Whether it is in light or not is unlikely to make a difference in sweetness. If they do not eventually sweeten up--and they now look ripe--and they do this next year as well, I would follow Drew's recommendation and find mulberry trees whose fruit you do like and graft onto your present tree. It is possible that the tree will sweeten up over 4 years in the ground or so, but it is more likely a bum tree.

It's a recipe I got from this site, but I don't know who to attribute it to. Here's the recipe I cut and pasted, with my addition of the sugar. SALSA VERDE 7 Cups chopped, cored, green Tomatoes 5 to 10 Jalapeno Peppers, seeded and finely chopped 2 Cups finely chopped Red Onions 4 Cloves Garlic, finely chopped ü Cup Sugar 1/2 Cup Lime Juice 1/2 Cup loosely packed finely chopped Cilantro 2 Teaspoon ground Cumin, more to taste 2 Teaspoon dried Oregano 2 Tablespoons Salt 1 Teaspoon freshly ground Black Pepper In a large stainless steel saucepan, combine tomatoes, peppers, onions, garlic and lime juice. Bring to a boil over medium-high heat, stirring constantly. Stir in cilantro, cumin, oregano, salt, sugar and black pepper. Reduce heat and simmer, stirring frequently, for 5 minutes. Remove from heat. Ladle hot salsa into hot jars, leaving 1/2 inch headspace. Remove air bubbles and adjust headspace, if necessary, by adding hot salsa. Process both sizes in a BWB for 20 minutes. Makes about six 8-oz jars or 3 pint jars Obviously I doubled the recipe to accommodate the amount of tomatoes I had to use. As I said, it's quite good, just a little sour for my taste. Steve

An older post but still a valid contribution. Sugars will help with sour and acidic foods. Apple is full of sugar, especially cider, but a natural sugar. I often us a bit of maple syrup. I just made a rhubarb chili. Very well knowing i would be dealing with a 'sour/tart' flavor. I used a couple grated carrots and an 1/2 apple i had leftover from a fruit salad... Still a bit tart and way too spicy by over-doing the chipotle, i sauteed another onion and a cup of edamame and a can of garbanzo beans...delish finally. With a dollop of yogurt, fetta, fresh cilantro cream, it paired well with the spicy tart chili. A left over chunk of parm rind is so good in the last 45 min of any hearty soup. I often add when i'm heating up a frozen qrt of homemade.

Probably it's the root stock that's grown when the top graft died. In other words, most named citrus varieties are grafted onto a root stock of another variety. Usually the root stock will be more vigorous or be better adapted to the soil, or better disease resistance. Anyway, often the top part dies for some reason and the bottom root stock will start to grow it's own leaves and fruit. Your's looks a lot like the root stock "Cuban Shaddock", it's a pomelo/lemon cross. it has better fruit than some root stocks and can be a decent lemon substitute. It's not a "Yuzu", for sure. Too bad your neighbors cut theirs down. "Yuzu" is a nice fruit. Yes it's sour, but so are lemons and limes. It's used for cooking and the peel is very aromatic. I love mine. I also like my "Cuban Shaddock", it's also good for cooking with.