Apples ripen and fall to undergo earthly decomposition. Transformation from a crisp fruit to disappearance in the soil ecosystem is a process of biochemistry, microbial action, insect activity and cellular breakdown.
An apple grows from a pollinated blossom. During its youth the apple receives nutrients through the tree’s roots and leaves. By the time it's fully ripe, it contains about 10-15% sugar.
Before decomposition even begins, the apple undergoes transformation on the tree. The ripening process is driven by the apple's own internal chemistry and cellular processes.
It includes:
Ethylene (C2H4) Production: This gaseous plant hormone triggers a cascade of changes within the apple. An unripe fruit typically contains low levels of ethylene. As the fruit develops, ethylene is generated as a signal to trigger the ripening process.
Starch Conversion: Complex starches are broken down into simpler sugars like fructose, glucose, and sucrose. This is why ripe apples taste sweeter than unripe ones.
Acid Reduction: The tart acidity of unripe apples diminishes as acids like malic acid are metabolized.
Cell Wall Degradation: Enzymes degrade the cell walls, particularly pectin, the gelid glue holding cells together. The apple tissue becomes softer and juicier.
Pigment Changes: Chlorophyll, the green pigment, breaks down to reveal underlying pigments like carotenoids (yellow, orange) and anthocyanins (red, purple), giving the apple its characteristic adult color.
Carotenoids and produce the orange pigment of carrots. Anthocyanins create the deep blue of blueberries.
Aroma Development: Volatile organic compounds (VOCs) are synthesized, enhancing the apple's aroma, an alluring signal to attract seed dispersers. Deer, bears, mice, turkeys, raccoons and many songbirds enjoy apples.
If chewed, the seeds release amygdalin, a bitter cyanide compound. Larger consumers enjoy the sweet flesh and swallow seeds without biting, having learned from taste experience. The seeds pass undamaged through the animal's digestive tract to be deposited on fertile ground.
The Fall and Initial Breakdown
The apple consists of many cells held together by cell walls, packed with organic compounds. When it hits ground, cells are broken and damaged. This triggers the release of enzymes and plant chemicals.
The apple's physical integrity is compromised. Bruises, breaks in the skin, and exposure to elements create entry points for the elements of decomposition.
Oxidation: Exposed apple flesh immediately undergoes oxidation, leading to browning. This is due to enzymes like polyphenol oxidase reacting with phenolic compounds in the apple tissue.
Insect Activity: Even before microbes arrive, insects find the fallen apple. Fruit flies (Drosophila) are drawn to the sugary scent. They lay their eggs within the fruit.
Female fruit flies can lay around 400 eggs, typically in batches of five. Eggs hatch into larvae in only 12 hours. The larvae feast on the apple tissue, facilitating the breakdown by creating entry points for more organisms.
Other insects, like ants and wasps, are also attracted to the sugary fluids. Nectar is scarce in fall, and these insects are often buzzing around or climbing on rotting fruit due to its high sugar content.
Their feeding behaviors form new surfaces microbes can access, increasing overall decomposition speed. Their movement aerates the soil, improving interaction among various decomposer species.
This creates an ideal environment for microbes like bacteria and fungi. These decomposers convert stored apple nutrients into forms other organisms can absorb and use.
Bacteria
Acetic Acid Bacteria: These bacteria break down sugars, producing acetic acid (vinegar).
Pectinolytic Bacteria: These bacteria are crucial for further breaking down pectin, softening the apple tissue.
Cellulolytic Bacteria: These bacteria break down cellulose, the main structural component of plant cell walls.
Bacteria like Pseudomonas and Bacillus quickly multiply in the soft tissue, consuming sugars and breaking down the fruit’s cell structure. Some bacteria can double their population in just 20 minutes.
Fungi
Molds: These fungi are often the first obvious sign of decomposition, appearing as fuzzy, colored patches on the apple's surface. Common mold species include Penicillium, Aspergillus, and Botrytis.
Of the Aspergillus spp. the most common are black Aspergillus, especially A. niger. Other species include A. flavus and A. fumigatus.
They secrete enzymes to break down complex carbohydrates, proteins and lipids. Their mycelia infiltrate the apple, where they reduce organic matter and release enzymes to digest cellulose and lignin.
These processes cause visible decay within a few days. At this point the apple takes on a smell noticeable to humans.
Yeasts: These single-celled fungi can ferment sugars. Producing alcohol and carbon dioxide in anaerobic conditions, they also contribute to the apple's characteristic smell of decay.
They can ferment aerobically, producing water instead of booze. The microbes work synergistically. Each breaks down different components of apple tissue to release the rich stash of nutrients.
Chemical Transformation
The deconstruction of the apple tissue includes a series of chemical reactions, driven by enzymes secreted by the microbes. The enzymes are biological catalysts to speed up the decomposition process.
Hydrolysis: Many enzymes involved in decomposition are hydrolases. They break down complex molecules by adding water. For example, amylase reduces starch to glucose by adding water molecules.
Oxidation and Reduction: These reactions cause the transfer of electrons, important for decomposition of many organic compounds.
The decomposition of an apple catalyzes a response system. The sugars and acids in the apple trigger a series of biochemical reactions. As the fruit decays, fermentation can begin.
Fermentation: In anaerobic conditions, some microbes can ferment sugars, producing byproducts like alcohol, lactic acid, and sometimes acetic acid. Acetic acid bacteria like Acetobacter prefer oxygenated environments.
That's why artisan apple cider vinegar must be twice-fermented. Yeast works under anaerobic conditions for the first fermentation, then aerobic bacteria turn the ethanol produced by yeast into acetic acid.
In low-oxygen conditions gases like carbon dioxide and methane are released. About 30% of the carbon in decaying organic matter is emitted as carbon dioxide, ideally absorbed by plants and algae.
As decomposition progresses, volatile compounds are released, further contributing to the smell. This scent attracts more decomposers, accelerating breakdown process and facilitating nutrient release.
Return to the Earth: Humification and Nutrient Cycling
As decomposition progresses, the apple gradually loses its structure and becomes a dark, amorphous material. This process of humification breaks complex organic molecules into more stable substances such as humus.
Humus: Humus is a mix of organic compounds contributing to soil fertility and structure. It improves water retention, provides nutrients for plants, and supports a healthy soil ecosystem.
Nutrient Release: As the apple decomposes, nutrients like nitrogen, phosphorus, and potassium are released into the surrounding soil. These nutrients become available to plants including the mother tree.
Microbes transform complex organic molecules into simpler forms plants can easily absorb. This ecological activity defines healthy ecosystems, supporting local flora and sustaining the broader food web.
The microbial community continues its work, decomposing the remaining material into humus. This nutrient-rich substance blends into the soil, ready to nurture new organisms and continue the cycle of life.
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