What are Harvest Maturity Indices? Why are they Important?

• Harvest maturity indices can be fresh produce’s physical, chemical, physiological, or chronological attributes.
• The choice of harvest maturity indices will differ and must be based on species and economics.
• The optimum methods to measure harvest indices are objective, quantitative, non-destructive, and easy to use.

Fresh produce suffers the maximum loss and waste. One reason is the unreliable harvest maturity indices used in the fresh produce supply chain. So, what are harvest maturity indices, and why are they important? This article will try to answer these questions and outline the various indices available for growers.

Figure 1: “Development stages of climacteric fruits.” Prasad et al. 2018. (Image credits:  http://www.fao.org/docrep/008/y4893e/ y4893e04.htm).

Need for Maturity Harvest Index

While maturity and ripeness may be synonymous in lay terms, they are not. For stakeholders in fresh produce supply chains, the two are distinct stages of fruit development. As fruits develop, they are first immature and still growing in size, mature, and then ripe; see Figure 1, which traces the development of a climacteric fruit.

Fresh produce are considered to be mature enough for harvest when they meet the following conditions:

• The fruits and vegetables have completed growth and development.
• Harvesting at this stage produces fresh produce that has or can undergo required changes, even postharvest, to develop quality that meets minimum consumer demands of size, taste, flavor, and appearance.
• The harvested commodities will maintain quality during the required storage and transport time, and the fresh produce will not be over-ripe.

Depending on a commodity’s stage of development, fresh produce at harvest can have physiological or horticultural maturity.

Horticultural or commercial maturity refers to any stage of development when the fresh produce is ready for an intended use. The maximum development of fruits may not be complete, and the harvested commodity may not ripen. Horticultural maturity includes physiologically immature, physiologically mature, and ripe stages. This stage differs for various foods and covers multiple parts of a plant, as shown in Figure 2:

• Leaves and stem as in lettuce, cabbage
• Inflorescences or flowers, as in cauliflower and broccoli
• Roots and tubers like carrots and garlic
• Immature fruits like cucumber and okra
• Developed fruits like apples, tomatoes, and berries.
• Seeds like beans and nuts

Physiological maturity: Fresh produce reaches physiological maturity when the fruit or vegetable is fully developed and mature enough to allow postharvest fruit development or maintenance of appearance, flavor, and adequate shelf life. Examples include climacteric fruits like apples, mangos, avocados, and bananas.

A harvest maturity index provides a means for growers to determine whether their fresh produce has attained the desired quality for harvest.

Harvest Maturity Index

The harvest maturity index is a measurement (or measurements) that indicates a food commodity is mature enough for harvest. These are used mainly in the fresh produce supply chains for fruits and vegetables. Optimum harvest maturity ensures fresh produce has an adequate storage lifespan and will meet consumer acceptance and taste at retailers.

Determining harvest maturity is not straightforward as it differs between fruits and vegetables and on the intended use:

• Harvest maturity is physiological maturity for climacteric fruits, when fresh produce is mature but not ripe, as in apples and bananas.
• In the case of non-climacteric fruits, harvest occurs close to ripe stages or horticultural maturity, as in strawberries.
• Vegetables have a harvest maturity index based on their horticultural maturity.

When harvest occurs, the various stages of maturity require different measurement points, which are reflected in the vast array of harvest indices available.

Figure 2: “Horticultural maturity in relation to development stages of the plant” Reid. Image credits: Maturation and Maturity Indices

Types of Harvest Maturity Index

Harvest maturity indices can be physical, chemical, physiological, and chronological parameters.

Physical Features

Physical parameters are the most commonly used harvest maturity indices. They typically include size, weight, internal and external color, firmness, juice content, ease of abscission, surface morphology, and specific gravity.

