I just did a search of 'Kosher' on steem, and came across a common misconception. Kosher food is not 'Blessed by a Rabbi.' It is food that have been certified as being made, packaged, and meeting the laws of Kosher, which can be very complex. There are also different levels of Kosher observance, which complicates it further.
The following is an excerpt from http://www.jewfaq.org/kashrut.htm which is a great starting point. There are volumes of books on the subject getting into the tiniest details. Anyway if you are interested please read on.
General Rules
Although the details of kashrut are extensive, the laws all derive from a few fairly simple, straightforward rules:
Certain animals may not be eaten at all. This restriction includes the flesh, organs, eggs and milk of the forbidden animals.
Of the animals that may be eaten, the birds and mammals must be killed in accordance with Jewish law.
All blood must be drained from meat and poultry or broiled out of it before it is eaten.
Certain parts of permitted animals may not be eaten.
Fruits and vegetables are permitted, but must be inspected for bugs (which cannot be eaten)
Meat (the flesh of birds and mammals) cannot be eaten with dairy. Fish, eggs, fruits, vegetables and grains can be eaten with either meat or dairy. (According to some views, fish may not be eaten with meat).
Utensils (including pots and pans and other cooking surfaces) that have come into contact with meat may not be used with dairy, and vice versa. Utensils that have come into contact with non-kosher food may not be used with kosher food. This applies only where the contact occurred while the food was hot.
Grape products made by non-Jews may not be eaten.
There are a few other rules that are not universal.
The Details
Animals that may not be eaten
Of the "beasts of the earth" (which basically refers to land mammals with the exception of swarming rodents), you may eat any animal that has cloven hooves and chews its cud. Lev. 11:3; Deut. 14:6. Any land mammal that does not have both of these qualities is forbidden. The Torah specifies that the camel, the rock badger, the hare and the pig are not kosher because each lacks one of these two qualifications. Cattle, sheep, goats, deer and bison are kosher.
Of the things that are in the waters, you may eat anything that has fins and scales. Lev. 11:9; Deut. 14:9. Thus, shellfish such as lobsters, oysters, shrimp, clams and crabs are all forbidden. Fish like tuna, carp, salmon and herring are all permitted.
For birds, the criteria is less clear. The Torah provides a list of forbidden birds (Lev. 11:13-19; Deut. 14:11-18), but does not specify why these particular birds are forbidden. All of the birds on the list are birds of prey or scavengers, thus the rabbis inferred that this was the basis for the distinction. Other birds are permitted, such as chicken, geese, ducks and turkeys. However, some people avoid turkey, because it is was unknown at the time of the giving of the Torah, leaving room for doubt.
Of the "winged swarming things" (winged insects), a few are specifically permitted (Lev. 11:22), but the Sages are no longer certain which ones they are, so all have been forbidden. There are communities that have a tradition about what species are permitted, and in those communities some insects are eaten.
Rodents, reptiles, amphibians, and insects (except as mentioned above) are all forbidden. Lev. 11:29-30, 42-43.
Some authorities require a post-mortem examination of the lungs of cattle, to determine whether the lungs are free from adhesions. If the lungs are free from such adhesions, the animal is deemed "glatt" (that is, "smooth"). In certain circumstances, an animal can be kosher without being glatt; however, the stringency of keeping "glatt kosher" has become increasingly common in recent years, and you would be hard-pressed to find any kosher meat that is not labeled as "glatt kosher."
As mentioned above, any product derived from these forbidden animals, such as their milk, eggs, fat, or organs, also cannot be eaten. Rennet, an enzyme used to harden cheese, is often obtained from non-kosher animals, thus kosher hard cheese can be difficult to find.
Kosher slaughtering
The mammals and birds that may be eaten must be slaughtered in accordance with Jewish law. (Deut. 12:21). We may not eat animals that died of natural causes (Deut. 14:21) or that were killed by other animals. In addition, the animal must have no disease or flaws in the organs at the time of slaughter. These restrictions do not apply to fish; only to the flocks and herds (Num. 11:22).
Ritual slaughter is known as shechitah, and the person who performs the slaughter is called a shochet, both from the Hebrew root Shin-Cheit-Teit. The method of slaughter is a quick, deep stroke across the throat with a perfectly sharp blade with no nicks or unevenness. This method is painless, causes unconsciousness within two seconds, and is widely recognized as the most humane method of slaughter possible.
Another advantage of shechitah is that it ensures rapid, complete draining of the blood, which is also necessary to render the meat kosher.
