3.3.1. Microbial spoilage of pasteurised vegetablejuice
In this paper, additional experiments were carriedout in endive juice to appreciate the variability due toexperiments with biological raw materials, and thusto give the modelling limits. First, the two primarymodels were applied to growth and pectic compounddegradation kinetics during proliferation of C.luteolaon endive juice (Fig. 5). The Baranyi modelfor growth as well as the linear model for pecticdegradationgave accurately fitted values. The experimental error observed in vegetable juice (Fig. 5)was equivalent to the one obtained in syntheticmedium (Fig. 1). Then, m, S and A parametersestimated in vegetable juices, were compared topredicted values obtained with the second modellingstep (Table 1). Specific growth rate, m, was ratherclosed to predicted values whatever the temperatureconditions. Likewise, the alteration percentage, A , pwas not so far from the predicted ones at 7 and 108Calthough the initial pectin concentration varied from0.8 to 1.3 g/ l. In contrast, the variability of thestability period data, S , was greater with vegetablejuices than with synthetic medium. This latter couldbe due to the heterogeneity of the pectin constituentin endives and also to the great complexity ofenzymatic mechanisms involved in vegetable spoilage.To validate the predictive model, it seems interesting to check if experimental conditions carried out inthe laboratory correspond well to food industryprocesses. Post-harvested vegetables are stored atlow temperatures for a few months. Severalpsychrotrophic bacterial strains are able to grow anddamage vegetables by producing mainly pectinolyticenzymes (such as pectin lyase, pectate lyase, pectatehydrolase) but also cellulosic enzymes. C. luteola is a psychrotrophic microorganism being able toachieve 10 cfu /ml after 1 month and to damage60% pectic acid after 2 months at 48C. Therefore, itseems to be representative of pectinolytic bacteriaspoiling vegetables in cool rooms. Moreover, trialsperformed in endive juices showed that resultsobtained in synthetic medium could be extendedalthough the variability of experimental data wasgreater. Further studies will focus on damage characterisation of various post-harvested vegetables suchas carrots and celery by analysing enzymatic activities at low temperatures.