It's a deep dive, but an example or two might help, a great deal of food is processed and handled under purely industrial conditions, with all of it strored for some time, and shipped long distances, this causes oxidation and reduction of the most volitile and flavorfull/coulorfull components, which then must be protected or added back in when it is packaged for sale. Hence a vast industry devoted to flavorings, and coulorants, where many of these substances that were used are now banned, at least in a patch work fashion. Lead and other heavy metals are added to alter flavor and coulor, they are cheap and readily availible and also I suspect work as preservatives, allowing unscrupulous middlemen to by up large quantities of substandard product, and alter it enough to make it appear good enough for retailers to sell. Then there are agricultural areas that have been poluted by industry, mining, war, or naturaly, or even by past agricultural "mistakes", and again unscrupulous people can make large proffits by concealing the origin of what they are selling. So I buy spices as whole unground items, and choose carefully from small ethnic groceries, that cater to people keeping culinary traditions alive, and know by sight and smell the quality of what they are buying, and are often very hapoy to share there way of determining the good from the bad, and precisely, what to look for.