The shochet is not simply a butcher; he must be a pious man, well-trained in Jewish law, particularly as it relates to kashrut. In smaller, more remote communities, the rabbi and the shochet were often the same person.
Draining of Blood
The Torah prohibits consumption of blood. Lev. 7:26-27; Lev. 17:10-14. This is the only dietary law that has a reason specified in Torah: we do not eat blood because the life of the animal (literally, the soul of the animal) is contained in the blood. This applies only to the blood of birds and mammals, not to fish blood. Thus, it is necessary to remove all blood from the flesh of kosher animals.
The first step in this process occurs at the time of slaughter. As discussed above, shechitah allows for rapid draining of most of the blood.
The remaining blood must be removed, either by broiling or soaking and salting. Liver may only be kashered by the broiling method, because it has so much blood in it and such complex blood vessels. This final process must be completed within 72 hours after slaughter, and before the meat is frozen or ground. Most butchers and all frozen food vendors take care of the soaking and salting for you, but you should always check this when you are buying someplace you are unfamiliar with.
An egg that contains a blood spot may not be eaten. This isn't very common, but I find them once in a while. It is a good idea to break an egg into a glass and check it before you put it into a heated pan, because if you put a blood-stained egg into a heated pan, the pan becomes non-kosher. If your recipe calls for multiple eggs, break each one into the glass separately, so you don't waste all of the eggs if the last one is not kosher!
Forbidden Fats and Nerves
The sciatic nerve and its adjoining blood vessels may not be eaten. The process of removing this nerve is time consuming and not cost-effective, so most American kosher slaughterers simply sell the hind quarters to non-kosher butchers.
A certain kind of fat, known as chelev, which surrounds the vital organs and the liver, may not be eaten. Kosher butchers remove this. Modern scientists have found biochemical differences between this type of fat and the permissible fat around the muscles and under the skin.
Fruits and Vegetables
All fruits and vegetables are kosher (but see the note regarding Grape Products below). However, bugs and worms that may be found in some fruits and vegetables are not kosher. Fruits and vegetables that are prone to this sort of thing should be inspected to ensure that they contain no bugs. Leafy vegetables like lettuce and herbs and flowery vegetables like broccoli and cauliflower are particularly prone to bugs and should be inspected carefully. Strawberries and raspberries can also be problematic. The Star-K kosher certification organization has a very nice overview of the fruits and vegetables prone to this and the procedure for addressing it in each type.
Separation of Meat and Dairy
On three separate occasions, the Torah tells us not to "boil a kid in its mother's milk." (Ex. 23:19; Ex. 34:26; Deut. 14:21). The Oral Torah explains that this passage prohibits eating meat and dairy together. The rabbis extended this prohibition to include not eating milk and poultry together. In addition, the Talmud prohibits cooking meat and fish together or serving them on the same plates, because it is considered to be unhealthy. It is, however, permissible to eat fish and dairy together, and it is quite common (lox and cream cheese, for example). It is also permissible to eat dairy and eggs together.
This separation includes not only the foods themselves, but the utensils, pots and pans with which they are cooked, the plates and flatware from which they are eaten, the dishwashers or dishpans in which they are cleaned, the sponges with which they are cleaned and the towels with which they are dried. A kosher household will have at least two sets of pots, pans and dishes: one for meat and one for dairy. See Utensils below for more details.
One must wait a significant amount of time between eating meat and dairy. Opinions differ, and vary from three to six hours after meat. This is because fatty residues and meat particles tend to cling to the mouth. From dairy to meat, however, one need only rinse one's mouth and eat a neutral solid like bread, unless the dairy product in question is also of a type that tends to stick in the mouth.
The Yiddish words fleishik (meat), milchik (dairy) and pareve (neutral) are commonly used to describe food or utensils that fall into one of those categories.
Note that even the smallest quantity of dairy (or meat) in something renders it entirely dairy (or meat) for purposes of kashrut. For example, most margarines are dairy for kosher purposes, because they contain a small quantity of whey or other dairy products to give it a buttery taste. Animal fat is considered meat for purposes of kashrut. You should read the ingredients very carefully, even if the product is kosher-certified.
Utensils
Utensils (pots, pans, plates, flatware, etc., etc.) must also be kosher. A utensil picks up the kosher "status" (meat, dairy, pareve, or treif) of the food that is cooked in it or eaten off of it, and transmits that status back to the next food that is cooked in it or eaten off of it. Thus, if you cook chicken soup in a saucepan, the pan becomes meat. If you thereafter use the same saucepan to heat up some warm milk, the fleishik status of the pan is transmitted to the milk, and the milchik status of the milk is transmitted to the pan, making both the pan and the milk a forbidden mixture.
