Simply put, the texture of food is the feeling of the food in your mouth. In practice however texture is not just one thing but lots of different sensations felt all at once. After all, food is made up of various constituent parts and structural elements. These components are arranged and combined into various structures, both micro and macro.
For example, the texture of tomatoes depends on many structural qualities, such as the proportions of flesh to seeds, size of cells, shape of cells, strength of cell wall, and strength of connections between cells.
There are various texture traits that have a strong impact on the pleasure of eating a tomato. Here are three examples:
Firmness
The firmness of a tomato is one of the most important factors in achieving a tomato with a pleasant texture – they can be too firm, or equally not firm enough. In broad terms, the fleshy elements of a tomato are the pericarp; the exocarp, essentially the outermost layer or skin, the mesocarp, the middle layers, and the endocarp, which directly surrounds the seeds.
Here are two ways the structure of the tomato affects firmness:
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The size of cells in the exocarp, or skin: the presence of small cells just under the cuticle, in the skin, of the tomato often results in a tougher skin and a firmer overall texture.
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The strength of cell walls and connections between cells in the mesocarp, or fleshy part of the tomato: the stronger these connections and cell walls are the firmer the tomato will feel.
Juiciness
This is the amount of liquid released by the tomato when crushed.
There are two ways the composition of the pericarp affects overall juciness:
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Water content of the cells: the more water content the cells have the juicier the flesh feels.
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Strength of the cell walls: this is also important, weaker walls break more easily, more readily release their liquid content and so result in a juicier texture.
Mealiness
This is the granular, powdery, soft, dry or coarse texture that tomatoes can have. The fleshy element of the tomato is responsible for this texture.
Below are two ways the structure of the layer contributes to mealiness:
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Shape of the cells: many of the cells in the pericarp are not round or square but stretched so that grouped together they look like bundles of sausages stood on end. It seems that the more elongated these cells are, the more mealy the pericarp feels.
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Strength of the cell wall and connections between cells: if the cell walls are quite strong but the connections between them weak the tomato has a mealy texture. The opposite, where the cell walls are weak but the connections between cells strong, gives a crisp texture like a crunchy apple.
The texture of a tomato is dependent on the physical properties of the tomato. Looking more closely at one of these properties – cell wall strength – I will explore how and why that changes during ripening.
The cell wall of a plant is a highly complicated structure, but it has three fundamental components:
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Tough cellulose fibres. These are mostly parallel but interwoven, like the fibres of a cotton thread, cellulose fibres provide much of the malleable strength of the wall.
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Hemicellulose molecules. Whereas cellulose is crystalline, strong, and resistant to hydrolysis, hemicellulose has a random, formless structure with minimal strength. By weaving around the cellulose fibres and bonding to them, hemicelluloses help bind the wall together.
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Sugar molecules called pectins. Pectins form an additional network of connections within the cell wall in which the cellulose/hemicellulose network is embedded.
The strength of cell walls in a tomato is attributable to both the original construction of the walls and the ripening process. The structure of this cell wall and its relative strength are heavily modified during ripening. The affect of ripening on the physical structure and corresponding properties of tomatoes is naturally of great interest to plant breeders
In relation to texture, the ripening of a tomato consists of the breakdown or digestion of the cell walls and the intercellular glue that binds them together.
During ripening enzymes are secreted into the gaps between cells and attack the structure of the cell walls and the molecules between them.
Different enzymes will control the breakdown of different aspects of the cell wall and binding elements, i.e. one enzyme might contribute to the break down of cellulose strands in the cell wall and another one might break the connections between cellulose strands and the pectin between cells.
The enzyme lycopersicon esculentum polygalacturonase (LePG) and expansin, a family of closely related nonenzymatic proteins found in the plant cell wall, specifically LeExp1, are involved in tomato ripening.
LePG is an enzyme that digests pectin chains. LeExp1 has a role that is less clear and is still being researched. It has been suggested that this protein is responsible for altering the connections between different molecules in the cell wall, disassembling the structure of the wall and possibly making it easier for other enzymes to access and digest these molecules.
In plants where both of these elements were removed the fruit were significantly firmer and less susceptible to deterioration during long term storage – yet they still appeared to ripen normally.
However, in tomatoes where just one of these components has been removed or knocked out, i.e. stopped from producing the protein they code for, there is little or no effect on the softening of the fruit during ripening. This suggests that a range of enzymes and other factors work together to influence texture.
The breakdown of plant cell walls that occurs during ripening is therefore not a simple process and more research needs to be done to understand the relationship between different enzymes.
The two genes – LePG and LeExp1 – code for proteins, which, working together as part of the ripening process, assist the breakdown of cell walls in tomatoes. This then leads to a softer cell walls and affects the overall firmness of the tomato.
It is therefore possible to follow a simple causative path from the presence of a couple of genes among the millions found in a tomato’s DNA to the firmness of the tomatoes that you put into your mouth.
It is also clear that these genes and the proteins they produce will have effects on other aspects of texture and that they are part of a complex ripening process that we do not, as yet, fully understand.
A greater understanding of the role enzymes and genes play in the breakdown of cell walls and other aspects of ripening in tomatoes will allow plant breeders to breed tomatoes with improved texture.
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