Using Dry Matter as a Measure of Maturity in Fresh Produce
March 18, 2019 at 11:57 pm | Updated March 18, 2019 at 11:57 pm | 5 min read
Fruit is transported over long distances nationally and internationally to cater to a broad market. This makes it necessary to harvest fruits as soon as they are mature to increase the time available for their transport and storage, thus improving profits. Determining fruit maturity is, however, not simple. There are several qualitative ways to fix the best harvest time for fruits and vegetables; however, in many cases, especially climacteric fruits, it is best to use precise instruments. Understanding what the instruments measure, which in many cases is dry matter, can help in their use.
What is Dry Matter?
The solid component of any fruit, leaf, stem, or roots without the water component is called dry matter (DM). Dry matter from fruit is the result of the accumulation of carbohydrates, starches, sugars, oils, proteins, antioxidants, vitamins, minerals, fiber, lipids, volatile compounds, etc., along with a build-up of structural carbohydrates like fiber and skin.
Phases in Fruit Development
There is a 100 fold increase in mass (and dry matter) of the fruit from the time a flower is fertilized to the time the fruit reaches maturity.
Cell division: This is the first phase of fruit development that follows fertilization. It occurs in flowers. During this time, carbohydrates are essential to building new cells.
Cell expansion: Once cell division stops, the cells expand, and with them, the entire fruit. The expansion occurs as the fruit acts as a sink for the various nutrients that are transported to it from the leaves where they are produced. The carbohydrates reaching the fruit now get stored as starch and sugar. As the fruit develops over time, the type and concentrations of solids that occur in the fruit will change.
Maturity: As the fruit reaches maturity, the stored starch is converted to sugar. The types of sugars can differ based on the kind of fruit. For example, in apples, the sugar is stored as sorbitol, and in grapes, as sucrose. The form in which the sugars are stored is important for the horticulture industry, and based on this difference, fruits can be classified as climacteric and non-climacteric.
- Non-climacteric fruits: The sugars are stored in their soluble forms, and the fruit need to remain on the plant to ripen, as seen in the case of peaches, melons, grapes, citrus fruits, etc. The fruit can be harvested shortly before ripening is complete.
- Climacteric fruits: In fruits where the sugars are stored as starch or oil content, the hydrolysis or conversion to sugar can happen after they are plucked and in storage, as in avocados, mangos, apples, pears, or bananas. These fruits can be harvested when they are mature but unripe and green, and they ripen post-harvest. In these fruits, two kinds of maturity are recognised.
- Physiological maturity refers to the phase when fruit can be plucked from the plant and will continue to develop post-harvest. This is the stage at which climacteric fruits are harvested. Estimation of maturity is important in the field and requires precise instruments.
- Horticultural maturity refers to the phase when the fruit has developed all the characteristics that make it ready for consumption. By this time, the fruit is already on the shelves in the market.
Fruit reaches its maximum size when it is mature. Further development of the fruit involves no additional accumulation of solids and thus dry matter.
Ripening: This phase is triggered by the production of ethylene, and it overlaps the later stages of physiological maturity. During this phase, the change in the composition of compounds continues. Moreover, structural changes also take place in the fruit. The fruit becomes soft and develops its color, aroma, and sweetness.
Dry Matter as a Measure of Maturity and Quality
Dry matter changes during the different stages of fruit development, and this correlation makes it a good indicator of maturity in both climacteric and non-climacteric fruits. Moreover, the dry matter accumulated by plants depends not only on its genotype, but also the field conditions in which the plants grow and the cultivation methods used. This increases dry matter’s reliability as a measure of maturity and fruit quality.
- Non-climacteric fruits: Besides dry matter, color, size, and firmness are additional criteria that can be used to judge the correct time to harvest non-climacteric fruits.
- Climacteric fruits: In climacteric fruits, dry matter is fast replacing traditional measures of fruit maturity such as firmness, as there are few external indicators. While dry matter is a good indicator, for climacteric fruits that ripen post-harvest, there can be some exceptions. In apples, firmness and starch index changed with late maturity stages more than dry matter did, making them better measuring tools than dry matter.
In avocado, dry matter is not the indicator of maturity. It is the oil content of the fruit that correlates with the maturing process and is found to increase as the fruit matures. However, measuring the oil content can be difficult or expensive.
On the other hand, it has been found that water content in avocados decreases as oil content increases. Thus, dry matter in avocados correlates with oil content, and so, by measuring dry matter, it is possible to judge if the fruit is mature enough to be plucked. A minimum oil content of 11.2% is optimal to guarantee maturity in avocados; this concentration of oil content can correspond to 22% to 26% of dry matter in different varieties of avocados.
The F-751 is a portable NIR analyzer designed to specifically evaluate the maturity of avocados based on their dry matter content.
How to Measure Dry Matter
In the laboratory, dry matter is measured by drying fresh fruit in an oven or dehydrator. Dry matter is the ratio of dry weight to the fresh weight (or weight of the fruit before drying).
In the field, handheld sensory instruments rely on near infrared (NIR) spectroscopy. This is a non-destructive way to estimate dry matter levels and does not affect the fruit tested. The F-750 Produce Quality Meter can be used on several fruit species. The readings are precise, and the instrument is also useful for scientific studies on fruit quality and maturity.
Improving Food Production
Harvesting fruit at different stages of maturity has an impact on the ultimate taste. Growers need to know the correct time to harvest fruit so that the quality of the fruit is not affected. Dry matter estimation tools are one of the modern applications designed and manufactured to make the production of fruits and vegetables easier and quality control more consistent.
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture
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