While we were in the near-by gourmet grocery store, my wife spotted "Japanese yam" from California. It is called satsuma-imo 薩摩芋 in Japan. The name "Satsuma" came from the old local jurisdiction which is now a part of Kagoshima prefecture 鹿児島県 where this type of yam was widely grown. Satsuma-imo, which supposedly originated from South America, is not endogenous to Japan and is said to have come to Southern Japan in 17th century from the Philippines via China. Compared to the more popular yam we usually get, Satsuma-imo has a redder skin and white/yellow flesh.
Satsuma-imo is associated with a fond nostalgic memory for my wife. Many years ago, we went to Sapporo 札幌 in the winter. It was very cold as most winters are in Sapporo and there was deep snow on the ground. We were visiting one of my colleagues, who was considerably older than we were. We were all sitting around the gas space heater which was the characteristic source of heat in Hokkaido houses at that time when my colleague's wife hopped up and said she had the perfect snack for us. She produced two large Satsuma-imo and wrapped them in aluminium foil. Then she popped them into a contraption on the heating stove (although details how she cooked the potato are foggy). She explained that this (yaki-imo 焼き芋 or grilled yam) was a favorite snack for young girls of her generation. According to her, they cooked them in the coals of the heat stove and ate them after school on cold days. We sat together in companionable conversation while they cooked and the air became filled with the sweet smell of potato. After some time she fished them out of the heat and opened the foil. She broke open the red skins. The white/yellow flesh was soft and exuded sweet smelling clouds of steam. We all fell on them; devouring the hot pieces we broke off with our fingers laughing as we popped them in our mouths puffing out breath to keep them from scalding our tongues. She was 100 percent correct they were the perfect snack for that day, weather and company. That was the first time my wife ever tasted yaki-imo so now whenever my wife sees or tastes a yaki-imo, she remembers that experience with nostalgia.
Since we never really compared how Japanese and regular yam taste, we cooked them identically and had a "yam tasting". Both yams were wrapped in aluminium foil and placed in our Weber grill next to the chicken we were cooking using indirect heat. The temperature was kept around 360F (I monitor both the grill temperature and the food temperature). In about the last 20 minutes, it went up to 380F. In the picture below (the second picture), the one on the left (red skin) is the Japanese yam and the one on the right is the regular yam both after cooking.
As you can see the Japanese yam has yellow flesh and the regular yam has red/orange flesh.
Upon tasting, the Japanese yam has a nice slightly firmer texture (or "hokkori" ほっこり in Japanese parlance) and it is sweeter than the regular yam. Both my wife and I ate at least half of the Japanese yam before she had time to make mashed yam with butter and soy sauce as a side for our chicken dinner.
The regular yam is a bit softer and wetter and not as sweet as compared to the Japanese yam.
My memory of Yaki-imo is that, on cold winter days in Sapporo, we often saw a man (usually fairly old) drawing a cart down the street selling grilled yams. In the cart, the yams were cooked over charcoal in a large earthen pot lined with small round stones. Thus, this type of yaki-imo is called "Ishiyaki-imo" 石焼き芋 or "ishi" means "stone" i.e. "stone grilled" yam. I remember the vendor simply calling "Ishiyaaaaaa-ki imooooo" as he pulled his cart along. In addition, I do remember the cart also had a steam powered whistle which made a continuous shrill whistling noise. Apparently, now ishiyaki-imo is being sold on the street in a similar manner but, the hand-drawn cart has been replaced by a truck. Jon has posted a video of this. Ishiyaki-imo is, of course, cherished in Japan by women of all ages. I found a comprehensive comparative study using different cooking techniques by a group of young women. I also found that special Yaki-imo aluminum foil is being sold in Japan, which absorbs heat efficiently shortening cooking time and enhancing the flavors of yaki-imo.