Jun 132012
 

Potatoes, fresh from the ground, prior to being cured then stored.

Potatoes are an excellent crop to grow, and to store.

Probably no other crop yields as much protein, or as many calories, when measured against the hours of work it takes you to grow, care for and harvest them.  Potatoes yield more calories per square foot of growing space than anything else, and more protein than anything except for legumes.

Depending on the soil and your growing strategy, you can expect yields from about 1.25 lbs per linear foot of potato row up to 4 lbs or more per linear foot (ie between about 20,000 and 70,000 lbs/acre – national commercial average of 41,300 lbs/acre).

Originating in South America, potatoes were brought to Europe in the late 1500s.  They have steadily grown in popularity and now are the world’s third largest cash crop (after rice and wheat).

Potatoes don’t need a lot of water or fertilizer.  And if stored in optimum conditions, you can get as much as ten months of storage once you’ve harvested them.  They can be grown across much of the US, and although they prefer cooler climates, they are even grown in Florida and Arizona.  They are grown commercially in 36 of the 48 Conus states.

Potatoes are also a key source of nutrition for most of us.  On average, Americans each ate 117 lbs of potato in 2010.

That’s not to say that potatoes are a perfect crop.  Of course, some environments are not suited for potato growing (and different types of potatoes grow better/worse in different conditions), and equally of course, as students of history will know (the various Irish and Scottish potato famines in the 1840s and 1850s) they are vulnerable to some types of bio-hazards (particularly viruses and pests) that can destroy entire growing seasons worth of potatoes.

As with anything else, we always encourage you to diversify the food products you grow so if one crop has problems, you will hopefully still have other crops unaffected by the problems impacting on the challenged crops.

But those considerations are for another time.  Tis article starts at the end of the growing of potatoes, and assumes you have already carefully harvested them at the ideal time.

There are four considerations to keep in mind when storing potatoes.  They need to be kept cold, dark, humid, and with a small amount of air circulation.

Remember that the harvested potatoes remain as living things.  They are made up of about 80% water and so need to be in a humid environment to prevent them drying out.  The cooler they are kept, the slower their ongoing aging will be, and keeping them out of the light will prevent the light stimulating the formation of bitter-tasting and poisonous glycoalkaloids.

Let’s look at their storage requirements step by step.

1.  Preparation for storage.

You should avoid washing the potatoes (ie keep them dry) unless absolutely essential.  If you must wash them, do so gently and not physically damage the potatoes (they are reasonably vulnerable to damage until their outer skin has a chance to harden) and ensure they are well dried.

It is recommended you should dig up the potatoes after a few days of dry weather, so they will be dry to start with, and leave the potatoes out in the field for a couple of hours so as to dry them further and to make it easier to clean them by brushing any dirt off them.

Treat potatoes as reasonably fragile before they have been cured (and still treat them carefully afterwards too).

2.  Curing the Potatoes

Before putting the potatoes into ‘deep sleep’ you first want to encourage them to adjust to their now out-of-the-ground experience.  You do this by curing them for a week or two, in a well ventilated humid and dark place, at a temperature of about 50 – 60 degrees, or a shorter time at a warmer temperature (ie 5 – 10 days at 59 – 68 F).  This will toughen up their skins and might help any harvesting damage to heal.  Humidity should be about 95% – 98% – as humid as possible without condensation forming.

3.  Storing the Potatoes

At the end of the curing process, you will want to inspect all the potatoes again, and if they are satisfactory with no sign of any type of infection or damage, transfer them to long-term storage.

This will also be in the dark, and also be very humid – as before, as humid as possible without allowing any condensation.  Actual water will cause the potatoes to rot.

Temperature is ideally cool to cold, but no colder than about 39 F.  If you plan on frying the potatoes, you might want to keep them a bit warmer – perhaps no cooler than 45 F.  Colder temperatures accelerate the speed of starches converting to sugars, and while in some vegetables, a sweetness is desirable, it tastes strange in potatoes.  Furthermore, if you then fry the potato, the sugars would burn.

Warmer temperatures encourage the potatoes to sprout and also accelerate the development of tuber diseases.

There should be a little air flow to enable the potatoes to ‘breathe’.

Be careful what else might be stored in the same area, or sharing the same air.  Some produce gives off ethylene gas which massively reduces the storage life of potatoes.  And the subtle smell given off by stored potatoes can be taken up by apples and gives them an unpleasant flavor.

Don’t store the potatoes piled too high – not only does this reduce the air flow into the potatoes in the middle of the store, but it also puts harmful pressure on the lower potatoes that might damage them.  It is suggested not to have potatoes piled more than 2 feet high.

From time to time during their storage (ie about once a month – more regularly if you are having storage problems) you should inspect them for signs of any rot or infection.  Remove any that show any signs of problems, before they start to infect other potatoes close to them.

Some people suggest removing adjacent potatoes if you find a potato that is going bad.  In a perfect world, this makes sense, but you need to trade-off between, on the one hand, disposing of too many potatoes unnecessarily just because of their proximity to bad potatoes, and on the other hand, of not throwing away infected potatoes that quickly get worse and pass the infection further on into your potato stock.

Probably what we’d do is create two supplementary storage places in your potato cellar.  One for actively going bad potatoes, and we’d eat those first (well, not the bad bits, of course).  Then the second storage place would be for suspect potatoes, and we’d eat those second, leaving the main bulk of the potatoes in the normal storage area(s).

Seed Potatoes

Store seed potatoes at a colder temperature than ones you plan to subsequently eat.

You can take seed potatoes all the way down to almost freezing.  You also want to keep the humidity high for them, too, but with seed potatoes a pale diffuse ambient light is recommended.

