Wouldn't you know the answer is right here on the forum, in a thread from 2005. http://www.glutenfre....php/t8147.html
Turns out lactaid (and other foods and enzymes) is made by using a mold called aspergillus. I wouldn't have immediately decided this was the villain, except that I took a B-vitamin a few days ago, had a bad reaction, and it turns out that yes, some vitamins are made with aspergillus. (Note: I've been gluten-free about 2.5 weeks, and have never had a bad reaction to this brand of B vitamins before.)
For more info, check the thread cited above. I'll also re-list the aspergillus-containing items from that thread:
PRODUCTS OF ASPERGILLUS FERMENTATION:
CHOCOLATE that includes chocolate liquor
MALT EXTRACT (MOST CEREALS)
LACTAID (MILK ADDITIVE)
SOME VITAMINS - SOLGAR AND JARROW brands ARE SAFE; MOST ARE NOT
CITRIC ACID (A VERY COMMON FOOD ADDITIVE; NOT DERIVED FROM CITRUS FRUIT)
SOFT DRINKS (Citric acid is made with the aspergillus fermentation process)
FRUIT JUICE (COMMERCIAL-OFTEN MADE FROM MOLDY FRUIT; MANY CONTAIN MOLD ENZYMES ADDED IN PROCESSING)
DIGESTIVE ENZYMES (PANCREATIC ARE ALRIGHT), OTHERS ARE NOT
CHOLESTEROL LOWERING ENZYMES
ACIDOPHILUS WITH F.O.S. (Acidophilus has very healing effects and promotes natural anti-fungals in the body, however, artificially produced F.O.S. is made with the aspergillus fermentation process. If you would like natural F.O.S., you can consume watermelon seeds and reap the same nutritional benefits without any aspergillus fermentation).
Enzymes and what the enzymes are made of:*
Alpha-galactosidase ----- Aspergillus niger
Amylase ----- Bacillus subtilis, Bacillus amyloliquefaciens, Aspergillus oryzae, Aspergillus niger
Amylase ---- (Malt diastase) malt
Amyloglucosidase ---- Aspergillus niger, Rhizopus oryzae
Beta-amylase ---- wheat, Bacillus spp.
Catalase --- Aspergillus niger
Cellulase --- Aspergillus niger, Trichoderma longibrachiatum (reesei)
Glucose oxidase ---- Aspergillus niger
Hemicellulase ---- Aspergillus niger, Trichoderma longibrachiatum (reesei), Aspergillus oryzae, Bacillus subtilis
Lactase ---- Aspergillus oryzae, Kluveromyces lactis
Lipase ---- Aspergillus niger, Arthrobacter ureafaciens, Candida cylindracea, Rhizomucor miehei, Rhizopus oryzae, Rhizopus delemar
Lysophospholipase ---- Aspergillus niger
Pectinase ---- Aspergillus niger, Aspergillus japonicus, Rhizopus oryzae
Phytase ---- Aspergillus niger
Protease, microbial ---- spergillus oryzae, Aspergillus niger, Aspergillus melleus, Bacillus licheniformis, Bacillus subtilis, Bacillus thermoproteolyticus, Rhizopus niveus
* As provided by member companies of the Enzyme Technical Association.
Beano was also mentioned.
Here are some links the original poster referred to:
Purely in the interest of science (har har), right now I'm testing whether I now react to chocolate liquor (which I confirmed is in my favorite Ritter Sport Marzipan bar).
A big thank you to Stephanie (skbird), who originally posted all of that incredibly useful information.