Fruit Respiration Impact on Fruit Quality

• Fruits continue to respire after harvest as they are still alive. Aerobic respiration that uses oxygen is the norm, but when oxygen levels drop, anaerobic respiration or fermentation starts.
• Aerobic respiration is necessary for the development of fruit quality and ripening.
• Excessive fruit respiration is detrimental in the postharvest stages, as it can cause fruit deterioration, reduction in sensory qualities, and rapid senescence.
• The optimal environmental conditions, temperature, oxygen, and carbon dioxide can differ for each species and cultivar. Controlled Atmosphere environments must be customized for specific commodities to ensure respiration rates are lowered but not stopped.

Fruit respiration is one of the main factors that control the shelf life of fruits and determines their quality. It is an aspect of fruit postharvest handling that has received attention for many decades and still is an area of focus, as the conditions in the supply chain are not ideal and result in vast amounts of annual fruit loss. Understanding the fundamental scientific principles behind this crucial fruit physiological process can help stakeholders formulate better-suited conditions for their commodities.

What is Fruit Respiration?

Harvested fruits are still alive and need to respire. Fruit respiration is the metabolic process that provides energy for keeping cells and fruits alive by maintaining internal processes. It involves the breakdown of various organic compounds to give simpler constituent molecules and energy. There are two main types of respiration- aerobic and anaerobic.

Aerobic Fruit Respiration

Aerobic fruit respiration is the default process, where a range of substrates are broken down using oxygen (O₂) to give carbon dioxide, water, and energy.

All the energy produced is not available as ATP for fruits to use. Most is lost as heat, called heat of respiration, and raises fruit temperature. In preharvest fruits, it is about 60% and can increase to 90% in postharvest fruits. Postharvest handling aims to reduce this respiration heat due to its detrimental effects on fruit physiology.

Carbohydrates, lipids, and organic acids are used as substrates in aerobic respiration. Glucose is the compound preferred for respiration, and other substrates are converted to glucose before use.

Anaerobic Respiration

Without oxygen, the fruit switches to anaerobic respiration or fermentation. Anaerobic respiration produces less energy than aerobic respiration. The same substrates are used, but the chemical reactions will vary and produce ethanol, which precedes carbon dioxide (CO₂) formation. The high concentrations of fermentation products cause physiological disorders like off-flavors, off-odors, discolored tissues, and necrosis.

The extinction point is the O₂ level at which anaerobic respiration starts and varies between species, cultivars, tissue types, fruit development stage, and maturity.

Respiration Rates

Fruit respiration rate is the rate of O₂ consumption and/or CO₂ production.

The respiration rate indicates how fast the sugars and stored resources in fruit are used up. Therefore, the respiration rate determines the fruit’s physiology and how long it can last. Respiration rates are directly correlated with fruit deterioration rate. Higher respiration rates lead to faster consumption of carbohydrates and quicker deterioration, loss of fruit quality, and shorter shelf life. Therefore, respiration rate is used as a shelf-life indicator.

Postharvest stage focus on lowering respiration rate to increase fruit shelf life. Fruit respiration is not the same and differs among fruits and is also based on storage temperature. The respiration rates for different fruits are as follows:

• Very low: dry fruits, nuts, and dates
• Low: apple, pear, kiwifruits, citrus, grape, pomegranate
• Moderate: banana, cherry, citrus, tomato, pear, tomato
• High: apricot, papaya, fig, ripe avocado, strawberry
• Very high: blackberry, raspberry, all berries

Fruit Quality Changes Due to Fruit Respiration

Fruit respiration is beneficial and detrimental to fruit quality, as is explained below.

Fruit Quality Development

Fruit respiration is crucial for fruit development, ripening, and good fruit quality. Respiration provides carbon skeleton intermediates for forming ripening enzymes, fats, sterols, flavor development, and pigment synthesis. Moreover, respiration maintains physiological processes leading to softening, aroma development, and astringent loss.

So fruit color, taste, aroma, and ripening depend on continued aerobic respiration. Therefore, postharvest storage conditions should be conducive to respiration, albeit at low levels.

If respiration is too high, loss of sugars and other compounds leads to loss of weight and sensory quality of fruits.

Effect on Ethylene Production

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

Another reason why respiration is necessary for fruits is its role in the ripening process.

Fruit respiration is part of the ripening process and is connected with ethylene production. An increase in respiration and CO₂ production is called climacteric rise and occurs simultaneously or just after ethylene production starts. The climacteric rise and increased ethylene production are seen during the ripening phase in climacteric but not non-climacteric fruits; see Figure 1.

However, the heat of respiration can increase fruit temperature, increasing ethylene production and hastening ripening. Therefore, postharvest management practices reduce respiration to slow ripening, maintain fruit quality, and extend shelf life until suitable retail conditions. Moreover, ripening would lead to senescence and fruit wastage.

Effect on Fruit Transpiration

Another result of the heat of respiration and fruit temperature rise is an increase in fruit transpiration. As a result, water vapor loss will increase, leading to weight loss of fruits, shrinking, shriveling, and loss of gloss.

Factors Affecting Fruit Respiration

It is possible to maintain optimum rates of fruit respiration by managing the factors that influence it. The factors are intrinsic and environmental.

Intrinsic Factors

The type of fruit is one of the main factors determining respiration rate. Fruit size, health, and age will also affect respiration rate.

