MSG solution is monosodium glutamate dissolved in warm water so it can be dashed into a drink the way you would dash bitters or saline. It belongs to the same family of bar tricks as a saline solution: it is not a flavour you taste on its own so much as a lever that makes the flavours already in the glass feel rounder, longer, and more present.
What It Actually Does
Glutamate is the compound behind umami, the savoury taste that parmesan, soy sauce, anchovies, ripe tomatoes, and aged sherry all share. On the tongue it registers as depth and duration rather than as a distinct flavour — which is exactly what a bright, acidic, fruit-driven sour tends to lack. Citrus and fruit syrup hit hard and then vanish; a dash of glutamate gives the finish something to hold onto.
On the safety question: MSG is classified as generally recognised as safe by the FDA, and the double-blind trials that have looked for the cluster of symptoms once labelled “Chinese restaurant syndrome” have not been able to reproduce it in people who report being sensitive to it. The reputation is a 1968 letter to a medical journal that outlived its evidence. Glutamate is also not exotic: there is more of it in a wedge of parmesan than in a dash of this.
The Build
Two grams of MSG into ten grams of warm water, stirred until it goes completely clear, then cooled before use. Warm water only — there is no need to boil it, and MSG dissolves readily at well below simmering. Make a small quantity: a dash is a fraction of a millilitre and a batch this size will outlast most home bars' patience for savoury cocktails.
Dosing
One dash from a dasher bottle is the working unit, and it is very easy to overshoot. Past about two dashes in a single drink the effect stops being “this tastes deeper” and starts being “this tastes like soup,” which is not recoverable. Build the drink, taste it, and add the dash last if you want to learn what it does — the before-and-after is the most convincing argument for keeping a bottle of it around.
Bottom Line
A seasoning bottle that costs almost nothing and does something no amaro or bitters can. Treat it like salt: invisible when it is right, impossible to ignore when it is wrong.