The Greening of Potatoes

When potatoes are exposed to the light, two things happen.  A green layer forms on the exterior of the potatoes, and glycoalkaloids are formed inside the potato.

Glycoalkaloids (in particular, solanine) are both bitter and poisonous.  All potatoes have a low level of glycoalkaloid in them, and they help to give potatoes their distinctive flavor/taste.  Light exposure causes more glycoalkaloids to form.

Some people think the green they see contains the glycoalkaloids.  This is not so – the green is harmless chlorophyll.  And so simply peeling off the green layer does not make the potato safe to eat.  The green chlorophyll is merely an indicator that correlates to the level/presence of glycoalkaloid in the potato as well.  If you see a lot of green, dispose of the potato and don’t eat it.

Even a day of sunlight can be enough to push glycoalkaloids up to unacceptable levels (something to think about next time you visit a farmers’ market in the summer and see stalls with displays of potatoes sitting in the sunlight).

Less than Optimum Storage Conditions

Not everyone will have a perfect potato store-room.  The key things for you to consider when storing potatoes are keeping the temperature as cool as possible, but generally above about 40 degrees, and to ensure there is no light in the storage area.

The next thing to optimize would be the humidity – the more the merrier – and a little bit of air flow.

Also, remember to check the potatoes when first storing them.  If they are your own potatoes, clean and cure them first, be sure they are dry, and remove any ones with any signs of infection.

A day of bright sunlight is enough to spoil a potato, so darkness is most important.  As for temperature and humidity, the warmer the temperature, the quicker the potatoes will age, and the drier, the more the potatoes will shrivel up.  Sprouting is usually the first sign of potatoes being stored too warm, and shriveling a sign of insufficient humidity.

You’ll see a number of internet sites with various suggestions about how to store potatoes if you don’t have a temperature and humidity controlled cellar.  Some of these sites have conflicting and even close to contradictory ideas; the kindest reason we can think of for this is that they are recognizing the compromises that have to be made and are more concerned with extending potato life from a week or two (if not stored well) up to a month or two (in slightly better conditions).

Boxes lined with straw or peat moss are a common theme.  If you do this, remember the need for potatoes to breathe and to get some fresh air, and also remember that while it is okay to keep the potatoes humid, you don’t want to get them actually damp/wet.

If nothing else, keep them in the coldest part of your house, and block the light from reaching them.

Some root vegetables allow for very simple storage – simply leaving them in the ground.  This is less appropriate for potatoes.

Summary

The humble spud should be an integral part of your food growing and storage plans.

It has everything going for it – reasonably easy to grow, gives a good food yield in terms of the amount of man hours needed to put in to growing them, is nutritious, and can be stored for up to 10 months.

The closer to our optimum storage guidelines you can get, the longer the storage life and the better the potatoes will be when you eat them.

  3 Responses to “How to Best Store Potatoes”

  1. I live in a climate where it tends to be very hot and dry, especially in the summer. Would a “root cellar” dug into the ground (say, lined with concrete block, or poured concrete) be sufficient to keep potatoes and other food relative cool in the summer? How deep down does it need to be (i.e. how deep a layer of earth over top, for it to stay cool like a cave), and how does one keep it humid-but-not-wet?

    I’m looking for old-fashioned, non-electric methods — if there was enough electric available to run a temperature-and-humidity controlled refrigerator unit, we wouldn’t need to be storing potatoes! I’m familiar with the methods used by the Amish in Pa. and by my “hill people” ancestors in North Carolina, but the climate is a lot different on the east coast, I’m not sure how well they will work on the hotter, dryer west coast. So, I’d love to hear from anyone who has experience with that out here.

    • Hi, Julie

      The good news is that soil temperatures very rapidly reduce as you dig down. You need depth for two reasons – first, to get down to the cooler earth, and secondly to have an insulating ceiling/roof on the top of your cellar.

      A foot or two of earth is probably enough for an insulating roof, and probably is also enough for you to get down to cooler soil. But there’s an easy way to find out.

      In the summer, simply dig down a way and stick a temperature probe/thermometer into the soil after you’ve dug down six inches, then after you’ve dug down a foot, 18″ and 24″. Keep digging down, as necessary, until there is no more temperature reduction and you’ll then have a good feeling for the thermal incline as a function of soil depth.

      Maintaining a cool temperature ‘passively’ simply by relying on cool earth walls is possible, but the ability of the walls to soak up heat is limited. You’ll want to be careful, when going in and out of the cellar, that you don’t let too much hot air in, and try to use low energy lights when you are in there – not only because light is the enemy of long term storage, but also because of the heat they give off.

      On the other hand, you also need to be careful to maintain moderately fresh air in the cellar, otherwise you might create other challenges and issues for storage life (and your own health while visiting).

      As for keeping the humidity high, I’ve not researched that, but off the top of my head, I’d have some large open tanks of water and absorbent material – towels, cloths, whatever – hanging down and into the tanks of water, to encourage water to wick up and then evaporate into the cellar.

      Controlling the amount of open area on the top of the water tank and the amount of ‘water wick’ in the air will help you to control the humidity.

      As I said, I’ve not researched this, neither have I done it myself, so I’d suggest you take these concepts with a grain of salt. I’m a bit worried about warmish/moist air encouraging mold and other nasty things, and I bet, if you or I did a bit of inspired Googling, we’d find some interesting information and helpful advice on the topic. I’ll add it to the list of things to research and write about, one of these days!

      • Thanks! …On the other hand, I’m guessing there are a lot of other foods that do better in DRY storage, which as you say tends to preclude mold and mildew; so maybe we would be better off just focusing on those for now, and not attempting to store a whole lot of potatoes in this climate. Or if we do, that will be “someday down the road”, as a kind of luxury item for this area…the way some people grow hothouse orchids!

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