What is Heat of Respiration?

Understanding and Managing Heat of Respiration in Fresh Produce

Have you ever considered that a crate of freshly picked strawberries or a head of lettuce brings not only its own temperature but also a continuously generated “internal heat” into the cold room where it’s stored? Even after being harvested, fresh fruits and vegetables continue to live, and just like humans, they breathe. This biological process is called respiration, and the heat it produces plays a vital role in cold chain management. This “heat of respiration” is the invisible enemy of refrigeration systems, and if not managed correctly, it degrades product quality, shortens shelf life, leads to food waste, and significantly increases energy costs.

What is Heat of Respiration? A Living Process

To sustain their life and maintain cellular functions, post-harvest products use their stored starches and sugars as fuel. With the help of oxygen from the environment, they break down these complex organic compounds, releasing energy, carbon dioxide (CO2), and water. An inevitable byproduct of this fundamental metabolic reaction is heat. The basic equation for the process is:

$$\mathrm{C_6H_{12}O_6} + 6\mathrm{O_2} \rightarrow 6\mathrm{CO_2} + 6\mathrm{H_2O} + \text{Heat}$$

In essence, this is the product slowly “burning” and consuming its stored energy, leading to aging. This process is also the engine behind natural senescence, which leads to the eventual decay of the product. If this generated heat is not efficiently removed from the environment, it increases the product’s own temperature. The rising temperature, in turn, accelerates the respiration rate, creating a vicious cycle. This cycle causes the product’s nutritional value, texture, and moisture to be lost rapidly, speeding up decay.

Why is it a Critical Factor for Refrigeration Engineering?

Heat of respiration is a dynamic and continuous variable that should never be overlooked in refrigeration load calculations. Its importance lies in several key points:

1. Additional and Continuous Heat Load: The refrigeration system must deal not only with a one-time load of lowering the product’s initial temperature (field heat) but also with the 24/7 heat of respiration load that the product generates throughout its storage period. This directly affects the runtime of compressors, defrost cycles, and overall system wear and tear.

2. Direct Quality Loss: A high respiration rate means the product is rapidly consuming its valuable assets. Sensitive vitamins like Vitamin C are broken down, the sugar-acid balance that determines flavor is disrupted, and the product loses water through accelerated transpiration (sweating), causing it to wilt and shrivel. This directly translates to a shorter shelf life and a decrease in marketable product quality.

3. Proper Equipment Selection and Design: A cold storage facility designed without accounting for the heat of respiration load will be undersized. An inadequate evaporator (indoor unit) will struggle to remove both the sensible heat and the respiration heat, leading to temperature fluctuations within the room and a failure to maintain setpoints. An undersized compressor will run continuously, rapidly shortening its lifespan and resulting in exorbitant energy bills.

Factors Affecting Respiration Rate and Heat

Every product has a different respiration rate, which is influenced by various factors. Understanding these factors is the key to determining the correct storage strategy and maximizing product life.

1. Temperature

   This is the most significant factor controlling the respiration rate. As a general rule, every 10°C increase in temperature approximately doubles or triples the respiration rate. This exponential relationship explains why pre-cooling—bringing products down to their optimal storage temperature as quickly as possible after harvest—is so vital. Protecting the cold chain from the very beginning allows the product to retain its internal energy reserves for a much longer period.

   • Example: According to ASHRAE data, sprouting broccoli generates about 60 mW/kg of heat at 0°C, but at 20°C, this value can soar to 1155 mW/kg. That’s nearly a 20-fold increase! Similarly, even an apple with a low respiration rate will produce more than twice as much heat at 10°C as it does at 0°C.

2. Commodity Type and Variety

   Different products, due to their genetic and physiological makeup, have vastly different respiration rates.

   • High Respiration Products: Asparagus, broccoli, spinach, fresh peas, mushrooms, and strawberries are examples of fast-growing, metabolically active, and delicate products. These are the most challenging products for refrigeration systems.

   • Low Respiration Products: Potatoes, onions, apples, citrus fruits, and nuts are more durable, slow-developing products that often have a natural period of dormancy.

   Furthermore, fruits are categorized by their post-harvest behavior as either climacteric or non-climacteric.

