Americans draw their cuisine from a vast array of cultures, but on Thanksgiving most of us will sit down to eat essentially the same foods: roast turkey with cranberry sauce, stuffing, mashed potatoes with gravy, green bean casserole, dinner rolls and sweetpotatoes topped with melted mini marshmallows, followed by pumpkin pie for dessert.

Although you may call them candied yams, if they were grown in the U.S., the soft, orange roots you've smothered in butter and brown sugar are sweetpotatoes. If you don't add butter and sugar, sweetpotatoes are a healthful sweet treat, loaded with vitamins A, B-6, C and E and potassium and manganese.

In California, sweetpotato classes come in four colors:

• Tan Jewell with orange flesh
• Light yellow Jersey with white flesh
• Purple Oriental with white flesh
• Red Garnet with deep orange flesh

Scott Stoddard, UC Cooperative Extension advisor who evaluates sweetpotato varieties for production in California, is trying to identify lines with red and purple skin that grow and store well. It is a high priority for the industry because the Garnet class dominates many markets throughout the state. "Apparently, Californians like well-tanned sweetpotatoes, especially in Southern California," said Stoddard, who is based in Merced County.

“It's the same eye appeal as when people choose peaches,” Stoddard explained. “People tend to choose peaches with more red blush to the skin. Just as we tend to choose more colorful peaches, red-skinned sweetpotatoes are very popular, even more than the traditional tan skin sweetpotatoes that dominate in the rest of the United States.”

“The red variety is in high demand in LA, the Bay Area and Seattle markets,” said Jason Tucker, a grower and vice president of the CA Sweetpotato Council. “With our climate and sandy soil, we can grow any variety in California. It gives us so much flexibility and we're looking for new varieties. We can grow distinctive varieties, such as the Oriental variety, to meet demands even in other countries like Korea and Japan.”

Louisiana State University and North Carolina State University have the only two sweetpotato breeding programs in the U.S. For more than 50 years, UC has collaborated with the two universities in the National Sweetpotato Collaborators Trial, in which varieties are grown and evaluated in several states. Because the breeders are mainly interested in sweetpotatoes with light-colored skin, they used to discard the others. In 1998, Stoddard began screening their castoffs for high yields and sweet flavor in other colors.

“Sweetpotatoes are expected to be on shelves 12 months a year so we need one that will store up to a year,” said Stoddard. Sweetpotatoes are harvested from July through October so he is looking for a red-skinned sweetpotato that offers better nematode resistance and holds up in storage longer. “The white ones and Orientals store well, but the reds break down in June and July.”

In addition to high yields and consistent flavor in a variety, Tucker said, “We are looking for varieties that maintain a high level of sugar, or sucrose levels, smooth skin and a consistent shape with a distinct color – purple, red or white.”

Before Stoddard introduces a new variety to a California field, he has the plant material virus-tested by Foundation Plant Services at UC Davis.

A purple sweetpotato with white flesh called Murasaki is a variety that emerged from Stoddard's test plots in 2008 and is now being grown commercially. Burgundy, released in 2014, is the first variety with red skin and orange flesh released in 30 years.  While Burgundy has better nematode resistance and improved storage characteristics, yields have failed to reach the numbers necessary to be commercially successful.

Sometimes when looking for a new red-skin cultivar, the UC Cooperative Extension advisor finds other desirable qualities. Stoddard is most excited about a tan-skin sweetpotato that he named Bellevue, after a road near the field where the variety was first evaluated and connects Atwater and Merced. “This variety has shown excellent yield potential combined with superior shape and skin quality in numerous test locations around the world, including North Carolina, Senegal, Israel and Australia,” he said. “It also has high nematode resistance and nice color.  But it's not red, though ironically it came out of the red-skin trial.”

“It takes a few years to see if they will be successful,” Stoddard said. “Things always show up once you move from a small plot to millions of plants, that's when their true personality comes out. We'll see how they do in different soil and in different areas. All varieties have their own personality.”

About 300 acres of Bellevue were grown in California in 2014. Although it may only be grown in California, Bellevue is patented by LSU. The varieties are patented by LSU and NC State to provide funding for their breeding programs.

“Scott is a highly valuable asset to Merced County growers, he provides all of our sweetpotato research,” said Tucker. “He brings new varieties from North Carolina and Louisiana to California to find a new variety that works well in our soil and climate. He analyzes them so we can make better decisions about which varieties we will grow.”

About 90 percent of the estimated 20,000 acres of sweetpotatoes grown in the state are in Merced County, around Atwater and Turlock, where the soils are sandy.