Where would one go to locate reputable scientific studies on different subjects?

There is no quick fix for this kind of research. Unless you are already working in the field of interest, figuring out what the consensus is on a particular topic will require reading many research articles. If you are not affiliated with an institution (college or university), this probably means going to Google Scholar (http://scholar.google.com/) and typing in key words to do with your question.

However, your question is not really a scientific one… it’s a little more complicated. You want to know what kinds of preservatives are needed for specific meats (so that’s a scientific question), and you want to know what preservatives fast food chains are using for those meats (This is more of a news- or business-related question. Scientific papers might not be able to help you here.). Then you want to know why they wouldn’t follow the guidelines, if they don’t (One might assume for monetary reasons, but is there any source that can accurately answer that question?).

Where I would start is to type the keywords preservatives “fast food” into Google Scholar. But note that some of the results will be books, which imply opinion, and some will be editorials, and some from periodicals that are not peer reviewed. So have a look not only at the articles, but also at where they were published. After reading a dozen or so papers, you might start to get an idea of what the consensus is… but it’s also important to see what other articles the ones you found are citing. Have a look through their references, and see if you can spot the same ones appearing over and over again. Those are likely to carry more weight.

Regarding access, you might not be able to read all of the articles from home without paying – try requesting articles through your library, or doing your research on the guest computers at a local university.

Thank you @glacial. Google Scholar is book marked.

Wow. First hit is an article saying a lawsuit ”...brought by a group of overweight children against the McDonald’s Corporation that seeks compensation for obesity-related health problems…

http://content.healthaffairs.org/content/22/6/207.short

That’s INSANE! These children are ordering what they want and their parents are paying for it! I don’t know where it went (I’ll look some more) but I hope that all common sense prevailed.

Actually, the best place to go for this type of information is… to the companies themselves. If you can’t find the specific information on their websites, you can always write to the Customer Service representatives that they generally also publicize on their sites and ask your specific question about ingredients.

For example, just off the cuff I did a look into McDonalds, who have an easy to find link for What we’re made of that seems to answer a lot of questions and invites readers to submit more if they have them.

Companies may from time to time not want to reveal certain facts about their products, but I don’t think that there are many in the US that will out and out lie to you. So that’s a good place to start, I think: just ask them.

EDIT: (Still at McDonalds) You can also do a search for “preservatives” at the main page to get these results and start digging there.

Just for fun: (Bolded words are in all three styles)

McDonald’s “Bakery Style Bun”:
Ingredients: Enriched Flour (Bleached Wheat Flour, Malted Barley Flour, Niacin, Reduced Iron, Thiamin Mononitrate, Riboflavin, Folic Acid), Water, Whole Wheat Flour, High Fructose Corn Syrup, Honey, Wheat Gluten, Contains 2% or Less of: Yeast, Soybean Oil, Maltodextrin (Dietary Fiber), Salt, Calcium Sulfate, Dough Conditioners (Wheat Flour, DATEM, Ascorbic Acid, Azodicarbonamide, Sodium Stearoyl Lactylate, Mono and Diglycerides, Enzymes, Calcium Peroxide), Ammonium
Sulfate, Natural Flavor (Botanical Source), Calcium Propionate (Preservative), Vitamin D3, Rolled Wheat (Topping).

Arnold Select Hamburger Bun ($4.75 for a package)
Enriched Wheat Flour (Flour, Malted Barley Flour, Niacin, Reduced Iron, Thiamin Mononitrate (Vitamin B1), Riboflavin (Vitamin B2), Folic Acid), Water, High Fructose Corn Syrup, Yeast, Soybean Oil, Salt, Wheat Gluten, Monoglycerides, Calcium Propionate (Preservative), Calcium Sulfate, Grain Vinegar, Monocalcium Phosphate, Datem, Soy Lecithin, Sesame Seeds, Soy Flour.

Generic local grocery store brand ($1.99 a package)
enriched flour bleached (wheat flour, malted barley flour, niacin, reduced iron, thiamin mononitrate, riboflavin, folic acid), water, high fructose corn syrup, yeast. contains less than 2% of each of the following: wheat gluten, salt, calcium sulfate, monoglycerides, calcium propionate (preservative), monocalcium phosphate, calcium stearoyl lactylate, sodium stearoyl lactylate, soy flour, enzymes, ammonium sulfate, citric acid, ascorbic acid, added as a dough conditioner

Seems Mickey D’s is actually a little “higher quality” than the generic grocery store brand.

You could try wikipedia. Scroll through the articles until you get to the references and foot note section at the bottom.