Hmmm. We sent your question off to the helpful people at the US Apple Association, and a couple of food scientists they contacted hypothesized the your friend must have been referring to the use of sulfur-based preservatives, or sulfites, in home canning.

Sulfites have historically been used to inhibit browning of fruits and vegetables, including apples (they are also used more broadly in food manufacturing and processing as a preservative, anti-bacterial agent, etc.). The FDA labeled sulfites as “generally recognized as safe” as a preservative and food additive in 1958. But, beginning in the 1980s consumer complaints of allergic reactions to sulfites began to surface. As a result, in 1986 FDA prohibited the use of sulfites on fruits and vegetables to be eaten raw (which is why you’ll see “No sulfites” notices on salad bars). The agency also required that food manufacturers list sulfite as an ingredient on product labels if found above a certain tiny threshold. Now, knowing all that, do you really want to add sulfites to your apples?

There are alternatives to sulfites to prevent browning. The US Department of Agriculture’s home canning guide recommends bathing cut fruits and vegetables in an ascorbic acid solution to retard browning. Ascorbic acid is typically found with canning supplies, or you can make your own by crushing Vitamin C tablets.