbiology are attractive because of their simplicity. For example, researchers have used the Gompertz model to estimate parameters of the model as function of the effects of substrate composition (salt concentration, pH, temperature) for the growth of Listeria monocytogenes (Buchanan et al., 1989). The accuracy in predicting growth depends on the number of parameters used in the sigmoidal model. Modified versions of the Gompertz equation can include three or more parameters to describe the behaviour of the bacterial growth curve and a modified version of the Gompertz model to describe the bacterial population growth curve was used by Zwietering et al. (1990).

The kinetic parameters derived by the Gompertz equation have been often used to calculate the shelf life of numerous minimally processed vegetables (Lanciotti et al., 1999, Riva et al., 2001, Corbo et al., 2004, Sinigaglia et al., 2003) taking into account that, according the French regulation (Ministere de l'Economie des Finances et du Budget, 1988), 5 × 107 CFU/g is the maximum acceptable contamination value at the end of the microbiological shelf life of these products. However, this method does not allow to estimate the standard error of the shelf life; as a consequence, the aim of this paper was to re-parameterize the Gompertz equation modified by Zwietering to insert, among the Gompertz parameters, that of the shelf life accompanied by standard errors. The model was tested by using it for the mesophilic bacteria cell load from different minimally processed vegetables (packaged fresh-cut lettuce, fennel and shredded carrots).

Section snippets

Characteristics of the samples

Data used to test the re-parameterized version of the Gompertz equation were from different minimally processed vegetables:

  • a)

    packaged cut lettuce salads and shredded carrots were processed at a local company producing “ready-to-eat salads” according to four different processes (Sinigaglia et al., 1999). The main differences among the four options were:

    • 1.

      treatment with a solution containing 150 ppm of free chlorine;

    • 2.

      treatment with a solution containing 100 ppm of free chlorine;

    • 3.

      treatment with a

Results and discussion

As reported above the Gompertz equation as modified by Zwietering et al. (1990) has been often used to determine the shelf life of minimally processed fruits and vegetables (Lanciotti et al., 1999, Riva et al., 2001, Corbo et al., 2004, Sinigaglia et al., 2003). The method generally adopted (Zwietering et al., 1990) consists in estimating the Gompertz's parameters by fitting the following equation to the experimental data:log(CFU)=K+A·exp{−exp{[(μmax·2.7182)·λ−tA]+1}}where: K is the initial

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