/oss/taxonomy/term/2230/all en /oss/article/general-science/why-does-lettuce-turn-brown <p>That’s an interesting question. <em>How</em><em> </em>lettuce turns brown is well known, but <em>why</em> this happens is a different story. Let's deal with the <em>how</em> first.</p> Mon, 21 Mar 2022 19:45:04 +0000 Joe Schwarcz PhD 9057 at /oss /oss/article/nutrition-you-asked/why-does-lettuce-turn-brown <p>That’s an interesting question. How lettuce turns brown is well known. But why this happens is a different story. Let's deal with the how first. The chemistry that takes place when lettuce leaves become brown is the same chemistry we see when an apple is cut and turns brown, when green guacamole turns brown or when a green olive ripens and turns black. All these reactions occur when chemicals in the fruit or vegetable called polyphenols react with enzymes called polyphenoloxidases. Enzymes are specialized protein molecules made within cells and serve as catalysts.</p> Fri, 04 Jun 2021 20:24:53 +0000 Joe Schwarcz PhD 8755 at /oss /oss/article/health/secret-more-flavourful-stew <p>The secret is the Maillard reaction! In 1912, Louis-Camille Maillard discovered that bread crust, coffee beans, beer and roasted meat all turned brown because of a chemical reaction between sugars and proteins they contained. But the reaction produced not only brown pigments, but it also produced flavours. Indeed, chemists soon made the first artificial flavour, that of maple syrup, by combining the amino acid serine with glucose. </p> Mon, 14 Jan 2019 16:14:13 +0000 Joe Schwarcz PhD 7533 at /oss /oss/article/you-asked/why-does-lettuce-turn-brown <p>That’s an interesting question. How lettuce turns brown is well known. But why this happens is a different story. Lets deal with the how first. The chemistry that takes place when lettuce leaves become brown is the same chemistry we see when an apple is cut and turns brown, when green guacamole turns brown or when a green olive ripens and turns black. All these reactions occur when chemicals in the fruit or vegetable called polyphenols react with enzymes called polyphenoloxidases. Enzymes are specialized protein molecules made within cells and serve as catalysts. In other words, they speed up chemical reactions. </p> <p>Normally within a cell the enzymes and the polyphenols are separated, but when cells are damaged as in cutting an apple or tearing lettuce, their contents leak out an the polyphenols and the polyphenoloxidase mix. The result is a reaction that oxidizes the polyphenols and allows them to link up with each other to produce a brown pigment. When lettuce leaves torn, some cells are damaged and the browning reaction begins. But even if there is no tearing, even if we have a whole head of lettuce, there will eventually be browning. That’s because aging of the cells also leads to damage and mixing of the polyphenols with the enzyme.</p> <p>The age old question about lettuce is what causes more damage, tearing or cutting?  Actually it seems not to make much difference. Some cooks argue that cutting is more destructive and leads to quicker browning but experiments do not bear this out. Some cooks say that they can taste metal in a salad if a knife has been used. I think this is culinary snobbery. In any case, if the lettuce is eaten soon after it is prepared, as far as potential browning goes, it doesn’t matter if it was cut or ripped. One more item about preparing lettuce. The dressing should always go on at the last moment because oil soaks into the leaves readily and makes them soggy. And noting that lettuce is soggy is not culinary snobbery.</p> <p>So that’s the how. What about the why? Actually we don’t know. The main theory that crops up is that the brown pigment has anti-fungal and insecticidal properties. In other words, the damaged fruit or vegetable is trying to protect itself.  he argument is that historically damage was caused by insects and fungi and that at the first sign of such damage the enzymes would swing into action to ward off further destruction. The fly in this ointment is that nobody has clearly demonstrated that the brown pigments really do have anti-fungal or insecticidal properties. But there certainly is some interesting chemistry there.</p> <p> Thu, 11 Oct 2018 17:00:00 +0000 Joe Schwarcz PhD 7388 at /oss /oss/article/you-asked/can-placing-avocado-seed-guacamole-prevent-discolouration <p>Whether you're a guacamole lover or not, everyone knows that the beautiful enticing green colour of fresh guacamole can quickly turn to a dismal unappetizing brown. Perhaps the reason that guacamole is traditionally made with peppers and tomatoes stems from the observation that these components retard the browning reaction. In terms of chemistry, the reaction is fascinating.</p> Tue, 29 Nov 2011 19:38:41 +0000 Joe Schwarcz PhD 1647 at /oss