Potatoes and nutritional density

I found out that if you were able to choke down 3 pounds per person per day you would get a bit over 1000 calories/day.   From reader Uninformed

It is worse than that.

 

I will be using "specific gravity" as a proxy for caloric density in much of this essay because it is relatively easy to measure specific gravity and that is what most of the literature reports. 

As with many things, details are important. In fact, from the lens of the subsistence gardener, details are very, very important.

The chart shown above is from MSU data. The bright red data point is the average of "red" boiler potatoes like Dark Red Norland and Red Pontiac and you would have to eat six pounds of them to get 1000 Calories. The bright yellow dot is the average for "round, white tablestock" which includes Yukon Gold. The elongated brown data point is the average for "russet tablestock" and the boomerang shaped data point is the average for "chipping stock" (three pounds to get 1000 Calories).

Potato chips are BIG business in Michigan. Cultural parameters were tweaked to hit a specific gravity of 1.085 for the chipping stock and the others fell wherever.

The big chip processors require a specific gravity of 1.080-to-1.085. Variation in moisture results in chips coming out of the fry-bath soggy or burned.

The second level of detail

Suppose you are a commercial producer of potatoes intended for the consumer's table and you are making variety and growing decisions.

The effect of irrigation on yield and specific gravity

Looking at the extremes, you have the choice of shipping 30,000 kg per ha (approximately 27,000 pounds per acre) with a specific gravity of 1.074 or you can ship 65% more product per unit area of land with a specific gravity of 1.066.

You get paid by the amount delivered and cosmetic appearance. You would be daft to not irrigate to maximize shippable product.

If you are choosing varieties, do you choose Russet Burbanks which might produce 30,000 pounds of US#1 per acre (S.G. of 1.075) or Norkoda Russet at 45,000 pounds of US#1 per acre (S.G. 1.065)?

Most people who buy potatoes at the grocery store are not informed enough to prefer Burbanks over Norkoda nor do they prefer a Kennebec with a specific gravity of 1.074 over a bag with a specific gravity of 1.066.

A specific gravity of 1.085 is not a "hard-stop"

Michigan chipping processors specify the 1.080-1.085 range of specific gravities because they know they will receive some shipments from growers who struggle to get over 1.080 due to soil, sun exposure, late rains and choice of variety. They would LOVE to raise the window to 1.085-to-1.090 or something higher because that would lower their energy costs and improve the throughput at the fryer vat. 

They could raise the window but then the rate of rejects...after frying...would be higher and that is expensive scrap because it is lost salable product.

As a subsistence gardener, it is not rocket science to increase the caloric density of your potatoes

Pre-planting decisions:

• Choose a site that gets great sun exposure. Just like sun ripens grapes that make good wine, sun makes potatoes. Sites with great sun exposure are rare in cities and suburbs.
• Ensure the site has adequate potassium.
• Ensure the site has enough nitrogen to grow a full canopy quickly. Sun that does not fall on leaves does not make nutrition. BUT, don't use any more nitrogen than that amount. This is an experience thing.
• Choose varieties that are noted for high specific gravity. Those are usually later varieties. Some catalogs (Example) list potatoes as "waxy" or "mealy". You want dry/mealy/floury potatoes to maximize caloric density. Specific gravities for potato varieties can be found on the internet but you have to be careful to not compare values from different sites to avoid apples/oranges comparisons.

Planting decisions:

• Plant early so you have full canopy during the longest days of summer.
• Plant closely enough together that they canopy quickly but far enough apart so that potato plants don't compete with each other. This will vary by site fertility, availability of irrigation and vigor of the variety you choose.

Growing decisions:

• Control weeds. Sunlight that weeds intercept cannot make potatoes.
• Control insects. The leaves they eat cannot intercept sunlight.
• Irrigate through early and mid-summer, if necessary, to ensure plants don't shut-down for lack of moisture.
• Back off irrigation in late-summer (mid-August for me) to ensure natural die-down.
• Harvest AFTER natural die-down. Your vines will squeeze every bit of goodness out of the vines and pack it into the tubers...if you let them. Some growers like to leave the tubers in the ground for a couple of weeks so they can harden up.

It is difficult to over-emphasize the importance of good sunlight. "Out west" blows the doors off eastern potatoes for specific gravity because they have clear skies and intense sunlight many more days of the year than us "back east". If a grower in Idaho or Washington is striving for high specific gravity rather than maximum tonnage, specific gravities of 1.120 are well within reach.

Recap

The nutritional density of potatoes can vary by a factor of three.

It is lunacy to grow, dig and store, cook and eat three times the mass of potatoes when the same gross amount of nutrition can be captured in half or a third of that amount.

Nutritional density of potatoes can be controlled, within limits, by the grower. The economic incentives for a subsistence gardener are significantly different than they are for a commercial farmer.

Attempting to survive solely on potatoes...without additional fats...is a huge amount of potatoes, even with the most nutritionally dense potatoes available.

Let me repeat, if potatoes are a cornerstone of your subsistence gardening plans, you need to also have a plan to add fats to your diet.