Why Color Remains an Important Fruit Maturity Index

• Fruit color can be used as a harvest index for nearly all fruits.
• Fruit skin color is the basis for consumer acceptance and marketability.
• Fruit skin color is a valuable indicator of maturity stages and is associated with other quality parameters, including nutritional value.
• Non-destructive methods to estimate fruit color in the supply chain and research range from simple color cards to sophisticated AI.

Several maturity indices exist to guide harvest time and judge ripeness for retailing purposes. They can be organoleptic, physical, and chemical. Skin color was the traditional maturity index, and despite the introduction of complex parameters and sophisticated devices, it still remains one of the crucial fruit qualities guiding the fresh produce industry. This article examines why color remains an important Fruit Maturity Index in determining the readiness of fruits for harvest.

Fruit Maturity Index: Key Indicators

Fruits undergo development, maturity, ripeness, and senescence.

The stage of maturity for harvest is vital in determining postharvest quality, storage, and marketability and can differ for each horticultural crop. Harvest usually occurs when the fruit is fully developed. Growers use maturity indices to indicate that the fruits are mature and ready for harvest.

Most fruits, except avocados, bananas, and pears, reach the best eating quality when allowed to ripen on the plants. However, this can cut transport and storage time. Therefore, harvest indices are a compromise between eating quality and marketing processes.

The maturity indices used are not restricted only to orchards and farms. They are used by handlers, packers, suppliers, and retailers.

There are few maturity indices in use as they must fulfill various criteria. A good maturity index must be able to do the following:

• It must be a quality parameter that correlates with the various stages of fruit development.
• The maturity indices must be simple, easy to perform onsite, and objective.
• Non-destructive indices are preferable to save produce.
• Moreover, the maturity indices must apply to all areas and years for a species or cultivar.

Fruit skin color ticks all these boxes, making it versatile as a maturity index.

Applicable to All Fruits

One of the reasons why external skin color remains so essential is that it applies to practically all fruits and most vegetables.

Table 1. Maturity Indices of selected fruits and vegetables, Reid, M.S. 2002. (Credits:

Maturation and Maturity Indices, Indian River Research and Education Center)

Several quality parameters are used to judge maturity, like size, shape, specific gravity, solidity, firmness, color, and internal composition. The indices suitable for a particular crop can vary. For example, dry matter is best for climacteric fruits, harvested when fully mature but allowed to ripen later when the stored starch converts to sugars. Dry matter is correlated with postharvest quality and consumer preference. However, non-climacteric fruits that accumulate sugars and not starch usually rely on soluble sugar content as a maturity index.

As Reid points out in Table 1, external color and size are used for all fruits and most vegetables.

Figure 1: Color change during ripening, Hathi et al. (2020). (Image credits: https://vigyanvarta.com/adminpanel/upload_doc/VV_1220_11.pdf)

Color Change Marks Different Fruit Development Stages

Color changes are salient developmental features associated with fruit maturity, ripening, decay, injury, and senescence.

The vital color change is the loss in chlorophyll content or greenness to reveal the other pigments in the fruit skin, like yellow carotenoids and red-purple anthocyanins, that give the characteristic ripe colors associated with fruits, see Figure 1.

This color transformation due to maturity and ripeness occurs in most fruits, except in some citrus varieties, squash, and mango cultivars. In these cases, internal color becomes essential.

Easy to Use

The color change as fruits mature makes them visually distinct from unripe counterparts, is easily observable, and serves as a clear visual cue for consumers, farmers, and distributors.

Uniformity in Harvesting: Standardizing harvest times based on color allows for more efficient and consistent harvesting practices. Farmers can use color as a guideline to determine when a crop is ready for harvest, helping optimize the timing of picking to ensure fruits reach the market at their peak quality.

Postharvest Handling: Color is essential in postharvest handling and sorting processes. It allows for separating ripe and unripe fruits, minimizing waste and ensuring that only mature fruits are sent to market. This is crucial for maintaining the harvested produce’s quality and shelf life.

Consumer Preference and Marketability

Fruit color, aroma, and firmness are among the first quality parameters consumers examine while choosing fruits and vegetables.

Consumers often associate specific colors with ripeness and optimal flavor. For example, many prefer bananas when they have turned yellow with a few brown spots, indicating optimal ripeness.

Color is a psychological factor of appreciation for people. Fruits that display vibrant and appealing colors are more likely to be chosen by consumers, contributing to higher sales and consumption. People will likely desire a red apple with flavor over non-red apples with flavor. Color is crucial for deciding among fruits of the same cultivar and even between varieties.

