How the Fruit Ripening Process Affects Freshness and Quality

• Respiratory rate, ethylene sensitivity, and production are the main criteria for differentiating ripening patterns.
• Respiratory peaks that trigger ethylene production start the ripening process in climacteric fruits.
• Ethylene sensitivity, production, and respiration hike are minimal or absent in non-climacteric fruits.
• Several fruits show varying degrees of ethylene sensitivity and production and defy neat classification in the two ripening types.

Ripening is a critical stage for marketing fruits and vegetables as it determines the quality that reaches consumers. Knowing more about ripening patterns can help plan optimal storage conditions and transport time to get the most out of any fruit. While two main ripening types are recognized, the distinction is not rigid, so find out more about the process in this article.

Fruit Ripening

Fruit ripening occurs in later phases of fruit development, preparing the fruit for consumption. The fruit flesh softens during ripening, the sugar content rises, and acids decrease. Volatile compounds that give fruits aroma are released, and flavor develops. The green color of immature fruits fades and changes into the colors characteristic of each fruit and cultivar.

However, not all fruits ripen in the same way. Fruits show varying respiratory and ethylene patterns during ripening, which form the basis of dividing them into climacteric and non-climacteric groups, see Figure 1.

Figure 1.: Generalized patterns of growth, respiration and ethylene during development, maturation and senescence of climacteric and non-climacteric fruits. Paul et al. 2012 (Based on Wills et al. 2007). (Credits: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3550874/)

Climacteric Fruit Ripening

Three main features characterize climacteric fruit ripening:

• Increased Respiration Rate: Climacteric fruits significantly increase respiration rate during ripening. This increase is often called the “climacteric rise” and is typically accompanied by a burst of ethylene production.
• Autocatalytic Ethylene Production: Ethylene production in climacteric fruits follows an autocatalytic pattern, where the initial production of ethylene stimulates further ethylene synthesis. This autocatalytic process accelerates ripening, leading to characteristic texture, flavor, and aroma changes.
• Ethylene is central for ripening: Ethylene plays a central role in the ripening process of climacteric fruits. It acts as a natural plant growth regulator, influencing various aspects of growth, development, and storage life. Ethylene initiates a cascade of signaling and metabolic pathways that facilitate ripening.
• Postharvest ripening: Climacteric fruits harvested when fully developed or mature will ripen off-tree. Suppliers use this property to extend shelf-life by using environmental conditions to control respiration and ethylene production and postpone ripening until a chosen time in the supply chain.

Recent research shows that respiration is not always a trigger for ethylene production. In some cases, as in bananas, the respiration rate increases after ethylene production begins and is influenced by environmental conditions. The speed and process of ethylene-induced ripening will vary widely depending on fruit species.

As the ripening process progresses, the fruit becomes more sensitive to ethylene. Even low ethylene concentrations can trigger biological responses in the fruit, initiating ripening-related changes.

Table 1: “Endogenous concentration of ethylene in selected fruits during ripening,” Paul et al. 2012 (Based on Burg and Burg 1962). (Credits: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3550874/)

Non-Climacteric Ripening

Non-climacteric fruit ripening differs significantly from climacteric fruit ripening in several key aspects:

1. Limited Response to Ethylene: Non-climacteric fruits produce little or no endogenous ethylene during ripening and do not respond to exogenous ethylene treatment, except for degreening citrus fruits and pineapples.
2. Respiration and Ethylene Patterns: Unlike climacteric fruits, non-climacteric fruits typically exhibit either no significant increase in respiration rate during ripening or only a transient increase, even after ethylene application.
3. Limited Cyanide-Insensitive Respiration: While mitochondrial cyanide-insensitive respiration contributes significantly to the rise in respiration rate observed in climacteric fruits, its presence is limited in non-climacteric fruits. This further distinguishes the respiratory patterns of non-climacteric fruits from those of climacteric fruits.
4. Dependency on Parent Plant: Non-climacteric fruits require attachment to the parent plant to ripen fully. Once detached, ripening either occurs very slowly or not at all, even if the fruit has completed its growth in size.

Non-climacteric fruits must be picked at the optimum stage of ripeness, as quality will not improve after harvest. Moreover, since they are ripe at harvest, fruits transition to decay within a short time, so their shelf-life is less.

