The Science of Sweet: How Fruit Ripens

September 7, 2020 steven

Before the development of refined sugar, if you wanted something sweet, you had limited options. Either you needed access to honey, or more likely, you used fresh or dried fruit.

In fact, that brick of fruitcake in the back of your freezer is an excellent example of old-style English baking that used the concentrated sugars, fructose and glucose in dried fruit to sweeten cakes and pies back when fruit was one of the few sources of sugar available.

This is an example of the value of fruit in the human diet. It’s everywhere, from our morning juice to the flavorings we love in ice cream and yogurt. It’s sweet and full of nutrients, too. You may take it for granted, but wholesome, plump fruit doesn’t happen by accident.

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Ethylene Gas and Ripening

The process that ripens fruits and vegetables is actually pretty complex, and even today some aspects of plant development and fruiting are not completely understood.

We do know ethylene is one of the biggest contributors to ripening. Increasing concentrations of this hormone, a hydrocarbon gas, signal cells to enter senescence, the ripening phase of development (from the Latin for “growing old”).

Ethylene is so essential to ripening it has been nicknamed the senescence hormone. You’ve probably used it yourself to artificially ripen slightly green tomatoes or rock-hard avocados by placing them near fruits like apples and bananas that release high concentrations of ethylene.

A number of contributing factors can trigger plants to release increased levels of ethylene gas and encourage fruiting, including high temperatures, injuries to the plant, harvesting of the fruit (in some instances) and even the presence of some types of micro-organisms.

In turn, ethylene prompts the development of plant enzymes that change the chemical composition of the developing fruit from the inside out by:

Converting starches to sugars
Altering the pectin in cells to make fruit softer and juicier
Breaking down the acids that give unripe fruit a sour or bitter taste
Altering chlorophyll and stimulating pigment changes that modify a fruit’s color as it ripens

As important as ethylene gas is to ripening in most plants, it doesn’t function alone. It depends on other initiating or regulating chemical interactions to operate effectively.

For example, auxin is a hormone most often associated with root growth and the development of buds. Auxin also facilitates the production of ethylene gas. Chemical inter-dependencies like this are important. They showcase the delicate balance taking place inside plants, which is necessary to produce a successful harvest.

Why Nutrients Matter When it Comes to Sweeter Fruits

At the root of ripening and most other plant functions, literally, are the nutrients plants need to flourish. The adage “nothing comes from nothing” is doubly true for growing things.

Although plants rely on photosynthesis to convert sunlight into food, they require plenty of minerals and trace elements to develop, flower and create flavorful, nutritious produce in the quantities we desire as gardeners. Lack of adequate nutrition can result in stunted plants or fruits, delayed ripening, flavor loss, inferior texture or a reduction in the nutritional values that make fruits and vegetables an essential part of the human diet.

Actually, this last part is a major concern. Over generations, shortsighted farming practices have reduced concentrations of naturally occurring trace elements over large areas of arable land, and measures to replenish those nutrients may be inadequate to the task of fully revitalizing the soil.

For example, by some estimates, more than 50% of the soil set aside for the growth of cereal grains worldwide is deficient in zinc, a mineral important for many plant enzymes to function properly.

Zinc deficiency can also reduce the amount of acetic acid in plant roots, making them susceptible to rot. This is only one well-known example of the potential drawbacks of soil depletion.

Research into the impact of trace elements on plant growth is ongoing, and our knowledge is still incomplete. Some experts list around 20 important nutrients that directly or indirectly aid in plant development. They’re broken into categories as macronutrients, secondary nutrients and micronutrients (needed in very small quantities).

We now know that some vegetable, fruit and herb varieties benefit from exposure to dozens of nutrients. The tomato is often used as a model for fruit ripening, and extensive testing has helped produce a nutritional profile that includes more than 50 minerals and trace elements that contribute to creating the optimally ripe, juicy tomato. There’s still much more to learn about the way plants use nutrients.

No single nutrient blend is all things to all plants, either. Plants will often have a set of changing nutritional requirements to meet different developmental milestones. Making many nutrients available to every plant at every phase of its development isn’t the answer, either. High concentrations of some minerals can be damaging or reduce a plant’s ability to use other elements in the soil, creating nutritional deficits even when trace compounds are present.

In hydroponic and aeroponic systems, understanding how nutrients impact specific plant varieties is important because all the nutrients necessary for plant development are being supplied as part of a closed system.

This is why there are so many nutrient “recipes” out there designed for different plants and hydroponic gardening goals. From a nutritional standpoint, most macro- and micronutrients perform multiple functions in the life cycle of a plant.

Here are a few examples of these nutrients and their fruiting-related functions:

Nitrogen: Increases seed and fruit production
Phosphorus: Aids in the formation of carbohydrates and sugars and encourages blooming
Potassium: Improves sugar production, texture, yield, pungency, vitamin content and weight in many fruits
Boron: Aids in seed development, fruit growth and sugar production
Calcium: Enhances cell stability
Copper: Aids in the development of key enzymes and in the utilization of proteins
Magnesium: Triggers enzymes necessary for growth
Molybdenum: Helps in the utilization of nitrogen
Silica: Enhances nutrient uptake
Sulfur: Promotes seed production
Zinc: Regulates the production of sugars and is important in the production of the enzyme auxin

Providing the nutrients necessary to ripen fruits and vegetables is like assembling the ingredients for a recipe, and like a good recipe, these separate ingredients work together to produce a delicious meal or snack that is greater than the sum of its parts, which is what successful gardening is all about.

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