Sweet Potatoes

By Farmer Richard

Sweet potatoes are a tropical plant originating in South America. The remnants of sweet potatoes have been found in Peru dating back 10,000 years and there is evidence of cultivation in Central America at least 5,000 years ago. Cultivated sweet potatoes spread to New Zealand, Polynesia and Africa. Today, Uganda is the second largest producer of sweet potatoes behind China.  In this country, sweet potatoes have been traditionally grown in the southeast. North Carolina is the leading producer, with California in second and Louisiana and Mississippi also being significant producers. 

Southern farms ‘plant’ selected sweet potatoes taken from last year’s harvest pretty close together in a bed of sawdust or peat moss.  The tubers send up green shoots which are cut off (called slips) and sent to us in bundles of 25 each.  They don’t look very good when we get them, but if we get them planted promptly, most of them will grow! 

Sweet potato blossoms

Farmer Richard digging sweet potatoes at our Harvest Party!

Researchers continue to experiment with new varieties. A new variety is created by cross pollinating flowers and planting 2 – 4 seeds that a flower produces.  We continue to trial them when the slips are available to us.  Our favorite slip producer is New Sprout Organic Farms. This year they offered some new varieties that we trialed. As we dug them this year, we looked at ‘marketable yield’, like tuber shape, set (how many tubers per plant), and color, both inside and out. Varieties ‘set’ 6-8 tubers in a banana like cluster from the main stem.  If 5-6 grow to a nice shapely size, it will be a good yield.  If only 2 or 3 fill out and one a 4 pound jumbo, maybe not as good.  It may be a photo opportunity at our harvest party when a 40 pound child joyfully lifts out a 5 pound sweet potato, but those jumbo’s may intimidate other CSA members who may not know how to cook a ‘monster’ that size or know how easily it will reheat in the oven. So, we try to avoid the ‘monsters’ by planting some varieties closer together, like 8 inches versus 12 inch to keep them to a manageable size!  Every variety has its learning curve. And of course every year has different growing conditions, so varieties need careful evaluation over time!

Plastic bed ready for sweet potatoes!

Newly planted sweet potato slip

Around the world there are 1,000’s of different sizes, colors and shapes of sweet potatoes, from white to yellow and orange to deep purple. But, since they are a tropical plant, we are very limited in what we can grow in Wisconsin. First, we use a system of dark colored plastic on a raised bed to hold extra heat in the ground and the plastic limits the rain water to a plant that thrives on limited moisture. We are limited to the varieties that will mature in 90-110 day range. That eliminates the purple flesh and white flesh varieties that we have tasted and would like to grow, but only produce stringy ½ inch thick roots when we tried them.  Andrea wants to develop our own breeding program for them since no one that we know is working on that! While we may have limited options to choose from, some of the new varieties from sweet potato breeding programs from North Carolina and Louisiana do/may work for us. We have several new varieties this year that we could use your help in evaluating!  We need a certain level of successful yield of shapely, not too big not too small tubers, but we also value flavor!

Once we were limited to only ‘Georgia Jet’ variety that would produce sizable yield in the North, but oh so ugly! Then came ‘Beauregard’ which, if planted close (8 inches), yielded pounds but had limited numbers of nice “saleable” shapely potatoes.  The plus to Beauregard is that it had good flavor! Then we found ‘Covington’, gave it 12 inch spacing and we got a much higher percentage of shapely tubers but don’t forget the flavor! We like naturally sweet sweet potatoes without added sugar or even maple syrup. We like the deep orange flesh color which has higher lycopene. But, there are other factors to consider. Different varieties of sweet potatoes have very different levels of at least 3 sugars, sucrose, maltose, and fructose. Each gives us a different perception of sweetness and they have different flavor profiles.   The sugars also change during the curing process.  Curing, yes that is also very important!

Harvesting sweet potatoes

Because Sweet potatoes are tropical, they are a perennial and never stop growing, so when we harvest them, their skin is very thin and delicate and can come off or be broken with any rough handling.  We gently lift and pull the banana-like cluster by the stem from the soil.  Then each bunch is placed (with cotton gloves) into the crate that will transport it to the ‘curing’ room.  Curing is a process we put the sweet potatoes through where we hold them at a high temperature of 85-90°F with 90-95% humidity to thicken the skin and heel any harvest scrapes.  The curing process also concentrates and converts the sugars. We measure the sugar with a refractometer and with the older varieties we generally see a Brix (unit of measurement) of 3-6% directly out of the field.  After 7 days curing, the Brix level increases to 10-12%.  That is a sugar level that I think says “add only a little butter and it is delicious!”

So, our ‘from the field’ Brix test on our new trial varieties is quite interesting!  Our recent ‘standard’ ‘Covington’ came in at the usual 3-5%, but several of the new varieties came in at 8-10% Brix. Wow, if that doubles in storage we have a whole new ballgame!  However, I was very surprised to measure the Brix after 6 days of curing and found that two of the varieties that originally had high levels had dropped!  What’s going on!?  The best I can conclude is that it’s not just a matter of looking at total sugars in the potato.  It depends on which sugars are in the potato and their ratios.  Sucrose gives us a stronger sense of sweetness, so even if the overall sugars are lower in a potato with a high percentage of sucrose, it might be perceived as being sweeter than another variety that had a high Brix level.  The bottom line is we have to eat them and evaluate each variety individually.  Please help us evaluate these new varieties!  There are other factors that Brix readings cannot account for. Differing levels of the different forms of sugar also may lend different flavor qualities to the different potatoes.  Also, when doing the pre-curing Brix test, I noticed quite a difference in texture, like when in the garlic press to squeeze them for juice for the refractometer; some were soft and juicy while others were much firmer and dry. These factors may affect cooking time and you may consider one texture more desirable than another. We want to know your observations; even a simple email would be appreciated.

Chart of Farmer Richard’s Brix testing. He tested two potatoes from each variety, each of those test numbers are listed in the corresponding cell.

Variety Name	Pre-Curing Brix Number	6 day Curing Brix Number	Richard’s visual notes
Covington	4.5/2.5	8.0	Light Orange
Bellevue	7.5/7.5	4.0	Dark orange/tan skin & small, but good yield
Burgundy	9.0/8.5	8.0….hmmm	Intense orange/ dark burgundy skin
Orleans	10.0/8.0		Orange/red
Carolina Ruby	8.5/9.0		Orange/dark red/some rot

Can we develop our own regional sweet potato variety, absolutely yes! Do we need a global distribution network with unknown inputs and unknown or known consequences, absolutely not! We can eat the best from our region with known inputs and know how it affects our environment and our fellow human beings!