Kosher status can be transmitted from the food to the utensil or from the utensil to the food only in the presence of heat, (including hot spices) or prolonged contact, thus if you are eating cold food in a non-kosher establishment, the condition of the plates is not an issue. I knew an Orthodox rabbi who would eat ice cream at Friendly's, for example, because the ice cream was kosher and the utensils are irrelevant for such cold food. Likewise, you could use the same knife to slice cold cuts and cheese, as long as you clean it in between, but this is not really a recommended procedure, because it increases the likelihood of mistakes.
Stove tops and sinks routinely become non-kosher utensils, because they routinely come in contact with both meat and dairy in the presence of heat. It is necessary, therefore, to use dishpans when cleaning dishes (don't soak them directly in the sink) and to use separate spoon rests and trivets when putting things down on the stove top.
Dishwashers are a kashrut problem. If you are going to use a dishwasher for both meat and dairy in a kosher home, you either need to have separate dish racks or you need to run the dishwasher in between meat and dairy loads.
You should use separate towels and pot holders for meat and dairy. Routine laundering kashers such items, so you can simply launder them between using them for meat and dairy.
Certain kinds of utensils can be "kashered" if you make a mistake and use it with both meat and dairy. Consult a rabbi for guidance if this situation occurs.
Grape Products
The restrictions on grape products derive from the laws against using products of idolatry. Wine was commonly used in the rituals of all ancient religions, and wine was routinely sanctified for pagan purposes while it was being processed. For this reason, use of wines and other grape products made by non-Jews was prohibited. (Whole grapes are not a problem, nor are whole grapes in fruit cocktail).
For the most part, this rule only affects wine and grape juice. This becomes a concern with many fruit drinks or fruit-flavored drinks, which are often sweetened with grape juice. You may also notice that some baking powders are not kosher, because baking powder is sometimes made with cream of tartar, a by-product of wine making. All beer used to be kosher, but this is no longer the case because fruity beers made with grape products have become more common.
Additional Rules
There are a few additional considerations that come up, that you may hear discussed in more sophisticated discussions of kashrut.
Bishul Yisroel
In certain circumstances, a Jew (that is, someone who is required to keep kosher) must be involved in the preparation of food for it to be kosher. This rule is discussed in depth under Food Fit for a King on the Star-K kosher certification website.
Cholov Yisroel
An ancient rule required that a Jew must be present from the time of milking to the time of bottling to ensure that the milk actually came from kosher animals and did not become mixed with milk from non-kosher animals. Milk that is observed in this way is referred to as Cholov Yisroel, and some people will consume only Cholov Yisroel dairy products. However, in the United States, federal law relating to the production of milk is so strict that many Orthodox sources accept any milk as kosher. You will sometimes see high-level discussions of kashrut address whether a product is Cholov Yisroel or non-Cholov Yisroel. See a more complete discussion under Cholov Yisroel: Does a Neshama Good on the Star-K kosher certification website.
Mevushal
Most kosher wines in America are made using a process of pasteurization called mevushal, which addresses some of the kashrut issues related to grape beverages. See The Art of Kosher Wine Making on the Star-K kosher certification website.
Kashrut Certification
The task of keeping kosher is greatly simplified by widespread kashrut certification. Products that have been certified as kosher are labeled with a mark called a hekhsher (from the same Hebrew root as the word "kosher") that ordinarily identifies the rabbi or organization that certified the product. Approximately 3/4 of all prepackaged foods have some kind of kosher certification, and most major brands have reliable Orthodox certification.
The process of certification does not involve a rabbi "blessing" the food; rather, it involves examining the ingredients used to make the food, examining the process by which the food is prepared, and periodically inspecting the processing facilities to make sure that kosher standards are maintained.
Kosher Certification Symbols These symbols are widely-accepted hekhshers commonly found on products throughout the United States. These symbols are registered trademarks of kosher certification organizations, and cannot be placed on a food label without the organization's permission. Click the symbols to visit the websites of these organizations. With a little practice, it is very easy to spot these hekhshers on food labels, usually near the product name, occasionally near the list of ingredients. There are many other certifications available, of varying degrees of strictness.
