Stop Hummus from darkening

Sure, a few tips below. Note that you're not going to be able to keep it from drying/darkening entirely; the below is intended to keep it as appetizing as possible despite this.

1. Use a higher % of tahini (sesame paste) than you might otherwise use. The extra oil content slows drying, and has the benefit of making the hummus creamier and better-tasting (although more expensive to make).
2. Also add some additional lightly salted cold water when making the hummus. The higher water content also slows drying.
3. When plating, cover the hummus in a heavy swirl of olive oil to prevent drying/oxidation.

If folks are going to be serving themselves, after which the hummus will sit out for a while, you can take either of these additional steps:

• Spritz the hummus with water or olive oil from a mister.
• Portion the hummus into several small containers, and uncover them one at a time as the others are used up.

Lemon juice and boiled onion slice.

I am here assuming that browning of hummus is from polyphenol oxidase, the same as for apples, taro, peaches and other fruit / vegetable products. I could not find anything to specifically say this was so but it makes sense.

https://en.wikipedia.org/wiki/Polyphenol_oxidase

If that is the case then there are two tricks you might try, both of which will be fine for consistency and flavor.

1. Lemon juice.

This is an old trick for apple slices. The citric acid is an antioxidant. Lemon juice is often in hummus anyway. Some lemon juice squeezed over the top before it is set out will reduce oxidation.

2. Onion slice. Inhibitory effect of onion extract on polyphenol oxidase and enzymatic browning of taro (Colocasia antiquorum var. esculenta)

In this study they heated onions at 100C and studied its ability to inhibit polyphenol oxidase (in taro and potatoes). Longer heat = more inhibition when extract was applied. Below graph from publication.

The authors thought reduced sulfur moieties acted as antioxidants. Other reduced sulfur chemicals (e.g. dithiothreitol) were also effective so that makes sense.

I propose that you could accomplish this effect for your hummus by boiling an onion, slicing it and setting a slice flat atop each bowl of hummus. For one the physical slice will occlude the hummus below from air. For two you will get the onion thiol compounds diffusing out into the hummus and serving as antioxidants. For 3 if some onion flavor gets into the hummus, onion flavor in hummus is good. For 4 this will look pretty especially if you use a red onion and drop a few pieces of parsley on top.

And if someone is hungry for the onion they can eat it! I would.