They are made from Muscat Grapes, which were the primary grape crop in Malaga and Valencia, Spain.
The problem with Muscat Grapes was that they weren’t seedless. There was extra work in processing in taking the seeds out, and with the raisin’s skin penetrated, the raisins got sticky and would stick together in the package. Consumers had to first separate them all before using. Now that there are seedless grapes such as Thompson which produce seedless raisins, both Muscat Grapes and raisins have fallen out of favour.
Muscat Grapes and raisins are now considered a specialty item, but they are still prized because their taste is still better than that of seedless alternatives such as Thompson.
The Muscat Grapes are treated with sodium hydroxide lye before drying. The seeds are removed by machines, by passing them through rollers with hooks.
The United States Department of Agriculture development programme in Fresno, California has recently (2000 & 2001) developed seedless Muscat Grapes (Selma Pete and Diamond Muscat), so production of Muscat Raisins may increase again as the vines get established.
Wine tasters often allude to the taste or aroma of Muscat Raisins when trying to describe something else. e.g. “with the intense lift of Muscat Raisins.”
Muscat Grapes were widely grown in Malaga and Valencia, Spain. Muscat Raisins were the ones most grown until Thompson seedless came along in 1872.