The most controversial certification is the K, a plain letter K found on products asserted to be kosher. A letter of the alphabet cannot be trademarked, so any manufacturer can put a K on a product, even without any supervision at all. For example, Jell-O brand gelatin puts a K on its product, even though every reliable Orthodox authority agrees that Jell-O is not kosher. On the other hand, some very reliable rabbis will certify products without having a trademark to offer, and their certifications will also have only a "K." Most other kosher certification marks are trademarked and cannot legally be used without the permission of the certifying organization. The certifying organization assures you that the product is kosher according to their standards, but standards vary.
It is becoming increasingly common for kosher certifying organizations to indicate whether the product is fleishik (meat), milchik (dairy) or pareve (neutral). If the product is dairy, it will frequently have a D or the word Dairy next to the kashrut symbol. If it is meat, the word Meat may appear near the symbol (usually not an M, because that might be confused with "milchik"). If it is pareve, the word Pareve (or Parev) may appear near the symbol (Not a P! That means kosher for Passover!). If it says "DE" next to the symbol, this stands for Dairy Equipment, and indicates that all of the ingredients are pareve but the food was prepared on equipment that was also used to process dairy ingredients. This is similar to warnings you find about possible cross-contamination of allergens. If no such clarification appears, you should read the ingredient list carefully to determine whether the product is meat, dairy or pareve.
Kosher certification organizations charge manufacturers a fee for kosher certification. This fee covers the expenses of researching the ingredients in the product and inspecting the facilities used to manufacture the product. There are some who have complained that these certification costs increase the cost of the products to non-Jewish, non-kosher consumers; however, the actual cost of such certification is so small relative to the overall cost of production that most manufacturers cannot even calculate it. The cost is more than justified by the increase in sales it produces: although observant Jews are only a small fragment of the marketplace, kosher certification is also a useful (though not complete) point of reference for many Muslims, Seventh Day Adventists and vegetarians. In addition, many people prefer kosher products because they believe them to be cleaner, healthier or better than non-kosher products. It is worth noting that kosher certifiers are not the only organizations that charge for the privilege of displaying their on a product: some charitable organizations allow manufacturers to display their logo in exchange for a donation, but unlike kosher certifiers, those charities do not perform any service in exchange for that payment.
Do All Jews Keep Kosher?
According to the 2000 National Jewish Population Survey (NJPS), 21% of American Jews report that they keep kosher in the home. This includes the vast majority of people who identify themselves as Orthodox, as well as many Conservative and Reconstructionist Jews and some Reform Jews.
Part of that 21% keeps kosher at home, but eat non-kosher food out of the home to one degree or another. Some will eat cooked food in a restaurant or a non-kosher home, as long as the meal is either vegetarian or uses only kosher meat and no dairy products. Some will eat non-kosher meat in restaurants, but only if the meat comes from a kosher animal and is not served with dairy products. Some will go... dare I say? ... whole hog and eat bacon cheeseburgers out of the home while keeping a strictly kosher household.
Even within the home, standards of kashrut that people employ vary. The strictest people will eat only foods that have reliable Orthodox kosher certification, eating only glatt-kosher certified meats and specially certified dairy products. They will not eat cooked food in a restaurant unless the restaurant has reliable Orthodox certification, and they are unlikely to accept an invitation to dinner from anyone who is not known to share their high standards. Others are more lenient, accepting less reliable certifications without question or "ingredients reading," accepting grocery store items that have no certification but do not contain any identifiably non-kosher ingredients.
As rabbi/humorist Jack Moline noted, "Everyone who keeps kosher will tell you that his version is the only correct version. Everyone else is either a fanatic or a heretic." (Growing Up Jewish, 1987). There is a lot of truth in this humorous observation. I have no doubt that some are calling me a heretic for even acknowledging the existence of lower standards, because kosher is kosher, and if you don't live up to my standards then you aren't keeping kosher at all.
Kosher Links
You can find more information about kashrut at the websites of major kosher certification organizations.
The Orthodox Union, which is responsible for "OU" certification, has some excellent information on its website, including a kosher primer, an explanation of their kosher policy, a philosophical discussion about "thinking kosher" and a questions and answers section. (Please note: the "Judaism 101" listed on some of their pages is not this website and has no connection with this website).
The Star-K Kosher Certification organization also has an excellent website. The wonderful thing about Star-K is, they give you an incredible amount of detail about the research that they put into determining whether a product is kosher. They tell you what products may be used without kosher certification, and they explain why such products can or cannot be used without kosher certification, giving complete detail about the research that went into making their determination. It also has articles about kashering appliances, and other useful information.
KosherQuest has a searchable database of kosher products as well as an extensive list of reliable kosher symbols and other interesting things.