As a staple of the average human diet, plants are a food source rich in a wide variety of nutrients, vitamins and minerals. Plant-based foods, commonly included in the human diet, are vegetables, fruits, cereals, legumes, seeds and nuts. To fully support their growth and health, human beings consume numerous plant parts (Figure 1
), including the stems (asparagus), leaves (spinach), inflorescence (broccoli), petiole (celery branches), axillary bud (Brussel sprouts), tubers (potato), roots (carrot), fruit (ripened ovary: tomato, apple), and seeds (almond, wheat and chickpea) [1
]. Plants provide humans with essential macronutrients (carbohydrates, proteins and lipids), micronutrients (vitamins and minerals), dietary fibers, and other phytochemicals (e.g., phenolic compounds, and phytosterols) [2
]. The nutrient composition and physicochemical properties vary between plant organs, partially due to their differing roles in both the developing and grown plant. For example, roots (e.g., carrot) store energy, often in the form of carbohydrates (starch), while ensuring water and mineral uptake from the soil [3
]. Seeds (e.g., almond and wheat) are also packed with macronutrients (starch, lipids and proteins) to sustain shoot development until it becomes a self-sufficient organism. The plant organs are composed of different tissues, such as parenchyma and xylem, and these tissues are made of more than one type of cells. Each cell within the plant is delimited by a cell wall that cements it to its neighbors. Within the living plant, these cell walls have specific chemical, physical and mechanical properties to fulfil a determined function (provide strength and structure to the cell, withstand turgor pressure, and permit interactions with the environment). For instance, parenchyma cells are spherical with thin cell walls while xylem cells are long with thick lignified walls. Once harvested and ready to be consumed, this diversity persists, which has a major bearing on the bioaccessibility and digestibility of nutrients and other phytochemicals contained within the cells. Bioaccessibility is defined as the proportion of a nutrient or food compounds that is ‘released’ from the complex food matrix and potentially available for digestion in the gastrointestinal tract (GIT) [4
]. Compound other than cell wall polysaccharides, such as antinutrients, also influence nutrient bioaccessibility and digestion, however these will not be covered in this review.
The plant-based foods we consume are generally classified as cereals, vegetables, fruits, seeds and nuts, and legumes, but this classification does not correspond to any specific botanical grouping. These foods exhibit a wide variety of plant structures and nutritional composition. As mentioned above, they play an important role in human health as they are a source of essential compounds: macronutrients that provide energy, vitamins and minerals, that act notably as cofactors for many enzymes and antioxidants, dietary fibers, and phenolic compounds [5
]. However, plants have developed strategies (synthesis of secondary metabolites and antinutrients) over the course of evolution to protect themselves from predators, such as birds, mammals, or harsh environmental conditions. With the emergence of agriculture, plants species have been cultivated and bred by humans. Certain desirable plant characteristics have been selected so that they become more edible and, consequently, through this process, some of the protective attributes of the plants may have disappeared [8
]. Nevertheless, in order to be digested in the human GIT, most plant-based foods need to be processed (e.g., milled, cooked, and/or fermented) [9
]. Indeed, the structure, in particular the cell walls (i.e., dietary fibers), of plant-based food matrices can be difficult to break down due to their resistance to the native enzymes of the human GIT.
In many plant-based foods, digestibility (and palatability) depends on reaching a particular stage of maturity [10
]. For example, as fruits ripen their cell walls soften, which eases mastication and the release of nutrients from the food matrix; at the same time, the polysaccharides within the fruit are hydrolyzed, leading to the release of monosaccharides, conferring sweetness [9
]. On the other hand, vegetable shoots and leaves are often harvested at a stage before secondary wall formation since this toughening of the cell walls, caused by acetylation and/or lignification, will decrease the palatability of the food, and hinder the bioaccessibility and digestibility of the cell content [10
In the present work, we discuss how the plant cell wall—an essential plant component—modulates nutrient bioaccessibility and digestion, and describe the main mechanisms by which it plays this role. We also report some of the current methods and technology used to characterize (in term of structure and physicochemical properties) the plant food matrix and the cell walls.