Switch out your existing tomato plant for sweeter varieties and optimize growth conditions. For sweeter tomatoes, choose varieties such as Sakura, Sun Sugar, and Rosada.
Planting early and harvesting it after it matures also makes your tomatoes sweeter. Even color plays a key role, with orange and yellow varieties tasting sweeter than red ones.
Excessive sunlight and high temperatures during the growth stage will also cause the fruit to turn sour, as will overwatering your tomatoes. Be sure to prune regularly as well.
To fix soil issues, amend the soil with organic compost and reduce nitrogen. Apply potassium and phosphorus during the ripening stage and water less when the fruit matures.