Color: The development of peel color remains the first means of non-destructive maturity check for growers and consumers alike. However, it is unreliable for climacteric fruits harvested when fully mature but still green. Degreening indices calculated by tools are available for fruits like citrus. Pulp color is examined for fruits where the peel color is destructive and doesn’t change during ripening. Color also varies according to cultivar and requires expertise or non-destructive precision tools for accuracy.

Firmness: Firmness measures the softening of texture. It can be measured by manual or penetrometer measurements. Determining firmness requires destructive sampling.

Weight and specific gravity are objective parameters and standard harvest maturity indices. Both require destructive sampling.

Surface morphology: Fruit attributes like wax or bloom development can be used as non-destructive indicators of optimum maturity—for example, wax in melons and white bloom in mangos.

Abscission is one of the oldest maturity indices, and the extent of abscission tissue or “slip” development is used.

Juice content: Measuring this parameter is necessary for fruits grown for processing and also requires destructive sampling.

Chemical changes

As the fruit develops and matures, it is a sink for various bio compounds like sugars, organic acids, starch, pigments, minerals, vitamins, phenols, oils, water, etc. Some of these compounds ‘ increasingly objective and quantitative measurements are used as harvest maturity indices. The chemical changes used as an index are soluble sugar content (SSC), dry matter content, and titrable acidity (TA).

SSC: Sugar content is used as a harvest maturity index for not only fruits but also vegetables. It can also be used for climacteric and non-climacteric fruits.

Titrable acidity: This harvest index measures the organic acids that make fresh produce sour. The organic acids will differ based on the species and cultivars.

Taste: Taste is measured by the ratio of SSC to TA, as even ripe fruits need some acidity, and only high sugar content is not optimal.

Dry matter content: Dry matter refers to all the solid content in fresh produce besides water. It is increasingly the most used harvest maturity index, especially for climacteric fruits, as it correlates highly with consumer acceptance. If optimum chemometric models are used, dry matter can be independent of genotype, season, and cultivation practices.

Starch content: Iodine tests are used to measure starch content.

The methods to measure the chemical harvest maturity index can be destructive or non-destructive. Destructive methods are refractometers for SSC, laboratory estimations for dry matter, and titration for acidity. New technology using near-infrared spectroscopy offers a non-destructive alternative that is also rapid, accurate, and easy to use.

Physiological changes

The physiological changes occurring during maturity and ripening can also be used as harvest indices. Ethylene production and respiration rates are the two parameters usually measured. These indices are unreliable due to individual differences in these rates within a species or cultivar. However, ethylene rates are used for apples and require laboratory testing.

Chronological features

For certain crops like vegetables or perennial trees with a short summer, days from planting or flowering can be used as the harvest maturity index. The index is imperfect but allows for planning and is still in use. Heat units accumulated over the season, factoring in weather, is another chronological index.

Table 2: Maturity indices for selected fruits and vegetables, Reid. (Image credits: Maturation and Maturity Indices)

Variations of a single harvest index will reflect chemical changes based on species, cultivar, year, region, and cultivation practices, for example, as in kiwifruits.

Choosing a Maturity Harvest Index

Since harvest maturity can occur at many plant and fruit development stages, choosing the correct index for a specific commodity is necessary. To do this, stakeholders will have to understand the unique characteristics of each fresh produce to decide which attribute best measures its required maturity stage.

Size and color are helpful for all fresh produce. Dry matter and SSC are suitable for climacteric fruits. However, only SSC is used along with size, color, flavor, and firmness for non-climacteric fruits harvested when ripe. See Table 2 for examples of the standard harvest maturity indices used for fresh produce.

Some standard features of an excellent, reliable maturity index are the following:

• Easy to use: Growers need an index that is easy and fast to use in the field and doesn’t require extensive training or expertise. The results must provide actionable insights.
• Objective: Harvest maturity indices that are objective and quantitative are more reliable than subjective indices. Several harvest index and measurement methods, such as charts for external color, can be cheaper but are subjective. However, external and internal color assessed by near-infrared spectroscopy can provide more accurate and objective results.
• Non-destructive: The method for analyzing the harvest index should be non-destructive, as maturity is monitored over time, and several fruits or vegetables can be sampled. Destructive methods will require harvesting the fruit before it is ready and has to be disposed of. Non-destructive methods allow the sampled fresh produce to grow and reach the required harvest maturity, increasing yield.

