Preharvest-Postharvest Interactions – the Case of Translucency and Colour in Tomatoes

Standard colour measurements can pose a serious problem for interpretation. For tomatoes the maturity at harvest does affect the colour that can maximally be obtained during postharvest storage. For more mature fruit up to 2 units (compared to 18 units tops) were found. This effect was ascribed to the level of a precursor produced during growth. More mature fruit also exhibit a more pronounced sensitivity to becoming translucent upon storage. This effect was also ascribed to the stage of development at harvest. Models were derived on plausible mechanisms to account for both effects of maturity at harvest in tomatoes. INTRODUCTION Preharvest information is the main determining factor for postharvest product response. It is generally accepted that postharvest quality attributes are determined by the growth conditions during preharvest. Even commercial companies start looking in that direction (Gorny, 2005). How these preharvest growth conditions affect the postharvest quality attributes is, however, largely unknown. The influence of preharvest growth conditions may be investigated by varying the stage of ripening at the moment of harvest and by comparing the postharvest response as a function of the stage of ripening. The stage of ripening at the moment of harvest is one of the most crucial aspects for understanding and describing postharvest behaviour (Tijskens et al., 2005). In this paper the results of two experiments on colour and translucency in tomatoes are investigated that use this system in one way or another. The first experiment describes the postharvest colour development of whole tomatoes as a function of the stage of ripening at the moment of harvest. The second experiment describes the postharvest development of both translucency and colour of cut tomatoes as a function of the stage of ripening at the moment of harvest. The aim of this paper is to show the need to assess and incorporate preharvest information regarding the stage of ripening in postharvest models for better product understanding. Also the propagation of the effects of harvest maturity from preharvest over postharvest to processing is highlighted. WHOLE TOMATOES Colour is a major postharvest quality attribute for whole tomatoes. Consumers do assess the quality of the product based, among others, on colour. Current practice in the horticultural chain is to harvest tomatoes just after they reach the breaker stage. Tomatoes in the breaker stage are unacceptable for consumption because of insufficient softness (too firm), insufficient color development (too green) and insufficient taste development. However, harvested tomatoes will mature during postharvest storage and during transport both in the chain and at the supermarket. The effect of the stage of ripening and the effect of the postharvest storage temperature on the postharvest color development is not known quantitatively. Here, color development over time and temperature of whole tomatoes will be investigated as a function of harvest maturity by applying repeated color measurements on individual tomatoes. Color data will be analyzed using a color model that expresses the extent of the postharvest coloring as a function of the initial maturity. Proc. IIIrd IS on HORTIMODEL2006 Eds. L.F.M. Marcelis et al. Acta Hort. 718, ISHS 2006 664 Colour Measurements of Whole Tomatoes Tomatoes (Lycopersicon esculentum ‘Bonavista’) were obtained from a commercial grower and harvested with either five or six tomatoes on the truss by comparing the third tomato of the truss with colour cards ranging from 1 (dark green) to 12 (dark red). A tomato truss was assigned either as green (colour card value 1-3), breaker (4-5) or light red (6-8). After harvest, tomatoes were transported within four hours to the measuring facility, individually labelled opposite of the calyx and separated from the truss to enable colour and firmness measurements. Four tomato batches, each consisting of 16 trusses, were harvested at three different harvest maturities (green, breaker or light red). Each batch was separated into four sub-batches and stored in the dark using climate chambers at 12.0°C, 16.0°C or 19.9°C and 70% RH. Image analysis was used for the RGB colour measurements. Colour was measured on individual tomatoes using a colour video camera in a controlled light environment (Schouten et al., 1997). After a measurement, the tomato image is separated from the background and the calyx and the light intensities for the red (R), green (G) and blue (B) colour are separately averaged over all pixels belonging to the tomato image. Tomatoes were measured every day (storage at 19.9°C) or every other day (storage at 12 and 16°C), starting one day after harvest (day 0) up to day 17. Colour Modelling of Whole Tomatoes The tomato colour model will only focus on the synthesis of red components in the tomato skin. The model assumes that a colourless precursor is converted into red components (Red) (mainly lycopene) by an autocatalytic enzyme or by ethylene (E). The amount of precursor at the moment of harvest determines the extent of postharvest colouring (Eq. 1). Red E 2 E prec r k + ⋅ ⎯→ ⎯ + (1) with kr (in day) the reaction rate constant for the colour change. Another assumption of the colour model is that the precursor production is light dependent (Eq. 2), just as was proposed for the cucumber colour precursor (Schouten et al., 2004). prec prec k ⎯ ⎯ → ⎯ (2) with kprec (in day) the reaction rate constant for the precursor production. By this combined mechanism the postharvest colour behaviour of tomatoes is now also dependent on the precursor accumulation during the preharvest phase as it assumed that tomatoes will be stored in the dark after harvest. To describe the postharvest behaviour of Red over time the ordinary differential equation derived from Eq. 1 by applying the rules of chemical kinetics needs to be solved analytically incorporating the initial conditions at harvest. Applying the fundamental laws of mass conservation the postharvest behaviour of colour of a tomato can then be expressed in terms of time after harvest (t), the biological age at harvest (tage), the initial colour at harvest (Red0) and the asymptotic colour value Redmax and Redmin at respectively plus and minus infinite time (Eq. 3). tage is the biological age expressed as the time (in days) that differentiates each tomato in terms of preharvest precursor accumulation. ) Red ) t k ((Red k t 0 age prec max min 0 min age prec max age prec max min age prec max r e Red ) t k (Red Red Red 1 Red ) t k (Red ) t k (Red Red(t) − ⋅ + ⋅ ⋅ − ⋅ − ⋅ + − + − ⋅ + − ⋅ + =