Making "long(er)-life" homemade mayonnaise

I make a pretty large quantity of homemade mayo and have never had a problem keeping it longer than that -- Good Eats uses a week for their recipe and for me it lives a month, easy, with no detectable reduction in quality. There really is enough acid and salt to deter most bugs if you like it strongly flavored (and let's be honest, mayo should be strongly flavored because it's a sauce made of fat and you might want to minimize the quantity applied). I'd also take the advice of stilltasty.com that while color, flavor, or texture may change it's generally still safe to eat refrigerated mayo even after it's notional "use-by" date.

In short, I strongly advise more salt and acid if you're worried (and also because it's tasty), but I also strongly advise a lack of worry.

To preserve mayo without chemical preservatives you're going to need to drop the pH. That usually means more vinegar, which is also going to alter the flavor. Now, you can experiment with quantities but you're still going to be altering the flavor. To counteract this you're going to need to flavor your mayo more strongly. Flavored oils are a way to do this, so are herbs and spices. Most commercial mayos have added sugar to offset the increase in acidity. At some point though you're going to have to quantify how much preservation you get out of a given vinegar concentration.

As mayo is made with raw eggs, its shelf life is limited by the risk of salmonella. Salmonella is killed by heating and acid.

Here is the safest method of preparing mayonnaise that I know of:

Method for assuring destruction of Salmonella spp. in egg yolk. Place egg yolk(s) in a small, stainless steel bowl. (The container must be large enough so that it can allow the egg yolk/acid mixture to be stirred or whisked as it is heated.) Place the container containing the egg yolk/acid mixture in a pan or bowl of water (such as a small double boiler) that is at a simmering temperature of 180 to 190F (82.2 to 87.8C). Heat the yolk/acid mixture to a temperature of 150F (65.6C). This will take about 1 minute. The mixture must be stirred or whisked constantly and the temperature measured frequently by using a micro-tip thermocouple thermometer (such as the Atkins 33040 ). Immediately remove the pan containing the yolk/acid mixture from the hot-water heat source. The yolk/acid mixture is now pasteurized and can be used in the preparation of mayonnaise and Caesar dressing. Recipes for these products should be checked, or recipes provided in this paper should be used to assure that there is the correct amount of acidity. As a starting point, the standard of identity for vinegar is 5% acetic acid. The amount of citric acid in lemon juice (bottled or freshly squeezed) is 4.7%. A typical mayonnaise should be prepared with 1 raw egg yolk per 8 ounces of oil and the acid concentration should be 1.4% of the aqueous phase as recommended by the FDA (CFR Title 21 Part 101.100).

Source: http://www.hi-tm.com/Documents/Mayonnaise.html

I don't know what the shelf life of this kind of mayonnaise is, but if this doesn't give you the duration you want, I doubt that there is another method to make it longer, except maybe using pasteurized eggs.

If you follow the method, make sure you are actually using a thermometer. It does not insure the safety if you don't reach the temperature mentioned, but if you get it a bit hotter, trying to make sure you reached it, your yolks will curdle. Also, note that the salmonella don't magically drop dead the second a threshold temperature is reached. They start to diminish, until they have all died. So I don't remove the mixture from the water bath, but make the mayonnaise in the water bath itself, giving it a longer time on the heat.

You're going to need to add vinegar (or lemon juice, or something acidic).

After making mayo with vinegar, just leave it out for a few hours before putting it in the fridge, so the acid has a chance to kill the bacteria in the egg.

You can keep it for a week after that, 2 weeks is at your own risk though. Personally, I'd risk it - but I don't decide whether to eat things based on use-by date - it's looks and smells good, it's probably fine.

Obviously this will change the flavour.

How about cultured mayonaise? I don't know the details, but have seen mention of this on some other websites... i think it gives a longer shelf-life because the "good bacteria" crowd out the "bad bacteria." Not to mention, it would be full of healthy probiotics -- added bonus!

Pasteurizing eggs at 56C for two hours, than shock chilled to 2-4C, or melange of eggs in vacuum bag at 56C for two hours, than shock chilled to 2-4C. Than using these according to a actual recipe, using sanitized equipment and while emulsifing, not rising temperature above 4C – using chilled equipement and chilled ingredients. These will for sure put you behind 3 days if stored in 2-4C range. But to claim actual shelf life, you would need tests. Do I use this technique? Yes. Do I claim one week shelf life? No? Is it hazzard? Yes.

EDIT: not proven shelf life of product, even done under HACCP deep knowledge and with precedence of simmilar recipes with proven shelf life, suppose to be considered hazzard at all times.

You can culture mayo by adding a tablespoon (per cup and a half of finished mayo) of whey right after you are done making the mayo and letting it sit on the counter for 7 hours. This will extend the fridge life to a few months.

If the freshly laid egg is clean (free from chicken poop--and most are. They don't get poop from being laid, just from the chicken's feet if they stepped in poop then entered the nest box), then the egg is protected by the bloom, which is a sealant the hen's body releases to protect the egg from spoiling. Really fresh, clean, free-range chicken eggs keep for 21 days on the kitchen counter. If washed (removes the bloom) then they should be refrigerated. Using such eggs would increase the length of time you can keep mayonnaise.

Im not sure I would use my home made mayo if it was more than a week old. I can taste that its changed after 3-4 days. I suspect that one of the suggestions here to add acid would be a good idea. However because youre making an emulsion, adding more vinegar or lemon juice will just water it down, and youll lose the firm gel youve worked so hard to produce. Instead, you might consider adding citric acid in granulated form to up the acid content.

Also limiting the exposure to oxygen extends its life, so if youre able, remove all air from whatever container youre using to store it.

Just my two cents...