Perfumers Apprentice made a great job to find out and explain more about Diacetyl contra Acetyl and Acetoin:
Custard Ingredients in Flavors
NEW UPDATE 07/20/14
There is another custard note that has not had as much attention as the others, and that ingredient is Butyric Acid. Butyric acid is found naturally in butter and milk, as well as certain cheeses, and it is sometimes used at low levels to accentuate a butter note. It is not as commonly used at as high levels as Acetoin and Acetyl Propionyl, because if overdone it can lend a rather unpleasant aftertaste to a flavor. There are companies who are now "re-working" vanilla flavors using it more in place of Acetoin and Acetyl Propionyl, but in reality Butyric Acid can also be irritating when inhaled. We have listed a new flavor, Vanilla Bean Gelato, which has no Acetoin or Acetyl Propionyl, and uses Butyric Acid instead.
Another update; Diacetyl can be found naturally in certain fruit extracts, so there is also the potential for trace amounts to be found in some natural extracts.
The potential may be less for concentrated flavors not made with natural extracts.
UPDATE 12/1/10 ..........
I have been doing more research, and I discovered that the molecule Acetoin can under certain circumstances "catalyze" into containing minute traces of the molecule Diacetyl. When I spoke to a chemist about this, she confirmed that this can happen during the production of the molecule, but is unlikely to happen after a flavor is blended. I then had a sample of Acetoin purchased from Vigon International analyzed by GC/MS and yes, it did have trace amounts of Diacetyl! When I looked at the online specification sheet that Vigon posts, it does show that the purity of that particular product is 93-100%. Nearly every molecule that is commercially sold is somewhere in this range for purity, and this seems to be an acceptable industry standard. (Vigon is a very reputable company!) Of course, Diacetyl is a perfectly acceptable Food Flavoring, so for the Flavor and Fragrance Industry there is absolutely no problem with this particular trace occurence in the production of Acetoin. It is completely normal. But for customers wanting to avoid even trace amounts of Diacetyl, this means that any flavor that contains the ingredient Acetoin can potentially also contain trace amounts of Diacetyl, even though Diacetyl itself was never added to the blend as an ingredient. Because of this new information, I will add to the flavor descriptions of those flavors that I know contain Acetoin, so that customers can avoid those flavors if they wish. Of course, if only a small amount of acetoin is in a flavor, then the potential likelihood of there also being diacetyl is very very small.
Introduction: I have received some calls from customers who wish to learn a bit more about some of the ingredients used in flavors. Specifically, there are 2 ingredients that have people curious. Acetyl propionyl and acetoin. Both of these ingredients are replacements for diacetyl, which is no longer being used by many (but not all) flavor manufacturers. There are some who still include diacetyl in their flavors, but it is not used as an ingredient in any of the flavors we offer. Athough acetyl propionyl and acetoin are regarded as safe for use in Food Flavors, there are studies being done as to whether there are problems with inhalation. I am no expert by any means on sensitivity and safety issues, I simply provide flavors that are Food Safe, but when customers call me with questions, I try to help in any way i can. Any time you have a flavor that has a vanilla-custardy type note to it, it will have one or both of acetyl propionyl and acetoin. There is also the possibility that there will be Diacetyl as an added ingredient, (the flavor manufacturing company that we purchase our flavors from is restricted by their insurance from using Diacetyl as an ingredient, because of employee environmental health issues, but this is not true of all flavor manufacturing companies). The flavor notes of diacetyl, acetyl propionyl and acetoin are rather obvious, and once a person learns how to recognize them, they are easy to spot. Just like once you know what cinnamon tastes like, you can usually tell if a cocoa recipe has cinnamon in it just by taste! Our vanilla custard is a good example. Pretty much by definition, a vanilla custard flavor, no matter who makes it, will have both acetoin and acetyl propionyl (or diacetyl) in it. Just like it would be hard to bake a cinnamon cookie with no cinnamon, it would be really hard (pretty much impossible) to create a vanilla custard flavor with no acetoin or acetyl propionyl (or diacetyl). If you would like to learn a bit how to tell the differences, you can compare our Vanilla Custard (which has both acetoin and acetyl propionyl), with our Vanilla Bean Ice Cream. The Vanilla Bean Ice cream does not have any acetoin in it, but otherwise is the same flavor as the Vanilla Custard. So by tasting the two, you can learn to taste the difference between them and learn what acetoin 'brings to the party'. I like to experiment by tasting flavors in Cool Whip. This is a good way to learn about ingredients. Remember that both Acetoin and Acetyl Propionyl are perfectly safe for food-use, so there is absolutely no problem at all with tasting these flavors in Cool Whip or water or any other food. Then you can taste our original French Vanilla Flavor, which has neither acetoin nor acetyl propionyl, and compare with our French Vanilla Deluxe, or the Vanilla Beach Ice Cream, or the Vanilla Custard, which has them. You can see how the flavors are different. By learning how to recognize these flavor components, you will be more able to recognize them in flavors by other manufacturers too! Sometimes only a little bit of these "custardy" notes are added, just as a subtle nuance. For example our waffle has a little bit, but not nearly as much as the Vanilla Custard. So to help people, we are marking the main flavors that we know contain these two ingredients, so that if you choose you can avoid them altogether! Unfortunately, we might not know of all the flavors that may contain tiny "trace" amounts, but we marked them as best we know.