• Fruit type: Commodity type and cultivars will influence respiration rate preharvest and postharvest.
• Maturity stage: Respiration is also connected with fruit maturity and increase as fruits mature. The fruit maturity at harvest will also affect the respiration rate postharvest, with more mature fruits showing a higher respiration rate.
• Size: Fruit size can influence climacteric respiration behavior. Small-fruited cultivars have a higher respiration rate than medium or large ones. For example, small-sized tomato cultivars’ respiration rate was 5-fold higher than others, and medium-fruited cultivars had a1.9 fold higher respiration rate than large fruits.
• Age /Storage time: With age, fruits start to ripen, associated with a higher respiration rate, as shown in Figure 1. After ripening, natural senescence will set in for fruits stored too long or in incorrect conditions. Senescence, especially in climacteric fruits, is associated with higher respiration rates.
• Microbial infection: Both climacteric and non-climacteric fruits will experience heightened respiration due to microbial infection during prolonged storage.
• Wounding: Any wound or bruising in fruits due to rough handling during harvest or transport causes fruit stress and will trigger ethylene formation that causes an increase in respiration rate. For example, the respiration rate of apple slices is 2-3 times higher than a whole apple in the same conditions.

Environmental Factors

Figure 2: Increasing respiration rate with rising temperatures in cherry cultivars, Crisosta et al. 93. (Image credits: HORTSCIENCE 28(2):132-135. 1993)

The critical environmental factors affecting fruit respiration are temperature, O₂, and CO₂ levels.

• Temperature: As temperature rises, the chemical reaction of respiration increases from 0°C–30°C, which is also called the physiological temperature range. For every 10°C rise in this temperature range, biological processes increase by 2-3 folds; see Figure 2. The physiological temperature varies with species and geographical regions, and tropical crops can tolerate higher temperatures up to 40°C.

Under extremely high temperatures, the enzymes involved in respiration break down, so the process stalls. The respiration rate will also increase if temperatures are very low due to the occurrence of physiological injury. For example, low temperature-sensitive fruits show high respiration rates below 10-12°C.

• Oxygen Levels: O₂ concentrations are critical as they are needed for respiration. As O₂ levels drop, respiration slows down. Though the O₂ levels in the atmosphere are 20%, fruit respiration can occur at O₂ levels of 1-2%. Reducing O₂ levels to zero is not recommended, as anaerobic respiration or fermentation can set in and affect the fruit’s taste, color, and texture.
• Carbon dioxide levels: There is 0.04% of CO₂ in the atmosphere. The effect of a concentration higher than the ambient CO₂ depends on the fruit type. In most cases, it slows respiration; therefore, it is one of the methods used in Controlled Atmosphere storage and MAP packaging to reduce respiration and ripening. A very high rate of CO₂ is, however, not desirable. CO₂ above 5% causes anaerobic respiration, decay, and irregular ripening. More than 7% CO₂ inhibits ethylene production and ripening of fruits.

Controlling Respiration

Respiration in postharvest stages can be controlled by manipulating the environmental conditions. Since respiration rate differs for fruit type and cultivar, there is no optimal set of environmental conditions. The storage conditions must be optimized for each fruit and cultivar for the best results.

• Temperature: For storage, distribution, and retailing, temperatures of 5–20°C are suitable for optimum respiration rates, depending on species. Low temperature-sensitive tropical fruits, such as avocado, banana, pineapple, citrus, mango, papaya, and tomatoes, should be stored above 10-12°C to avoid chilling injury.
• Oxygen and Carbon dioxide levels: O₂ levels below 8% and CO₂ above 1% are ideal for reducing respiration rates. Higher CO₂ levels also reduce senescence and fungal growth. Below 2% O₂ and/ or over 5% CO₂, some fruits like bananas, pears, mango, and tomato suffer from irregular ripening.

To control respiration, constant monitoring of the two gases, O₂ and CO_2, is necessary.

Measuring Fruit Respiration

Fruit respiration is measured by estimating O2 and/or CO2 levels in the atmosphere of the storage room/container or MAP (Modified Atmosphere Packaging) packages. Using sensors is the standard method in commercial supply chains.

Felix Instruments is an industry leader that produces a range of precision gas analyzers for non-destructive and real-time oxygen and carbon dioxide measurements. The gas analyzers measure O2 with electrochemical sensors and CO2 with pyroelectric and infrared sensors.

• The F-901 AccuStore & AccuRipe is a fixed device suitable for monitoring and regulating gas levels in storage and ripening rooms.
• The F-920 Check It! Gas Analyzer and F-950 Three Gas Analyzer can be used in the entire supply chain.
• The F-940 Store It! Gas Analyzer is suited for storage rooms and retail shops.
• The F-960 Ripen It! Gas Analyzer helps ripen rooms and can be operated from outside if internal connections exist.

Reducing Fruit Respiration and Food Loss

Controlling and reducing fruit respiration is one of the main criteria of postharvest handling to minimize loss of fruit quality, weight, and decay. Around 14% of food is lost before the retail stage during storage, transportation, and distribution, and the % of perishable fruits lost or wasted is 45%. There is a lot of scope to improve the supply chain conditions and thereby reduce fruit loss by designing storage environmental conditions customized for species and cultivars.

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