Fruit texture: a complex combination of physico-chemical parameters

The texture of fleshy fruit (apples, tomatoes, etc.) is an important quality criterion for all those involved in the sector, from producer to consumer. It determines the fruit's suitability for transport and storage, is a source of appreciation or rejection by the consumer and determines the processing methods used. Texture is the result of a combination of factors associated with tissue development and cellular metabolism, depending on the development and physiology of the fruit.  Because of this dependence on the biology and environmental conditions of the fruit, this quality characteristic is subject to great variability, which needs to be controlled to limit losses, particularly with regard to changes in climate and production methods.

The research carried out by the team aims to identify the contribution of cell walls at different scales to fruit texture (mechanical properties) in order to help breeders identify favourable genetic traits for the textures required by the various players in the industry, and to facilitate the choice of raw materials and processes by processors. These studies also involve the search for gentle extraction methods for parietal polymers in order to establish the links between their fine structure and their functions in terms of the mechanical properties of the walls and the texture of the fruit.

The work focuses mainly on apples as a model for fleshy fruits, and aims to understand the structural mechanisms underlying their various mechanical properties. It focuses on the role of 1) water and its compartmentalisation in the tissues, 2) polysaccharides and particularly hemicelluloses, their interactions and distribution in the wall, and 3) ions and secondary metabolites on the mechanical properties of the walls during fruit processing.

1) Mechanical properties of fruit flesh: role of water and cell walls

The mechanical properties of turgid fruit flesh are both viscous and elastic when deformed. The viscous character (loss factor) corresponds to the capacity of the flesh to dissipate the energy of deformation into heat. The elasticity (elastic modulus) relates to the deformation energy restored after deformation and can be compared to the firmness of the flesh.

The hydration of pectins within the walls and their distribution between the cellulose fibres also play a role in fruit firmness. Solid state NMR techniques carried out on apple walls have shown that the more these pectins are hydrated in a dispersed network of cellulose fibres, the firmer the fruit (Lahaye et al., 2020). The result is that not only do the nature of the polysaccharides and their interactions control the viscoelastic properties of the flesh, but also their distribution within the walls.

2) What role do hemicelluloses play in the mechanical properties of walls?

Hemicelluloses comprise three families of complex polysacccharides: xyloglucan (XyG), galactoglucomannan (GgM) and glucuronoarabinoxylan (GaX). They are known to interact with cellulose via hydrogen bonds. These hemicelluloses show great structural variability during the development of the fruit and in relation to its genetics (Guillon et al. 2017; Minoia et al., 2016; Dheilly et al., 2016; Lahaye et al., 2014; Lahaye et al., 2012a; Lahaye et al., 2012b; Galvez-Lopez et al., 2011). Cependant, le rôle de cette variabilité reste inconnu notamment sur les propriétés mécaniques des parois et des tissus. La synthèse de la faible proportion de GgM et ses modifications intervenant lors de phases précoces de développement de la pomme avec l’expression de gènes codant des mannane-synthases et glucanases (Dheilly et al., 2016) raise questions about their role in the evolution of the mechanical properties of the walls and the perception of contrasting textures. This early synthesis and remobilisation of GgMs is not limited to apples, but is also found in another member of the Rosaceae family, cherries, raising the question of the genericity of this stage in fruit development and its function at the time of initiation of cell expansion. These results indicate that the genetic dependence of texture is revealed not only at the time of ripening of the fruit, but from the earliest phases of its development.

 In order to study the relationships between the fine structure of hemicelluloses and their function on the mechanical properties of the walls, work has been initiated to extract hemicelluloses in their ‘native’ state from tomato and apple (Ray et al., 2014; Assor et al., 2013). Unlike conventional methods using alkaline extractions, the solvent based on dimethyl sulphoxide combined with lithium chloride enables the acetyl ester groups within these hemicelluloses to be preserved.  These groups play a key role in the conformation of the molecules and in interactions with cellulose in model systems. (Jaafar et al., 2019).

However, the extension of this work to the different families of hemicelluloses and the role of their fine structural variability has led to a re-evaluation of their production, which requires the extraction of pectins beforehand. Work has begun to evaluate the contribution of new solvents that are part of the principles of green chemistry. Various natural eutectic solvants have proved effective in facilitating the extraction of pectins (Chen & Lahaye 2021; Chen et al., 2021) from apple pomace. Similar approaches are being used to obtain hemicelluloses and cellulose.

3) Mechanical properties of fruit and processing methods

The mechanical properties of cell walls evolve throughout fruit development through the action of various classes of proteins and enzymes on the structure and interactions between parietal polysaccharides. However, oxidation phenomena are potentially induced when the fruit is mechanically destructured with the presence of compounds initially compartmentalised in the cell. This is particularly the case during the grating stage of apple pressing, which releases trace metal ions (iron, copper), phenolic compounds and cell walls and exposes them to atmospheric oxygen. A study of the distribution of metal ions and phenolic compounds in apples has shown a variability linked to genetics, but also to the year of harvest (Vidot et al. 2019, 2020a). This variability could have a decisive influence on pressability from one harvest to another, since a variation in iron content can transform phenolic compounds into anti- or pro-oxidant agents for pectins (Vidot et al., 2020b). Such degradation of pectins would result in changes to the mechanical properties of apple pulp and lower juice availability.