Therefore, growers should focus on improving fruit skin color development.

Marketability and Aesthetics: The visual appeal of fruits is crucial for marketing and sales. Consumers are more likely to purchase fruits that have bright colors, as this is often associated with freshness and quality. The aesthetic qualities of fruits, including color, contribute to the overall marketability of the produce and even influence a person’s opinion of the rest of the stores’ goods.

Fruit Color is Associated with Other Quality Parameters

Color is a critical indicator of fruit maturity for several reasons, and it plays a significant role in assessing the overall quality and ripeness of fruits.

Taste and Flavor development: Color changes during the postharvest stages for climacteric fruits. Several researchers have shown that color change is connected to soluble sugars, citric acid, titrable acidity, and flavor content. Tables 2 and 3 show changes in internal composition and associated color transformation (IC* or Color Index),

• The sugar to acidity ratio decides taste. The color shift can signify the accumulation of sugars and the reduction of starches, resulting in a sweeter and more flavorful fruit. This is particularly relevant for tomatoes, berries, and stone fruits. For example, color changes along with hexose sugars and sorbitol levels in sweet and tart cherries.
• The flavor is formed by volatile compound combinations in the fruit skin. These compounds change during ripening and are associated with color. For example, green apples have aldehydes and alcohol, while ripe ones have esters.

Antioxidants and Nutritional Value: A fruit’s color can sometimes indicate its nutrient content. For example, red in tomatoes and peppers is linked to antioxidants like lycopene. Anthocyanin, another antioxidant, is also responsible for the color development of most fruits from red to purple. This connection between color and nutrient content allows consumers to use color as a proxy for assessing the nutritional value of fruits.

Tables 2 and 3: ” Tables 2 and 3 show the average experimental values of the physical-chemical, mechanical and organoleptic properties of the Papay (Maradol Roja var.), the guava (Enana Roja EEA1-23 var.) and the tomato(Vyta var.)such as: pH, soluble solids content (SCC), firmness and IC *, calculated for the different ripeness stages,” Molina et al. 2019. (Credits: https://www.redalyc.org/journal/932/93261708002/html/).

Since color changes are associated with internal composition transformation during ripening, the Color Index is an excellent non-destructive method that horticulturists can use to establish different stages of fruit maturity.

Objective Method

Some of the standard non-destructive methods used for fruit color estimations are listed below:

Visual Inspection: Harvest maturity assessment based on visual observation and color charts is subjective and prone to misinterpretation but is still commonly used to determine maturity. Using standardized color charts, such as those provided by organizations like the International Commission on Illumination (CIE), can help quantify and compare color changes.

Reflectance Spectroscopy: Portable spectrometers measure the reflectance of light from the fruit’s surface across different wavelengths, allowing for precise color analysis. Near-infrared (NIR) Spectroscopy can assess internal quality and predict changes in fruit color.

Felix Instruments’s NIR-based devices, the F-750 Produce Quality Meter for a wide range of fruits and vegetables, and the F-751 series, customized for mangos, kiwifruits, avocados, and melon, can measure external color. It is used throughout the supply chain and for horticulture research.

Chlorophyll Meters: Measure the chlorophyll fluorescence emitted by plant tissues. Decreases in chlorophyll content that occur during ripening influence fruit color.

Digital Imaging: Capture images of fruits using cameras or scanners and analyze them using image processing software. This method can provide quantitative color information. Color histograms can analyze the distribution of color intensities in images to track changes over time.

Colorimeters are computer vision systems that measure the color of an object by comparing it to standard color and using the Color Index (CI*).

Multi-and Hyperspectral Imaging: Non-destructive methods for assessing internal and external fruit characteristics.

Machine Vision and Proximal Sensors: Machine vision and proximal sensors, such as cameras, are alternatives for large-scale fruit color determination. Machine learning algorithms or AI can automate fruit detection and measure skin color within defined perimeters.

Choosing a method depends on accuracy, cost, and application requirements.

Understanding Limits of the Fruit Maturity Index

However, color alone is not enough as a maturity index. It is best used in combination with fruit firmness or internal composition. Some fruits do not change color during maturation. Waiting for the development of color can lead to late harvesting and harm flavor development and storage duration, for example, in apples. Similarly, plums develop skin color after harvest, and firmness is a better indicator of maturity.

Therefore, while color is a valuable and straightforward harvest index, it is best used with other quality factors to ensure optimal flavor and consumer satisfaction.

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Fruit quality – how do fruit get their flavor? Penn State Extension. (n.d.). https://extension.psu.edu/fruit-quality-how-do-fruit-get-their-flavor

 

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