Aspartame. Polarized light micrograph of crystals of Aspartame, an artificial sweetening food additive. Aspartame is a registered trademark for L-alpha-Aspartyl-L-phenylalanine methyl ester, and is also known under the trade name Nutrasweet. Its main uses are in canned drinks and foods, and as a consumer-added sweetener in the form of tablets and granules. It contains the amino acid phenylalanine, and must carry a warning on the product packaging. This is because people who suffer from the condition phenylketonuria can not metabolize phenylalanine, and over-consumption can lead to mental retardation. Magnification: x25 at 35 mm size.