Elsevier

Food Chemistry

Volume 197, Part A, 15 April 2016, Pages 66-74
Food Chemistry

Perception of bitterness, sweetness and liking of different genotypes of lettuce

https://doi.org/10.1016/j.foodchem.2015.10.105Get rights and content

Highlights

  • Consumers could discern differences in taste that were quantified biochemically.

  • Ratio of bitter:sweet compounds determines bitterness perception and liking.

  • 8-Deoxylactucin-15-sulphate contributes most strongly to bitterness perception.

  • Glucose was most highly correlated with sweetness perception.

  • There is a genetic basis to the biochemical composition of lettuce.

Abstract

Lettuce is an important leafy vegetable, consumed across the world, containing bitter sesquiterpenoid lactone (SL) compounds that may negatively affect consumer acceptance and consumption. We assessed liking of samples with differing absolute abundance and different ratios of bitter:sweet compounds by analysing recombinant inbred lines (RILs) from an interspecific lettuce mapping population derived from a cross between a wild (L. serriola acc. UC96US23) and domesticated lettuce (L. sativa, cv. Salinas). We found that the ratio of bitter:sweet compounds was a key determinant of bitterness perception and liking. We were able to demonstrate that SLs, such as 8-deoxylactucin-15-sulphate, contribute most strongly to bitterness perception, whilst 15-p-hydroxylphenylacetyllactucin-8-sulphate does not contribute to bitter taste. Glucose was the sugar most highly correlated with sweetness perception. There is a genetic basis to the biochemical composition of lettuce. This information will be useful in lettuce breeding programmes in order to produce leaves with more favourable taste profiles.

Introduction

Sesquiterpene lactones are anti-feedants and phytoalexins produced by lettuce (Lactuca sativa L.). Selective breeding against the bitter taste imparted by them has reduced presence of these compounds in domesticated lettuce cultivars dramatically (Wink, 1988). Many modern varieties do still contain perceivable quantities of sesquiterpene lactones and this is particularly relevant, with a move away from iceberg-type head-lettuce to bagged lettuces, which contain fewer high yielding, sweet cultivars and more red-leaved varieties, which typically contain much higher concentrations of the bitter compounds (Price, DuPont, Shepherd, Chan, & Fenwick, 1990). The perceived bitterness is enough to reduce palatability and consumption in a westernised diet, where fruit and vegetables are already under-consumed (Casagrande et al., 2007, Rogers and Pryer, 2012). It is widely believed that this bitterness can be counteracted by sweetness (Bartoshuk, 1975, Keast and Breslin, 2003); an improvement in flavour is therefore likely to be a consequence of manipulating both factors. Although sensory perception of individual sugars (Pangborn, 1963) and SLs (Price et al., 1990, Seo et al., 2009, Sessa et al., 2000) has been previously assessed and sensory perception is well established in the case of sweet compounds, assessment of SL bitterness is sometimes contradictory and has not been considered with regard to tastant mixture suppression. Here we assess the interaction between sweet and bitter components within the natural food matrix of lettuce and additionally compare perception data to consumer liking.

Lettuce is a suitable crop in which to pursue flavour improvement as it is widely eaten across Europe and North America. Lettuce also contains a range of beneficial secondary plant metabolites, including, phenolics, ascorbate, α-tocopherol, lignans, as well as SLs (García-Macías et al., 2007, Oh et al., 2009); consequently, improving the flavour should increase consumer intake. Phytochemicals present in lettuce have been suggested as having a range of biological functions, from analgesic, anti-inflammatory, anti-tumor, and gastroprotective effects of the sesquiterpenoids (Giordano et al., 1990, Guzman et al., 2005, Sayyah et al., 2004), to a cognitive effect of phenylpropanoid flavonoids (García-Macías et al., 2007, Spencer et al., 2009). Additionally lettuce, particularly the romaine type, is a source of iron and potassium and a good source of dietary fibre, folate and manganese, vitamins A, B1, B6, C, K, and omega-3 fatty acids (Belitz, Grosch, & Schieberle, 2009). Bitterness in lettuce is not thought to be linked to the beneficial biological effects of the same molecules, owing to distinct functional groups in the compounds (Behrens et al., 2009, Brockhoff et al., 2007, Chadwick et al., 2013) and so it is feasible to balance the reduction of those most bitter SLs while maintaining or increasing those with greatest biological function.

Sweet and bitter tastes are sensed through the binding of the tastants to G-protein coupled receptors located within papillae on the tongue. Sugars bind to type 1 receptors (T1R) (Meyers & Brewer, 2008) and bitter molecules to type 2 receptors (T2R) (Meyerhof et al., 2010). Whereas there are just two T1R receptors involved in sweet perception (T1R2/T1R3), there are 25 T2Rs responsible for binding a broad range of bitter molecules. Whereas some T2Rs are generalists and bind to a wide range of structurally diverse molecules, others are specialists, binding to a narrow range of compounds (Meyerhof et al., 2010). SLs have been found to activate the T2R46, a generalist receptor (Brockhoff et al., 2007). Within the population, it is common to categorise individuals as “bitter sensitive” or “bitter blind”, and 25% of the population are “bitter blind”; however, this categorisation is due to polymorphisms of the Tas2R38 gene (Mennella, Pepino, Duke, & Reed, 2010). The receptor T2R38 is a specialist receptor, binding to thiouracil groups (as found in Brassica vegetables) and not to SLs. We therefore propose that “bitter blindness” resulting from Tas2R38 will not effect consumer perception of bitterness in lettuce.

We hypothesise that consumers are able to accurately detect sweetness and bitterness in lettuce as imparted by the compounds of interest. We also propose that taste interaction between sweetness and bitterness, as well as the absolute concentrations of the compounds, will have a significant effect on taste perception and liking. Additionally, it is broadly believed that consumers prefer foods which they perceive as sweet. To most consumers, a major factor in purchasing habits is liking for taste (Enneking, Neumann, & Henneberg, 2007) and so ultimately this will be the chief factor in delivering a positive change in consumer habits.

Section snippets

Plant material and growth conditions

F9 recombinant inbred lines (RILs) were supplied by the Michelmore lab (Genome Center, UC Davis, USA) and 102 RILs plus their parents, L. sativa cv. Salinas and the wild L. serriola UC96US23, were propagated by A.L. Tozer. For these studies, plants were grown under glasshouse conditions at The University of Reading and watered once or twice daily in accordance with the weather. The glasshouse temperature ranged from 17 to 30 °C. Seedlings were transferred from seed trays to 3½″ pots with

Sample selection

Lines within the mapping population were selected for extreme values in concentrations of sugars, total SLs, and for specific SLs according to previously reported bitterness ratios. This was done to maximise qualitative data from a small number of samples; hence, while others were selected for overall profile, RILs 41 and 122 were selected on account of having particularly high concentrations of lactucin-15-oxalate, which was reasoned to be the most bitter individual SL, based on correlation

Discussion

Sesquiterpene lactones in a natural food matrix can impart a bitter taste to consumers, with our consumer panel reliably scoring samples correctly in terms of bitterness with regard to the quantities of their determining compounds, as derived by biochemical measurement using HPLC. While it is known that there is great variety in the detection threshold between individual SLs, there is some disagreement as to which SL is the most influential on taste (Sessa et al., 2000, Van Beek et al., 1990).

Acknowledgements

MJC would like to thank both A L Tozer Ltd and the BBSRC for their financial backing and support of this work via an Industrial CASE studentship BB/G017670/1.

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