Álvaro García
Cell walls are primarily composed of complex carbohydrates and other polymers that give them their strength and rigidity. The main components are cellulose, hemicellulose, and lignin:
- Cellulose is a polysaccharide made of glucose units linked together in long chains, forming strong microfibrils that provide structural integrity.
- Hemicellulose is a branched polysaccharide that binds with cellulose fibers, helping strengthen the wall but is more easily broken down.
- Lignin is a complex polymer that provides additional rigidity and resistance to degradation, especially in mature plant cells. It acts like a glue, binding cellulose and hemicellulose together and making the cell wall more rigid and less permeable.
The cell contents on the other hand consist mainly of water, dissolved nutrients, and various organelles that perform essential functions. Some of the key components include:
- Cytoplasm, a gel-like substance in which organelles are suspended.
- Chloroplasts, responsible for photosynthesis.
- Vacuoles, which store nutrients and waste products.
- Starches, sugars, and proteins that the plant has synthesized or stored for energy.
What do lactobacilli want?
Some strains naturally occur on plant surfaces, consuming available sugars, but their growth is limited by the small amounts found there if plant cell walls remain intact. For better proliferation, lactobacilli need access to the cell contents, which contain starch and sugars. Disruption of plant material—through chewing, chopping, or grinding—partially breaks down the cell walls, giving lactobacilli more access. However, chemical barriers like the cuticle, fibers, and microfibers still restrict access. Effective fermentation depends on lactobacilli overcoming these barriers to access and ferment the sugars inside, as faster fermentation increases lactic acid production, which inhibits undesirable bacteria from proliferating.
Why add exogenous plant-degrading enzymes?
Most enzymes in live plants support growth and development, such as those involved in carbon fixation, sucrose synthesis, starch production, and nitrogen assimilation. Active degrading enzymes, if abundant, would compromise structural integrity, making plants vulnerable to collapse and pathogens. Plants rely on cellulose and hemicellulose in their cell walls for strength and protection, and enzymes that break these down are typically found in microorganisms and fungi, and not in plants. However, plants produce small amounts of these enzymes during aging, fruit ripening, or seed germination, were controlled cell wall breakdown aids processes like softening fruit or seedling growth.
Key plant-degrading enzymes
As mentioned, they are not normally found in plants but in microorganisms and fungi. Enzymes like cellulase, xylanase, and beta-glucanase are extracted and utilized for various industrial and agricultural purposes. These enzymes play a crucial role in processes such as silage fermentation, where they degrade plant cell walls composed of cellulose, hemicellulose, and other complex carbohydrates. Commercial products express enzymes like xylanase, cellulase, and beta-glucanase in enzyme units that typically correlate to their activity. An enzyme unit is a standardized measurement used to quantify how much substrate the enzyme can convert per minute under specific conditions.
- Cellulase breaks down cellulose, a key structural component of plant cell walls, into simpler sugars like glucose. An effective product should have at least 170,000 units of cellulase, capable of breaking down 170,000 units of cellulose into glucose per minute.
- Xylanase targets Xylan in plant cell walls, breaking it down into xylose and other sugars, which loosens the cell wall and makes more sugars available for fermentation. An effective product should have at least 630,000 units of xylanase, capable of breaking down 630,000 units of Xylan per minute.
- Beta-glucanase breaks down beta-glucans in plant cell walls, especially in corn silage and cereal grains, increasing wall permeability and releasing more sugars. An effective product should have at least 730,000 units of beta-glucanase.
Cell walls are composed of complex carbohydrates like cellulose, hemicellulose, and lignin, which provide structural integrity but also limit microbial access to internal nutrients. Lactobacilli, which are crucial for fermentation, need access to cell contents, such as starches and sugars, to produce lactic acid efficiently. While plant cell walls are naturally resistant to breakdown, adding exogenous plant-degrading enzymes—like cellulase, xylanase, and beta-glucanase—can significantly enhance the process. These enzymes break down cellulose, xylan, and beta-glucans, loosening cell walls and making more sugars available for fermentation. With today’s technological advancements, it’s not enough to rely solely on microbial inoculants; integrating these enzymes improves fiber breakdown, exposes more cell contents, speeds up pH reduction, and ultimately enhances silage preservation and feed quality.
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