Respiration and post harvest cooling

So many of us have fond memories of berry picking with the family. Jump in the car, head to your best berry patch, and fill up the pails (and tummies!) with sweet saskatoons. Pails are usually dumped straight into an empty cooler or box and stuck in the back of the car for the ride home, usually with a few errands along the way. Later that day or possibly the next, it’s time to deal with the berries. They never seemed to taste as good as that first bite off the tree. Your memory isn’t tricking you – the sugars and other plant nutrients in berries and many other fruits and vegetables can disappear very quickly after picking. The good news is, you can slow down this process to keep your fresh produce tasting fresher, longer.

Inside of every living plant cell is a storage area for water, sugar, starch and other important nutrients that are required to keep the plant cell healthy. When your produce is still outside attached to the rest of the plant, sunlight, water, and the healthy growth of the plant help keep the storage area in all of the cells full. Once you pick your fruit or vegetable, it has only whatever is in the storage area of each cell to keep the cells alive. As these storehouses of supplies dwindle, the cells die and your produce begins to first taste poorly and then, once there aren’t enough nutrients to keep the cells functioning properly, your produce begins to spoil. If you want your produce to stay fresher and more nutritious longer, you may need to slow down this process.

Plants that use up this storage quickly are said to have high respiration rates. Respiration rates also influence how quickly your plants use up the water in the cells, which is called transpiration. This means that high respiration rates lead to higher transpiration rates so plants wilt quicker than they should. Because they’re emptying their nutrient storage so quickly, plants with high respiration rates don’t taste as good for as long as plants with slow respiration rates, nor do they tend to store well.

Don’t be fooled by a good-looking plant. If the respiration rate is very high, your produce might still look good but taste bland – as any of us who have bought disappointing corn or asparagus have experienced.

All living cells respire, so most produce will keep longer if it’s kept cool. Your fridge is likely an ideal temperature for most, with the main exceptions being pineapples, tomatoes and bananas, which are better stored at room temperature. Everything else would likely keep longer in a cooler space. Fridge space is not unlimited though, so how do you prioritize what goes in the fridge and what gets stored elsewhere? The faster a plant usually respires (empties its cell storage), the more important it becomes to slow it down to maintain freshness.

Here are the respiration rates of common fruits and vegetables:

• Very high respiration rate – important to cool quickly to maintain freshness: broccoli, sweet corn, spinach, mushrooms, most berries
• High respiration rate: cauliflower, strawberries, Brussels sprouts
• Medium respiration rate: cherries, pears, lettuce, tomato, banana
• Lower respiration rate – less critical to cool quickly: apples, grapes, potatoes, citrus, onions

Respiration is strongly tied to air quality, ripening, plant stress and temperature.

We modify air quality using different techniques during preservation (think fermentation or dehydration) and it’s something we definitely consider for storage (ethylene being a major factor), but home growers don’t tend to concern themselves with air quality during harvest.

As plants ripen, respiration tends to increase so your plant’s stored nutrients are being used up faster. That is, green tomatoes respire less than red ones. The riper the fruit, the more important it is to manage your post-harvest cooling. Some fruits don’t ripen after picking and some do. Check out our article on ripening to learn more LINK.

Injured or stressed plants tend to respire more quickly than healthy ones. Damaged plants also don’t store well. Be careful when you’re harvesting and know what you plan to do with your product before you harvest it. For example, if I’m picking potatoes for eating tonight, I scrub them clean in the garden. If I’m picking potatoes for long term storage, I’m careful to not damage the skin so they don’t get scrubbed until before they’re eaten. While any damage can potentially increase respiration rates, damage causes other issues that affect long term storage such as infection or increased water loss.

As I’ve been suggesting throughout this article, for home gardeners at harvest time, our main control is over temperature. Ideally, harvest your produce on cooler days and have a plan to cool it off soon after picking. Don’t pick it and leave it lying on the grass or stick it into a cooler or box where it gets baked at whatever the ambient temperature is. Pick it, cool it quickly, and either store it or process it soon after harvest to keep your food at its sweetest and most nutritious.

It depends on how durable the produce is. Immersing in cold water, ice baths, misting, and refrigeration are all good options. Large scale berry picking operations typically have refrigeration at the harvest site. Grocery stores frequently use misting. For my own harvest, carrots get hosed off soon after picking and aren’t left outside in the sun for long. When we go cherry picking at our friend’s house, I bring along a large tub half full with cold water with ice in it, pick the berries into large freezer bags, and drop full bags into the ice water for the ride home. On my best days, I process the cherries as soon as we get home. If I was picking raspberries, I know they’d likely get crushed doing this so I’d lay them on a bed of ice or simply put them on cookie sheets straight into the fridge or freezer, depending on what I planned to make with them.

For ideas on how to keep your produce fresher, longer, check out our growing pages for suggestions for each plant.

USDA Agriculture Research Service Agriculture Handbook Number 66. The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks. Revised February 2016.