Ponderosa lemon
The ponderosa lemon originated in roughly 1887, and is believed to come from a chance seedling grown in Hagerstown, Maryland. It was later named and introduced to the nursery trade in 1900. Though often referred to as a cross between the lemon and citron, a recent genomic analysis showed it to solely contain pomelo ("Citrus maxima") and citron ("Citrus medica") DNA, perhaps being an F2 hybrid. Thus they cannot derive from true lemons, which have a mandarin orange component.
Preserved lemon
Although lemons contain citric acid, most citric acid is produced by fermentation uses microbes that can convert sugars into citric acid, which is the most important organic acid produced in tonnage and is extensively used in food and pharmaceutical industries. It is produced mainly by submerged fermentation using "Aspergillus niger" or "Candida" spp. from different sources of carbohydrates, such as molasses and starch-based media. The food and beverage industries use this acid extensively as a food additive globally.