I apologize for the gaps in articles lately. I’ve undertaken some life changes and writing a blog article isn’t something that always fit into the time allotments this past while. But never fear, I’m working on ways to make more regular posts.
It’s harvest time, huzzah!!!
August is that magical time of year when our crops really start to hit their stride and the literal fruits of our/your labours are taken off the field. However, without careful handling and special attention, those eagerly anticipated products can go from beautiful to unmarketable in a very short period of time. It’s disheartening and discouraging.
Once a crop is off the plant, everything has its magical sweet spot of post-harvest requirements, including a combination of temperature and humidity, and even the concentration of the air. Properly combined, you can stretch the post-harvest period to its maximum and extend the storage life to a certain extent.
For many different fresh products, the temperature before you even get to storage (pre-harvest or pre-cold-storage) can be as important a factor in post-harvest handling as your “cold” storage temperature. That factor is called “Field Heat”.
What is Field Heat?
Field heat is essentially the difference in the temperature of the harvested crop and the optimal storage temperature of that same product. So, for example, if you harvest a berry at an ambient temperature of 22°C and the best temperature for extended cold storage is 1°C, your field heat is 21°C. What all of this means is that field heat is the heat energy that needs to be removed from the produce before it can be stored.
What affect does field heat have on crops?
The kicker with field heat is that until that heat energy is consumed, and the product reaches its optimum temperature, the product is deteriorating more rapidly.
Fresh produce is deteriorating from the moment it is harvested. It is respiring and aging and going further and further away from perfection. It is losing water and undergoing other (likely undesirable) changes. The shelf-life is dropping. Generally, the warmer the product and surrounding temperatures, the faster the rate of decline and deterioration. This is particularly true for anything that can handle cold storage temperatures, but still applicable even for crops that don’t like cold temperatures. They will still deteriorate slower in cooler temperatures; you just can’t throw really cold temperatures at them.
The literature says that “the deterioration of fresh produce doubles for every 10°C above the optimum storage temperature”. That means that the longer the crop sits at a warmer temperature, the faster it is breaking down, and the less time you must sell it, use it, or do whatever you need to with it.
I dug into the “bible” of post-harvest handling (USDA Handbook 66 – The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks) to pull together a summary table of the rates of respiration (mg CO2/kg/hr) for some commonly grown veggies and fruits. You can see the big shift in respiration rates as the temperature increases.
Rate of Respiration (mg CO2/kg/hr) at different storage temperatures for different fresh products
| Vegetable |
Optimum Temp (˚C) |
0˚C |
10˚C | 20˚C | Vegetable | Optimum Temp (˚C) | 0˚C | 10˚C | 20˚C | ||||
| Snap beans |
7 |
20 | 58 | 130 | Onion | 0 | 3 | 7 | 8 | ||||
| Broccoli |
0 |
21 |
81 | 300 | Garden Pea | 0 | 38 | 86 | 271 | ||||
| Carrot | 0 | 15 | 31 | 25 | Potato | 6-8 | ? | 12 | 24 | ||||
| Cucumber | 10 | ? | 26 | 31 | Radish | 0 | 16 | 34 | 130 | ||||
| Garlic – bulbs | 0 | 8 | 24 | 20 | Spinach | 0 | 21 | 110 | 230 | ||||
| Head lettuce | 0 | 12 | 31 | 56 | Sweet Corn | 0 | 41 | 105 | 261 | ||||
| Leaf lettuce | 0 | 23 | 39 | 101 | Zucchini | 5-10 | 25 | 67 | 164 | ||||
| Fruit | Optimum Temp (˚C) | 0˚C | 10˚C | 20˚C | |||||||||
| Blueberry | 0 | 6 | 29 | 70 | |||||||||
| Black currant | 0 | 16 | 42 | 142 | |||||||||
| Raspberry | 0 | 17 | 35 | 125 | |||||||||
| Rhubarb | 0 | 11 | 25 | 49 | |||||||||
| Strawberry | 0 | 16 | 75 | 150 | |||||||||
What can you do about it?
In the end, the quicker that a product can be brought to the optimum storage temperature, the longer potential shelf-life it has. Obviously, you are rarely going to be able to harvest crops at 0°C without some serious issues but harvesting in the cooler parts of the day is preferred over harvesting at the hottest parts of the day.
Alternatively, you need to plan to use technology to bring the temperature down as fast as possible. There are different strategies (with different costs), ranging from forced air (cooler with good air movement/fans) up vacuum cooling, ice slurries, etc.
Ultimately, harvest at as cool a temperature as you can and then don’t delay at getting the product into the cold.
And, as with all recommendations and strategies, they don’t apply equally to all crops. So, you’ll have to make some adjustments based on what you are dealing with.