Importance of Harvest Maturity Indices

Harvest maturity indices ensure that fruits and vegetables are harvested at the optimum stage and not too early or late. Growers, as well as other stakeholders in the postharvest supply chain, benefit from the use of reliable harvest maturity indices. Some expected benefits are as follows:

Determines harvest time: The first advantage of using the harvest maturity index is fixing the correct time for harvesting fruits and vegetables to ensure required postharvest quality and extended shelf life. It also allows growers to maximize yield from their crops.

Improves quality: The optimum harvest maturity index ensures fresh produce meets consumer acceptance. Early harvest results in fruits and vegetables lacking the required quality, such as color, ripeness, size, nutritional value, etc. Meanwhile, late harvest leads to quick senescence and shorter storage time.

Less biotic spoilage: Fruits picked early or late are more prone to postharvest diseases and physiological disorders than those picked at proper harvest maturity.

Helps with compliance: Strict regulations ensure quality in national and international trade in fresh produce. For import, fresh produce must meet harvest maturity indices, often regarding SSC and dry matter content.

Less loss: A good harvest maturity index provides growers and suppliers objective information for sorting and grading commodities. It ensures that batches have commodities at similar maturity to reduce uneven ripening and senescence due to ethylene generation. Moreover, when fruits and vegetables meet consumer acceptance, retailers face less rejection and less food loss.

More profits: Choosing and using a reliable harvest maturity index can increase production by reducing loss at each stage of the supply chain, improving profits.

Enhances sustainability: Less fresh produce loss, better quality, and the nutritional value of fresh produce have several sustainability benefits for the supply chain and any society. They increase food security, reduce hunger and malnutrition, and increase the income levels of fresh produce stakeholders. Moreover, less food loss reduces the environmental impact of agriculture and horticulture.

Non-destructive Instruments

Growers and other stakeholders have used near-infrared (NIR) spectroscopy-based devices in fresh produce for the last two decades to measure chemical parameters like dry matter content, SSC, and TA. Felix Instruments Applied Food Science offers a general device for fresh produce and other customized devices for avocados, mangos, kiwifruits, and melons. These fruit quality meters have helped growers in decision-making and improved fruit quality.

Sources

EAGRI.ORG. (n.d.). Lecture 2: Maturity indices, harvesting and post harvest handling of fruits and vegetables. Retrieved from http://www.eagri.org/eagri50/HORT381/pdf/lec02.pdf

Kumar, S, Singh, R.P., Rizwanullah, M, & Kumar, P. (2023). Different Maturity indices

of Fruits and Vegetables Crops. In Current Trends in Horticulture (pp.101-118)Publisher: Biotech books. Retrieved from https://www.researchgate.net/publication/ 374380146_Chapter_8_Different_Maturity_indices_of_Fruits_and_Vegetables_Crops

Maiti, R., Thakur, A. K., Gupta, A., & Mandal, D. (2018). Postharvest management of agricultural produce. Research trends in bioresource management and technology, 137-166

Prasad, K., Jacob, S., & Siddiqui, M. W. (2018). Fruit maturity, harvesting, and quality standards. In Preharvest modulation of postharvest fruit and vegetable quality (pp. 41-69). Academic Press.

Reid, M.S. (n.d.). Maturation and Maturity Indices. Retrieved from https://irrec.ifas.ufl.edu/postharvest/hos_5085c/reading%20assignments/kader-6-maturation%20and%20maturity%20indices.pdf

Sharvesh, S. (n.d.). Maturity indices of fruits and vegetables. Retrieved from https://www.slideshare.net/slideshow/maturity-indices-of-fruits-and-vegetables-252237810/252237810