A by-product or byproduct is a secondary product derived from a production process, manufacturing process or chemical reaction; it is not the primary product or service being produced.
A by-product can be useful and marketable or it can be considered waste: for example, bran, which is a byproduct of the milling of wheat into refined flour, is sometimes composted or burned for disposal, but in other cases, it can be used as a nutritious ingredient in human food or animal feed. Gasoline was once a byproduct of oil refining that later became a desirable commercial product as motor fuel. The plastic used in plastic shopping bags also started as a by-product of oil refining. By-products are sometimes called co-products to indicate that although they are secondary, they are desired products. For example, hides and leather may be called co-products of beef production. There is no strict distinction between by-products and co-products.
In economics In the context of production, a by-product is the "output from a joint production process that is minor in quantity and/or net realizable value (NRV) when compared with the main products".Wouters, Mark; Selto, Frank H.; Hilton, Ronald W.; Maher, Michael W. (2012): Cost Management: Strategies for Business Decisions, International Edition, McGraw-Hill, p. 535. Because they are deemed to have no influence on reported financial results, by-products do not receive allocations of joint costs. By-products also, by convention, are not inventoried, but the NRV from by-products is typically recognized as "other income", or as a reduction of joint production processing costs when the by-product is produced.World Trade Organization (2004): United States – Final dumping determination on softwood lumber from Canada, WT/DS264/AB/R, 11 August 2004.
The International Energy Agency (IEA) defines by-product in the context of life-cycle assessment by defining four different product types: "main products, co-products (which involve similar revenues to the main product), by-products (which result in smaller revenues), and waste products (which provide little or no revenue)."
In chemistry While some chemists treat "by-product" and "side-product" as synonyms in the above sense of a generic secondary (untargeted) product, others find it useful to distinguish between the two. When the two terms are distinguished, "by-product" is used to refer to a product that is not desired but inevitably results from molecular fragments of starting materials and/or reagents that are not incorporated into the desired product, as a consequence of conservation of mass; in contrast, "side-product" is used to refer to a product that is formed from a competitive process that could, in principle, be suppressed by an optimization of reaction conditions.
Common byproducts bagasse and molasses from sugar production bran and germ from flour milling buttermilk from butter production distillers grains from ethanol production fly ash and bottom ash from coal combustion glycerol from soap or biodiesel production lanolin from wool processing lees from wine fermentation pomace from fruit juice or olive oil production saw dust from lumber production slag from smelting ore straw from grain harvesting trub from beer fermentation vinasse from sugar or ethanol production whey from cheese production
See also Main product By-product synergy Circular economy Side reaction
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