I've heard that skimmed milk has more added chemicals than whole milk. Is this true?
The prescriptionist scientist in me rejects the question as meaningless! The descriptivist in me compels me to try to answer the question you meant to ask.
Both whole milk and skim milk (in the USA) may be fortified with vitamins A and/or D.
Sharon Gerdes wrote in Dairy Foods (Feb 2009):
The dairy industry has been adding vitamin D3 to milk since 1932. In the 1940s, the dairy industry also began fortifying with vitamin A.
They can also be fortified with other nutrients such as Calcium.
The fortification is optional, so it depends on the source. So, to see what additives are in your preferred brands of milk, check the labels. Before you discard the ones with more Vitamin A and D, ask if you are really concerned that the risks of extra vitamins outweigh the benefits - ditto with the fat content.
However, Cornell University's Milk Facts suggests a motivation for additional Vitamin A fortification in skim milk: because it is naturally lower in vitamin A, it needs more added to bring it to the same levels as whole milk.
Whole milk is considered a good source of vitamin A. Vitamin A is a fat soluble vitamin that is found in the fat phase of milk. The vitamin A content that occurs naturally in 2%, 1% and skim milk is less than in whole milk because of the lower fat levels. Nutritional concerns about consumption of lower fat milk in the late 1970s led to the required fortification of vitamin A in lower fat milks. To achieve the nutritional equivalence of whole milk, lower fat milks should be fortified to 300 IU vitamin A per 8 oz serving. The FDA encourages fortification to a level of 500 IU of vitamin A per 8 oz serving, which is 10 % of the recommended daily allowance (RDA).
Letting my inner prescriptionist off his leash for a moment:
In science, the word "chemical" means the same as chemical substance.
(Ref: High school chemistry, plus the above Wikipedia article that states: 'the term chemical substance is a precise technical term that is synonymous with "chemical" for professional chemists'.)
Under this definition, everything is either a chemical substance or a mixture of chemical substances. (Okay, not everything. Not light or heat or love or skepticism, but everything made of solids, liquids, gasses or plasma. Everything you can touch, taste or smell.)
So, 1kg of whole milk is 100% chemicals, and 1kg of skim-milk is 100% chemicals. No difference.
In the USA to be called "skim milk," it must have vitamin A and D added to replace what is lost during skimming. (They are fat-soluble, and hence lost when skimming off the fat.)
That is, according to the Food and Drug Administration's definition, skim milk is milk with reduced fat but not reduced vitamin content. (It isn't defined as simply milk that has been through the process of skimming off cream.)
If skim milk is produced by skimming off the fat without replacing the vitamins A and D, then it must be called "imitation skim milk" or "imitation milk product."
The challenge aims to vindicate the right of the Creamery to use an honest, clear label on its all-natural, additive-free, pasteurized skim milk. The Creamery cannot do so in Pennsylvania because of FDA regulations mandating that skim milk sold across state lines may only be called “skim milk” if other ingredients are added to it. Pure skim milk without additives is banned by the FDA regulations from being described as skim milk and must instead be labeled as “imitation.”
While I certainly agree with the position that everything is inherent chemicals, I don't believe that you can hand wave away the prescriptivism vs descriptivism argument, or claim that it's just about being "scientifically accurate" when it comes down to the question of "what does skim milk mean." (Though that distinction may have some value in the case of other, more esoteric terms.)
Skim milk is usually made by centrifugation of whole milk. Otherwise the fatty cream is allowed to rise naturally, and skimmed off. Neither process involves adding new chemicals to the milk, or anything other than separation based on physical properties. If you're counting types of chemicals in your milk, whole milk will be slightly higher, as it still contains all those fats and fat soluble materials that make up cream. As Oddthinking states, both whole and skim milk may be fortified by vitamins A and/or D.