   • Climacteric Fruits: These fruits, such as bananas, apples, avocados, tomatoes, and peaches, continue to ripen after being harvested. During the ripening process, they exhibit a sharp increase in respiration rate, known as the “climacteric peak,” which is triggered by the production of ethylene gas. This is particularly important in mixed-produce storage, as the ethylene produced by a batch of ripening apples can trigger the ripening and spoilage of other sensitive products.

   • Non-Climacteric Fruits: These products, including strawberries, grapes, cherries, and citrus, remain at the same level of maturity as when they were harvested. Their respiration rates are relatively stable and tend to decrease slowly over time, making their storage management more predictable.

3. Atmospheric Composition:

   The levels of oxygen (O2) and carbon dioxide (CO2) in the surrounding environment directly affect respiration. Controlled Atmosphere (CA) and Modified Atmosphere (MA) storage technologies are based on this principle. Lowering the oxygen level to low levels like 2-5% and increasing the carbon dioxide level to above-normal levels like 3-10% chemically slows the respiration reaction. Low oxygen limits the “fuel” available for the reaction, while high carbon dioxide acts as a brake, suppressing the enzymes involved in respiration. However, this balance is very delicate; excessively low oxygen or excessively high carbon dioxide can lead to anaerobic respiration (fermentation), causing undesirable flavors and odors in the product.

4. Physical Condition and Damage:

   Cut, bruised, abraded, or otherwise injured products increase their respiration rates as a defense mechanism. Injury initiates a “healing response” in the plant tissue, which is an energy-intensive process. This response locally increases respiration and ethylene production at the site of the wound. This is the scientific explanation behind how one bad apple spoils the whole bunch; the ethylene gas produced by the injured apple triggers the ripening and decay process in its neighbors. Therefore, gentle handling of produce from harvest to storage is critical for preserving quality.

How to Calculate the Respiration Heat Load

To estimate this critical part of the refrigeration load, engineers use experimentally determined respiration rate data. This data is typically expressed in mW/kg (milliwatts per kilogram) or BTU/ton/24 hours.

Simple Calculation Example:

Let’s assume we want to store 10,000 kg of Golden Delicious apples in a storage room at 5°C.

1. Find the Data: According to ASHRAE tables, the respiration rate for Golden Delicious apples at 5°C is approximately 16.0 mW/kg.

2. Calculate Total Instantaneous Heat Generation (in Watts):

   • Total Heat = Mass of Product × Respiration Rate
   • Total Heat = 10,000 kg × 16.0 mW/kg = 160,000 mW = 160 Watts

3. Meaning and Daily Load: The refrigeration system must handle a continuous heat load of 160 Watts (about as much as two old-fashioned light bulbs) just from the apples breathing, in addition to all other heat gains. Considering this load continues for 24 hours:

   • Daily Energy Load = 160 W × 24 hours = 3,840 Wh = 3.84 kWh/day. This is the extra energy the refrigeration system must consume every day just to counteract the heat of respiration.

For more precise calculations, empirical formulas such as the one below, developed by ASHRAE and using temperature-dependent coefficients (f and g), are also available:

$$W (\text{W/kg}) = \frac{10.7f}{3600}\left(\frac{9t(\text{°C})}{5}+32\right)^{g}$$

This formula allows for the creation of a dynamic and precise heat load profile for different products according to temperature changes. This is especially important for systems designed for situations where the temperature changes rapidly, such as in pre-cooling, or where storage conditions are not constant.

Conclusion: The Art of Managing the Invisible Heat

Heat of respiration is an unavoidable process inherent to the nature of fresh produce. However, modern cold chain management is built on slowing down this process and controlling its effects. Rapid pre-cooling immediately after harvest, precise temperature and humidity control throughout storage, proper air circulation to prevent the buildup of ethylene and carbon dioxide, and advanced controlled atmosphere systems are the primary weapons in the fight against this “hidden heat.” Accurately understanding and calculating the heat of respiration is not just an engineering necessity for designing efficient and reliable refrigeration systems; it is also a moral responsibility to reduce food waste on a global scale, maximize product quality and nutritional value, and create a sustainable food supply chain.