Every process has waste. This by-product may be toxic, as from certain industrial plants, due to heavy metal or chemical concentrations but generally most are useable, at least in theory and often in practice.
Many of the remains of processed foods are used to create other foods. For example, after the juice of grapes are extracted to produce regular wine, their pulp and skins are fermented to produce a brandy called grappa. Likewise, coffee can be made from date pits. By adding sugar to orange peels, a wonderful sweet is produced. Potato skins are fried to create a light crispy snack. In Israel in falafel stands, the corners of pita cut off in order to stuff them are fried and offered to customers as a treat. In a similar fashion, the holes of donuts and broken bits of pretzels are available for sale.
Other food bits are additions to other dishes. Bread crumps are sometimes literally bread crumb and used in cheese cake and crumbles, to name a few items. Gelatin is a key ingredient in puddings and is produced from animal parts. Water in which corn cobs have been soaked serves as a thickener in soups. Celery greens are a source of salt. The best fish stock is made from the shells of lobsters, crayfish or shrimp. Whey, a by-product of the process of making yoghurt, is put into food dishes by Icelanders instead of wine.
In some cases, the food by-product is used for cosmetics. Walnut shells are grounded into a powder that serves as a base of makeup. The waste products of the olive oil production process, called Orujo, are also used in beauty products. People apply beet skins to their face to improve skin quality.
Finally, some industrial waste products have further uses. Sawdust creating during wood cutting is an essential ingredient of particle board. Precious diamond dust produced during the cutting and polishing process is used to coat drills. Likewise, bits of animal furs cut during the preparation of coats are seamlessly patched together. Even today, waste fabric is the base material for the most expensive paper. One source of ethanol for vehicles is the cooking oil in which french fries and other fried items are cooked.
Whether because resources are limited or profits are needed, people are incredibly ingenious in finding uses for the most useless by-products. They take the waste out of waste products.