Understanding Maturity Indices to Ensure Optimum Postharvest Quality of Horticultural Crops
Timing is Everything: Knowing the right time to harvest
The rate of quality changes increases rapidly as soon as the fresh produce is detached from the parent plant. The factors determining the appropriate maturity stage for harvesting the horticulture produce are critical in order to ensure the ideal shelf life. The quality of the produce at the retail end and its marketability to the consumer are also directly associated with harvest maturity. Harvesting immature fruit produces low quality and erratic ripening. The vegetables harvested too early may remain green for a more prolonged period, but they lack quality (e.g., texture, taste, sugar level, oil content, etc). Similarly, delayed harvesting of fruit and vegetables raises the risk of postharvest diseases and decay, leading to poor quality and low market value.
The majority of the crops in the world are harvested manually, and the harvesters determine whether the harvested plant parts (e.g., fruits) are sufficiently mature to be harvested or not. Therefore, they need to have a thorough knowledge of assessing and recognizing the appropriate maturity level, considering various factors that define the market readiness of certain produce.
What is Maturity?
The words maturity and ripeness are usually used as synonyms in our day-to-day conversations. Still, in the case of fruit and vegetables, they are distinct terms representing different stages of development. Maturity refers to the stage at which the edible part of the crop has completed its natural growth and development. On the other hand, ripeness is the stage when the crop becomes suitable for consumption and achieves its optimal taste and texture, as determined by its natural characteristics.
Maturity in horticulture produce is generally categorized into two types:
- Physiological maturity – the stage of completion of the plant's life cycle and its capacity for reproduction, and
- Horticultural maturity - the stage of development optimal for marketing or consumption
The physiological and horticultural maturity stages can be the same or different for a specific crop. For example, the horticultural maturity of okra is when it is partially developed, green, and tender, while the physiological maturity stage is when the okra pods are dried and split open to disperse the seeds. In the case of the French beans, both physiological and horticulture maturity is the same when the pod is dried and about to split. Contrarily, if all pods of the French bean are consumed, then the horticulture maturity would be in the immature tender pod stage.
Harvest maturity is when produce is ready to be harvested. Ideally, it should be the stage that ensures sufficient shelf-life of the produce and remains in prime condition when it reaches the consumer. The produce should also develop into market-acceptable size, with characteristic flavor or appearance, and be free from any toxins.
Maturity indices
The maturity index for a commodity is a measurement that can be used to assess harvest maturity and judge whether a particular commodity is ready to be harvested. They are different for each crop and can be subjective in deciding the right harvest stage to meet market requirements and ensure adequate shelf life along the distance to market. Usually, several methods have to be combined to assess the proper stage of maturity of produce.
1. Prominent methods of assessing fruit maturity:
- Physical factors:
Peel color, aroma, dimensions, floral component retention, mature outer leaves, desiccation of plant parts, hull splitting, and fruit plumpness.
For example, the peel color of tomatoes is used to judge the harvest time. Mature green stage fruit is harvested for long-distance markets, while red ripe stage or breaker stage fruit is harvested for local markets or canning.
- Mechanical assessment:
Ease of detachment, firmness test, density, and grading
For apples, a fruit firmness of more than 63 to 76 N (14 to 17 lbs) is recommended for the export market. The values depend upon the cultivar, but the firmness below these values receives a lower premium or is entirely rejected.
- Biochemical analysis:
Measuring levels of total soluble solids (TSS), titratable acidity (TA), the ratio of TSS to TA, and starch content
For example, pineapple fruit with TSS values of 13 to 17 Brix is preferred in European markets.
- Computation:
Counting the days from flowering to picking, heat units
For example, the optimum harvest maturity of Kiwifruit in New Zealand is usually about 23 weeks after the first appearance of flowers.
- Physiological evaluation:
Measuring the levels of carbon dioxide, ethylene, organic volatile compounds, etc.
2. Limitations of different maturity indices:
The common limitations associated with all maturity indicators include the variations caused by different nutrition levels, the impact of climate and seasonal changes, fruit position on the plant, the nature of the soil, levels of soil moisture, techniques of pruning, and the application of hormones or other chemicals. A few examples of common constraints of various maturity standards
- Physical factors:
Small-scale farmers mostly rely on visual assessments to judge harvest maturity, and their judgment can be inconsistent among different harvesters. In the case of large farms, it can be laborious and unreliable.
Various factors influence the color changes of horticultural produce and might not consistently indicate the established standards for a specific use. The fruit from plants in high-nitrogen environments tend to retain a green hue longer than usual.
Excessive moisture loss might also affect other visual indicators, such as the retention of floral parts, the presence of mature outer leaves, the desiccation of plant parts, or the plumpness of the fruit, further complicating the assessment of maturity.
- Mechanical assessment:
Assessing the maturity by relative ease of separation of the fruit from the tree could also be subjective as in certain cases of high nitrogen supply. Even improper irrigation schedules in the plants can make the fruit separate more easily, even before proper maturity.
Determining fruit firmness is a practical approach used in several fruit crops, and a range of pressure testers is available commercially for this purpose. The amount of force required to puncture the fruit pulp is the measure of fruit firmness, but in some cases of excess nitrogen fertilization, the fruit texture turns soft prematurely.
- Biochemical analysis:
Analysis of TSS, TA and TSS: TA ratio works well in citrus and mango fruit, while a starch test is commonly used in apples to determine maturity. However, factors such as sunlight, temperature, and fertilizer usage may induce variations in acidity and sugar levels in fruit
- Computation:
Growing degree days (GDD), also known as growing degree units (GDU), are units of heat accumulation used to estimate the rates of development of plants, such as when a flower will bloom or a crop will be fully mature. However, these calculations are true only under similar environmental conditions and only within a specific temperature range
- Physiological methods:
The maturity of the produce can be predicted using the respiration and ethylene production data at different picking times, but this requires high capital, sophisticated equipment, and technical expertise to operate and analyze it.
Conclusions:
The maturity of horticultural products at the time of harvest significantly influences their postharvest quality throughout the supply chain. To ensure high-quality produce for both consumers and processors, it is essential to utilize accurate and efficient maturity indices. There are various methods to assess maturity, and relying on a single maturity index is not advisable for determining the maturity of fruits. Instead, it is recommended to use a combination of multiple indices, as each one provides valuable insights into the maturity stage of the horticultural produce, helping to reduce errors caused by variability related to individual commodities, seasons, and growing locations. Maturity indices also vary based on factors such as market destination, shipping duration, and growing regions. Different producing countries may implement specific maturity standards. Additionally, the distance to the market and the associated transportation time are critical factors to consider when making harvest decisions.
References:
Erkan, M., & Dogan, A. (2019). Harvesting of horticultural commodities. In Postharvest technology of perishable horticultural commodities (pp. 129-159). Woodhead Publishing.
Reid, M.S. (2011) Maturation and maturity indices. In Postharvest technology of horticultural crops (3rd edition). University of California, Agriculture and Natural Resources.
Further reading
Pre-Cooling: Essential for Improving Postharvest Management of Horticultural Crops
Kiwi Yield, Harvest, and Storage
Irradiation Technologies in Vegetable and Fruit Storage
How to store tomatoes to minimize post-harvest losses
Okra: Harvest, Yield, Storage, and Post-harvest handling
Cherries Harvesting and Yield per Hectare – Do you pick cherries with the stem on or off?
Avocado Harvest, Yield per hectare and Storage
Cherries Harvesting and Yield per Hectare - Do you pick cherries with the stem on or off?