What the Nose Knows - Avery Gilbert [109]
At the leading edge of technology, biological tissue is used as the odor sensor. For example, researchers can insert a mammalian odor receptor gene into yeast cells, which then manufacture the receptor and install it on their own cell surface. A tiny shred of the yeast cell membrane—including an intact, functioning receptor—is cut out and anchored to a chip that produces an electronic signal whenever the receptor is activated.
In a different approach, researchers use bacteria cells to produce odor receptors and then paint receptor-laden cell membrane fragments onto a tiny quartz crystal. The vibrational frequency of the crystal changes along with the weight of the layer coating it; this setup—known as a quartz crystal microbalance—is so sensitive it can tell when the receptors in the layer of bioslime have latched on to odor molecules, increasing its weight. An English company is using this technology to detect explosives. Another group has gone further and integrated entire rat olfactory cells into a semiconductor chip. They call this setup an olfactory neurochip, but it’s really a rat-machine hybrid.
University-based scientists in France have pushed hybridism a step further: they have inserted a human odor receptor gene into yeast cells, which then express functioning human receptors for the odor molecule helional. The modified yeast cells become biosensors for helional. This is a technologically elegant but somewhat disturbing achievement: a combination of human DNA controlled by a foreign organism, which in turn is enslaved to a machine. Is this really a direction we want to pursue?
At some point in the development of these fusions of silicon and biology, the question becomes not whether the e-nose can replace the human nose, but whether we want it to. Would I let an e-nose sniff-scan me for lung cancer? Sure. Would I use a robotic odor sentinel? Maybe, especially if I had a BO problem. But do I really want my refrigerator to tell me, “I’m sorry, Avery, I can’t let you eat those cold cuts”?
Genes of Scent
Supermarket tomatoes have no flavor. It’s a common complaint, and a valid one. Commercial tomato varieties have less sugar, acid, and aroma than the wild type. On the other hand, they have better color, yield, disease resistance, and physical toughness, or what growers like to call “shippability.” (Tomatoes are picked while still hard and green, to help them survive the trip to the store.) The guiding principle for tomato breeders is that it is better to look good than to taste good.
Help may be on the way: as scientists decipher the genetics of flavor chemical production in plants, they open the door to bioengineered flavor enhancement. One research group has discovered genes for the enzymes that are the first step in the biochemical production of phenylethyl alcohol, a key ingredient in tomato aroma. When overexpressed in transgenic tomato plants, these genes give the fruit ten times more rose alcohol, making it more fragrant than the ordinary variety. Another scientific team recently created a tastier tomato by altering the gene controlling a key enzyme involved in aroma production. They took the enzyme gene from lemon basil and inserted it into a tomato plant, where it modified biochemical activity to produce higher levels of key aroma molecules. This is cool science, but the proof of the pudding is in the eating, and here the new transgenic tomato is a winner: It was preferred by panelists in taste tests.
Rather than import genes from other plant species, genetic engineers may decide to pluck useful ones from so-called heirloom tomatoes, the distinctive-looking and interesting-tasting varieties prized in farmers markets across the country. Heirloom