Characterisation and Evolution of Organoleptic Properties in Solanum Lycopersicum Cultivars

Knight, Matthew (2018) Characterisation and Evolution of Organoleptic Properties in Solanum Lycopersicum Cultivars. Doctoral thesis, Northumbria University.

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Abstract

Public dissatisfaction with tomato flavour has resulted in extensive research into methods of chemometric and sensory profiling, crop improvement and novel breeding strategies for future cultivar development. This work focused on characterising the intrinsic content of flavour and taste active compounds, their origin, localisation, evolution and biosynthesis across multiple cultivars using targeted instrumental analysis and non-targeted metabolomics approaches with the aim to better understand cultivar specific clustering driven by genetic differences as expressed by the metabolome.

Metabolomic classification successfully discriminated all 10 cultivars used in this study with 134 discriminant features upregulated in one or more cultivars as determined by ROC. A total of 7 compounds were tentatively identified based on the Sum formula Identification by Ranking Isotope patterns Using mass (SIRIUS) workflow. Those compounds were primarily related to tryptophan metabolism including the flavonones hesperetin, naringenin and prunin. Targeted analysis for taste and flavour active compounds including sugars, nucleotides, amino acids and volatiles was conducted to further understand the inherent biochemical traits that might lead to sensory quality differentiation. Discriminant classification using the volatile constituents alone allowed for a 91.7% successful classification of 252 samples into their respective cultivar groupings, with only a single cultivar proving challenging to classify.

Other than the inherent genetic variation between cultivars, the biochemical changes occurring during ripening is a significant determinant factor of the overall quality of the ripe fruit. Metabolomic profiling of 3 cherry cultivars across different stages of ripening revealed a relationship that was comparable during ripening transition. The transitionary ripening stages, namely the transition from ‘Mature Green’ to ‘Breaker/Turning’ or ‘Orange’ showed the most significant difference and highest degree of flux, due to the rapid conversion of many cellular components and processes. This includes rapid catabolism of GABA (~30% decrease), tripling of glutamate, doubling of the organic acid and sugar contents and the differential patterns of volatile generation. Fruits showed progressive generation of organoleptically important compounds throughout ripening, with less significant differences notable between ‘Table Ripe’ and ‘Light Red’ than in the earlier stages of fruit ripening, however a shift in fruit quality was still notable.

Localisation of compounds in the different parts of the tomato fruit is important in flavour and taste delivery. Hence, the distribution and abundance of flavour and taste active metabolites was determined in three distinct tissues of tomato fruit across three cultivars. This approach complemented the more commonly used homogenisation, enabling further exploration into the roles each tissue plays in the organoleptic profile of fresh tomato. The importance of the locule fluid, although accounting only for a small percentage of total fruit weight, was highlighted by the markedly increased concentrations of organic acids and umami amino acids, 2-2.5 and 2.3 times greater than that of the flesh, respectively, with comparable sugars. This likely indicates that the tissue is significant to the intense gustatory sensations associated with the first few seconds of fresh tomato consumption.

It is important that investigations in certain tomato fruit biochemical traits are somehow linked with measurable sensory attributes, in order to assess the intensity and acceptability of the cultivars. The distinct chemical profiles for 10 cultivars were then correlated to equivalent sensorial data creating simple binary predictive models. Adjusted R2 values indicated 93.7%, 80.3% and 82.8% coverage of the populations of data for the models, respectively. Sweetness, sourness and umami all showed very strong, significant relationships following linear regression between the analytical and sensory datasets. Finally, understanding and opinion of fresh tomatoes was elucidated though market research and open-forum sensory analysis. The findings indicated that part of the poor perception of tomato flavour may be due to inappropriate post-point of purchase handling, where 75% of surveyed households still store tomatoes in the fridge which is known to impede the formation of desirable organoleptic components.

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