Relative Humidity: An Often Forgotten Factor for High-Quality Produce

So, you’ve gotten your fruits and veggies from the field, but how do you get them to the fork in freshly-picked shape?

Adding to the already predominant supply chain initiative of good temperature control for maintaining the post-harvest freshness of fruits and vegetables, Sensitech Inc. affirms that keeping other factors at the forefront of cold chain technology is just as necessary for effective transportation; from packing, all the way to the store shelf.


Sensitech Inc., a leading global cold chain technology company, joins us to discuss the importance of preserving a suitable relative humidity in the tracking, measurement, and control of fresh produce shipments throughout the supply chain. 

Q: So, what exactly is relative humidity, and why is maintaining it key for shipped fresh produce?

A: Relative humidity (RH) is the moisture content of the atmosphere, expressed as the ratio of the actual moisture content to the total possible moisture content without condensation forming. Because colder air is more capable of holding moisture than warmer air, RH is temperature dependent. Knowing the RH inside of a shipping container can be crucial for maintaining freshness, especially for produce with high water content like lettuce, cucumbers, mushrooms, strawberries, and watermelons. Both low RH and high RH may have negative impacts on produce quality, particularly if RH levels are out of the recommended specifications for long periods of time.

Q: Why should suppliers and receivers in the industry look to avoid low RH in their produce?

A: Produce handled at RH levels lower than its relative water content may experience dehydration, shriveling, decreased gloss, decrease in market value, and higher susceptibility todisease. This may lead to brand erosion for the supplier and the retailer, as quality is a key driver for shopper behavior. In addition, for products sold by weight, water loss can lead to salable weight loss and reduced profit. A 2 percent weight loss may not affect the appearance quality of a product, but the cost of that lost weight can be significant. For example, for a pallet weighing 500 kg, 2 percent accounts for a loss of 10 kg per pallet. If the product is valued at $10 per kg, at retail pricing this weight loss results in a loss of $100 per pallet.

Q: What kind of distinguishable effects can you see in produce with low RH?

 A: Low RH can be seen in fresh produce in a variety of ways (e.g. shrinking of the skin or shriveling) and water loss due to low RH can also be affected by the following characteristics:

SKIN: An apple, for example, will lose moisture at a slower rate than a head of lettuce due to its skin thickness.

SURFACE/VOLUME PROPORTION: Leafy vegetables, like lettuce or spinach, which typically have a high surface-to-volume ratio, are more susceptible to dehydration than cylindrical-shaped produce, such as a potato.

DAMAGE: Cuts, lesions, and marks on the surface of the product due to mishandling can lead to higher water loss and increase the product’s susceptibility to disease and decay.

VAPOR PRESSURE DEFFICIET (VPD): VPD is the force which drives water from the product to the surrounding air. The VPD is influenced by the difference between the temperature of the produce, and the temperature and RH of the surrounding air. Higher temperature and RH differences lead to higher water loss.

Q: So, alternatively, does this mean too high of an RH also has an effect on fresh produce?

A: On the other end of the spectrum, produce handled at RH levels higher than its relative water content is likely to encounter condensation, disease development, or physiological disorders, resulting in an increased rate of product senescence. When the RH of the atmosphere reaches saturation (100 percent), water vapor will condense on the surface of the produce. The temperature at which condensation forms is called the dew point, and it is the most useful humidity parameter when dealing with condensation problems. Psychrometric charts are often used to determine the relationship between temperature, RH, and dew point. Unwanted condensation on products may encourage faster disease development or physiological disorders, resulting in decay and loss of product. Condensation can also damage labels or cardboard packaging materials, resulting in problems when unloading the cargo. 

Q: Are high RH levels considered undesirable for all fresh produce items?

A: High RH levels (85-95 percent) are usually recommended for transporting most fresh fruits and vegetables in order to prevent moisture loss. However, produce such as dried onions, garlic, dates, dried fruits, and nuts need to be transported in conditions of lower relative humidity. For example, the optimal RH range for dried onions and garlic is 65-70 percent.

Q: Can you tell me what kind of benefits the supply chain can expect to see with a properly maintained RH?

A: Maintaining both temperature and RH at the ideal recommended specifications (specific for each produce type) during transport and storage is vitally important to ensure freshness and an increased shelf life. Seriously dehydrated or decayed produce is generally considered unmarketable and rejected. A well-maintained RH, along with other factors within the supply chain can optimize freshness, reduce food waste, and thereby maximize profit margins.

This research and focus on extensive cold chain technologies, from temperature control to the proper hydration of each product, allows Sensitech Inc. to offer the supply chain a set of proficient and comprehensive solutions for preserving fresh produce throughout the entirety of its travels; giving “from farm-to-fork” a whole new perspective.