For processing tomato, fruit soluble solids content, pH, and paste viscosity are the major quality traits. According to distributors and retailers, fruit quality is essentially defined by shelf-life and firmness. During recent years, these criteria have been taken into account for breeding but have led to flavorless fruit. For consumers, quality is defined by the traits governing fruit attractiveness and fruit flavor. Nutritional quality (e.g., antioxidants, vitamins) is also important (See Sect. 1.14) but cannot be directly evaluated by consumers. Fruit size, shape, color and firmness are the first quality traits which attract consumers by visual and tactile stimulation. When eaten, the fruit expresses its organoleptic quality which involves the senses of taste and smell. Organoleptic quality is complex as it results from a combination of aroma, taste and texture components. These traits are often difficult to measure by methods other than sensory analysis. Some of the quality components are associated with each other, which makes the analysis even more complex. However, some of the major components of tomato flavor can be assessed by physical or chemical measurements. For example, flavor depends on the ratio of sugars and acids, and aroma is related to certain volatile compounds (Baldwin et al. 1998).
The large diversity in fruit size and shape of tomato illustrates the genetic variability available in this species. Each cultivar is characterized by a particular aroma profile, and a specific range of taste and texture. Genetic variation is the major source of fruit quality variation (Stevens 1986; Causse et al. 2003), but fruit organoleptic quality is also influenced by external factors such as the environment (Dorais et al. 2001) and conditions during fruit storage (Stern et al. 1994).
Identifying "robust" quality QTLs is a prerequisite for molecular breeding or for their molecular characterization. This robustness can be assessed by experimental studies of their stability over years, over different external conditions and over genetic backgrounds. These "robust" QTLs can also be identified by comparing data from different experiments and/or obtained with different populations. Molecular characterization of QTLs to date has been performed by positional cloning, but new genomic tools are starting to be applied for identifying candidate genes related to QTL variation.
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