The Chemistry of Food & Taste | Dr. Harold McGee

McGee initially dismissed the copper bowl as an old cook's tale when he couldn't find a chemical explanation in the literature. After seeing an 18th-century illustration of a French kitchen explicitly showing eggs whipped in a copper bowl, he tested it personally. The difference was 'tremendous' across multiple sensory dimensions. This protocol is a direct example of traditional practice winning out over incomplete scientific understanding—scientists of the time had made claims that were simply wrong. McGee also adds that copper's ability to prevent sucrose breakdown is why it remains preferred for jam-making, further illustrating the metal's unique food chemistry.

Wine experts first observed that raw grapes, when chewed and held, slowly developed flavors characteristic of the finished wine. McGee explains that many foods contain 'conjugates' where an aroma molecule is attached to a sugar. Our saliva contains enzymes that cleave these bonds, liberating the aromatic part. Furthermore, Maillard reactions during cooking generate such conjugates. Thus, even after cooking is done, the food continues to 'cook' in the mouth chemically. McGee uses this as a central argument to eat more slowly: the leftovers on the palate after swallowing can also evolve, delivering a longer, more layered sensory finish.

McGee explains that hot water extracts soluble material from coffee grounds. In a typical brew, about 20% of the bean's mass ends up in the cup, but the composition changes dramatically over time. Small, pleasant molecules extract first; larger, astringent, and bitter polyphenol and polysaccharide fragments come later. The temperature of the water acts as an accelerator—hotter water pulls those large molecules out faster, akin to extended time. He suggests a simple experiment: place a filter with coffee over several cups and move the drip to a new cup every 30 seconds. Tasting these sequential brews directly demonstrates the transition from desirable to harsh extraction, allowing the drinker to pinpoint when to stop for their own palate. This can guide decisions about grind size (finer grinds extract faster) and water temperature.

McGee cites his grandmother's habit of salting grapefruit and the wider practice of adding salt to coffee. Sensory research at the Monell Chemical Senses Center confirms that salt directly dampens bitter perception. This push‑pull relationship is a basic feature of taste coding. He also notes that this is distinct from the gradual process of retraining taste preferences, which takes weeks to months; salt offers an immediate fix. It is a practical, zero‑calorie way to make very dark coffee, grapefruit, or certain vegetables more palatable.

Onions contain inactive sulfur‑based defense molecules. When the tissue is cut, enzymes rapidly convert these precursors into volatile irritants. Because they are airborne, simply cutting further away or wearing any eye‑sealing goggles physically blocks them. Rinsing with water removes the freshly generated molecules from the cut faces before they can vaporize. McGee also mentions that some cultivars (e.g., Maui onions) naturally lack the responsible sulfur compounds, making them entirely tear‑free. This protocol distills the three practical approaches validated by the underlying chemistry.

McGee explains that NASA scientists first elucidated this because of the obvious need to control gastrointestinal gas in enclosed space missions. Beans contain intermediate‑sized carbohydrates (oligosaccharides) that human enzymes cannot break down. They pass intact to the colon where microbes ferment them into CO₂ and hydrogen. Soaking leaches some out; the boiling step removes more. However, he also highlights a modern perspective: these molecules are prebiotics that support gut microbiome health, and the body often adapts after a few exposures, lessening discomfort without processing the beans so rigorously.

When researching egg foams, McGee read that cooks recommended copper bowls. Finding no chemical explanation, he assumed it was an old cook's tale. An 18th-century engraving showing a boy whipping eggs in a copper bowl made him reconsider. He bought an expensive copper bowl and ran a side-by-side comparison. The result was a completely different foam—different color, texture, and mouthfeel. This experience reshaped his entire approach: from then on he would test culinary lore rather than dismiss it, and he came to see that traditional knowledge often rests on chemical truths that science catches up to later. He also notes that copper's ability to inhibit sucrose breakdown is why it is used in jam-making.

Wine experts first noticed that chewing a raw grape gradually released flavors reminiscent of the finished wine. It turns out many foods contain 'conjugates'—aroma molecules bound to sugars—that our salivary enzymes cleave, setting free volatile aromatic compounds. McGee points out that Maillard reactions during cooking generate such conjugates as well. Therefore, the sensory experience of a meal continues to evolve over 20–30 seconds after the food enters the mouth. This means that eating quickly forfeits a substantial portion of the flavor potential. He views this as one of the strongest arguments for slowing down, chewing thoroughly, and paying attention to how tastes develop and linger.

Researchers stained and counted taste buds on the tongue and found a wide spectrum, from very few to so densely packed they were almost uncountable. Those at the high end were dubbed super-tasters. While the name sounds desirable, super-tasters are so sensitive to bitter and sour that many common foods overwhelm them. McGee used a bitter test strip in his classes at the French Culinary Institute; chefs were often upset to learn they were not super-tasters because they valued being a strong taster. In reality, super-taster chefs tend to dial down flavors, resulting in food that normal tasters perceive as bland. Thus, there is no 'best' taster—awareness of one's own sensitivity is critical for anyone who cooks for others.

McGee notes that observing wild primates today shows they will preferentially pick fermenting fruit and consume it, suggesting that the pleasurable and psychoactive effects have been part of our lineage for millions of years. As archaeological techniques for detecting residues in pottery improve, the dates for intentional fermentation keep being pushed back to the very beginnings of farming in multiple regions, including China and the Middle East. He speculates that the practice probably started simply: fruit gathered, forgotten, and then noticed to smell and taste interesting, leading to deliberate fermentation.