The word kosher is derived from the Hebrew word Kashrut, Kaf-Shin-Reish. Kosher literally translates as “fit, proper or correct to eat” and refers to those foods which are permitted to eat under Jewish law.

In Judaism, the world of the living must be separated from the world of the dead. In kosher, this is illustrated by the separation of Meat and Dairy. Meat represents Death and Milk or Dairy, represents Life. As a result, one of the most important aspects of kosher—though not the only aspect—involves separating meat and dairy.
The basics of kosher
1.The first requirement of kosher is that Meat (Fleishik)and Dairy (Milchik) will do not mix under any circumstances. Kosher cooks use separate pots, pans, plates, utensils, and ensure that this does not happen. This comes directly from the Torah which prohibits—no less than three times—“boil[ing] a kid in its mother’s milk” (Ex. 23:19; Ex. 34:26; Deut. 14:21). Therefore meat and dairy are not mixed in dishes, the meal, or in the cooking and serving process. Indeed, kosher households often keep two sets of dishes for cooking and serving meat and dairy.

Not only that, but one must wait between consuming dairy and meat together—longer for meat first, shorter for dairy first. (There are mixed opinions on this matter, but the minimum requirement seems to be 3 hours for meat.)

2.The term parve refers to “neutral” foods like vegetables, fruits, fish and eggs, which can be used with both dairy and meat (Some people do not eat meat and fish on the same plate or in the same meal, considering the practice unhealthy.)

3.The term treif refers to all animals or products that are not to be eaten at any time. These are a series of animals forbidden in the Torah, and include land (mammals and reptiles), air (birds) and water (fish and crustaceans). You can also think of this in terms of life and death: treif animals tend to be predators or scavengers.
•    Specifically, treif animals in the land category lack cloven hooves and do not chew their cuds—pigs, camels, and rabbits are treif. Cows, sheep, goats, deer and bison are all kosher provided they are slaughtered by a Shochet kosher butcher in accordance with Jewish law. (Animals that die of natural causes are never kosher.)
•    Treif  animals in the water include anything without fins or scales: shellfish like lobsters and shrimp, but also fish without fins—eels or scales–sharks, and catfish.
•    Also considered treif are most birds of prey, reptiles, insects, rodents, and amphibians. Only domesticated birds like chickens, turkey, duck, pheasant and so on that graze or are fed (which do not hunt or scavenge) are considered kosher.
•    Kosher law also excludes some  modern additives. For example, glycerides, polysorbates and rennet (used in making cheese) can be made from non-kosher animal products. Baked goods made with powdered or frozen eggs may contain ovum or fertilized eggs—both considered treif. Other treif additives are gelatin and “natural flavors’’ or colors. Some red dyes are derived from beetles and or other insects.
•    Kosher foods can become treif by touching something that is un-kosher. If meat or dairy (or items designated as dairy or meat) touches the opposite, the item becomes non-kosher
•    Grape juice (including raisin juice and wine) products made by non-Jews is also considered treif. In Torah times, wine was often sanctified by pagans, and so the practice of cooking or boiling wine developed in order to render the wine unfit for ritual purposes. Cooked or boiled wine is known as mevushal wine. Thankfully, flash pasteurization has made this process better and now there are many good wines that are kosher. Even canned fruit salad containing grapes has to be kosher and bear a hechsher symbol, which I’ll explain next.

How do I know if it’s kosher?
The rules above, of course, can only be applied if you know where your food comes from, for example, your backyard garden or a kosher butcher. But given that so much food consumed today is processed, a labeling system has developed to classify processed food as kosher. These labels are known as hechsher. One of the most famous hechsher symbols in the United States is that of the Orthodox Union in New York City—the U with a circle around it. If you haven’t seen this symbol before, start looking—it appears on many products. The general rule with processed food is if it doesn’t have a hechsher, it’s not kosher. Otherwise, there’s no way to know where the food originates. In fact, kosher food has become popular even among non-Jews for this reason: as safety concerns about the food supply arise, the hechsher comes to mean “safe.” So perhaps the meaning of “fit to eat” means something to everyone, not just Jews!

Some hechsher symbols are trademarked and therefore cannot be used without permission of the certifying organization. The letter K is not trademarked and therefore is not a hechsher!

Kosher meat
As I mention above, kosher meat, or Fleishik must be treated in specific ways when slaughtered and butchered. First, the inspector needs to make sure the animal was not sick. The term glatt (which translates to “smooth”) is used to describe animals with uninfected lungs.

The animal must be killed in a specific way, known as shechitah: the throat is cut with a quick, deep stroke with no unevenness. Many observers find this method of death to be quick and humane, particularly when compared with the “assembly line” methods of non-kosher slaughterhouses.

The animals also have to have their blood drained within 72 hours of the slaughter. Then the meat is salted to draw out more blood.

Finally, some fat and nerves are not kosher. The sciatic nerve, which runs through an animal’s hindquarters and its adjoining blood vessels may not be eaten. Because removing this nerve is difficult and time-consuming,  kosher slaughterers only use the front quarters of the animal, selling the hind quarters to non kosher butchers.

Kosher dairy
In the US the federal laws relating to the production of milk are so strict that many people consider any dairy bearing a kosher symbol to be kosher. Cholov yisroel is dairy that is supervised by a Torah observant person at all stages of the milking and production.

Utensils and serving dishes
If hot meat or dairy touches a utensil, it becomes forever associated with meat or dairy. This is done only with the presence of heat. So, if you cook a chicken soup in a new pan, that pan must always be used for meat, not dairy.

Kosher vegetables and fruits
For the most part, fruits and vegetables are kosher. Fruit and vegetables fall into the category of parve, meaning they’re neither meat nor dairy. However, if the fruits or vegetables contain insects, they are not kosher. In fact, the Torah specifies that all vegetables and fruits should be checked for insects ( Leviticus11:41-44). Of course, some fruits and vegetables are more likely to have insects than others: broccoli, Brussels sprouts, asparagus, leafy greens, celery, scallions, strawberries, raspberries, and blackberries are all likely to contain insects. To ensure these items are kosher, you need to wash them in a kosher food safe vegetable wash solution or with water mixed with a very small amount of soap. Wash the produce in the prepared water. Remove the produce. Inspect the water for very small insects. If you find insects, repeat these steps until there are no insects.

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