The Spectrum In-between

Some fruits exhibit characteristics that blur the traditional boundaries between climacteric and non-climacteric ripening behaviors. These fruits can display climacteric and non-climacteric traits depending on factors such as cultivar or maturity. Here are some examples:

Based on Maturity: Some fruits respond to exogenous ethylene application only when fully mature and show climacteric ripening by softening and changing skin color, as in guavas. Immature green guavas will not ripen. Also, the maximum rise in respiration and ethylene production occurs when fruits are entirely ripe, unlike climacteric fruits, when they happen at the beginning and start the ripening process.

In tomatoes that are considered climacteric, only fruits that are 40% mature are sensitive to ethylene. Fruits harvested at 20% maturity do not respond to ethylene and remain unripe.

Based on Cutivars: Another trend is the occurrence of climacteric ripening only in some cultivars. For example, different melon varieties exhibit ethylene levels variations during fruit development, ripening, and shelf life. Some varieties show poor shelf life, while others display good shelf life. Certain melon varieties, such as ‘cantaloupe’ and ‘charentais,’ exhibit climacteric behavior, indicating a role for ethylene in their ripening process.

Based on Harvest: Avocados are climacteric. However, the fruits produce only trace amounts of ethylene while attached to the tree, possibly due to an unknown inhibitor. After harvest, the repression is removed, and the fruits produce ethylene and ripen.

Sensitivity to Ethylene: Several non-climacteric fruits like strawberries, grapes, and citrus will produce some ethylene and carbon dioxide due to higher respiration rate but will not show climacteric ripening. Some non-climacteric fruits will respond to exogenous ethylene application.

These examples highlight the complexity of fruit ripening processes and the need to consider individual fruit varieties or cultivars while planning storage and transport conditions.

Applications of Ripening Type

Understanding the ripening pattern of the fruits is very important for planning management strategies during fruit development, determining their optimum harvest date, as well as designing postharvest storage practices:

1. Optimum Harvest Time: Knowing whether a fruit is climacteric or non-climacteric helps determine the optimal harvest time. Climacteric fruits like apples and bananas ripen after harvest and can be picked slightly underripe. Non-climacteric fruits, like strawberries and cherries, do not ripen further once harvested, so they must be picked when fully ripe.
2. Postharvest Handling: For climacteric fruits, strategies can be implemented to control ripening, such as using ethylene inhibitors or controlled atmosphere storage. Non-climacteric fruits require different handling techniques to maintain their shelf-life and quality.
3. Shelf Life: The rate of respiration and ethylene production, which are associated with ripening, impact the shelf life of fruits. Understanding the ripening type helps predict how quickly fruits deteriorate and allows for better distribution management.
4. Loss Reduction: Knowledge of ripening type enables growers to implement effective strategies to reduce postharvest losses due to decay and spoilage. By optimizing harvest timing and postharvest handling practices, losses can be minimized, maximizing profitability.

Measuring Ethylene

Since ethylene is crucial for all climacteric and some non-climacteric fruits, measuring the gaseous phytohormone is essential to track the ripening process. Felix Instruments Applied Food Science offers small, portable, precision electrochemical sensor-based instruments for ethylene measurement in real-time in the fresh produce supply chain:

• The F-900 Portable Ethylene Analyzer and The F-940 Store It! Gas Analyzer for estimation in controlled atmosphere (CA) facilities and headspace in MAP.
• The F-920 Check It! Gas Analyzer for MAP, storage, and ambient measurements.
• F-950 Handheld Ethylene Analyzer and F-960 Ripen It! Gas Analyzer for CA, ripening rooms, and MAP.

Information on ripening types can guide the maintenance of ideal ethylene levels at different steps in the supply chain to optimize marketability and profits.

Sources

Capino, A., &  Farcuh, M. (2024, March 11). Ethylene and the Regulation of Fruit Ripening. Retrieved from https://extension.umd.edu/resource/ethylene-and-regulation-fruit-ripening/

Paul, V., Pandey, R., & Srivastava, G. C. (2012). The fading distinctions between classical patterns of ripening in climacteric and non-climacteric fruit and the ubiquity of ethylene-An overview. Journal of food science and technology, 49(1), 1–21. https://doi.org/10.1007/s13197-011-0293-4

Tripathi, K., Pandey, S., Malik, M., & Kaul, T. (2016). Fruit ripening of climacteric and non-climacteric fruit. Journal of Environmental and Applied Bioresearch, 4(1), 27 – 34.