Is tomato sauce safe if the water evaporated in the canner

If you are seeing this effect after the jars have been in storage for a long period, do not eat the contents! This is a sign of botulism due to improper canning; the bacteria often (but not always) produce gas as they grow spores.

If this is happening immediately after the canning process, it is probably because you are not creating a proper vacuum seal. There are three accepted methods for doing this: Thermal exhaust, mechanical sealing (i.e. using a chamber vac) and steam displacement AKA steam injection. See the link for more information; the second two require specialized equipment, so thermal exhaust is what's normally done in a home setting.

In the thermal exhaust method, you get the contents very hot (71-82° C), which causes them to expand and release gases (air and carbon dioxide). After sealing and cooling, the subsequent contraction will create a vacuum seal. This is the most common method of home canning.

If you dump in hot brine and immediately seal the container, you are doing the opposite of this. Since the heat takes a while to distribute, you are causing this initial expansion when the jar is already closed, and this will force more gases up into the headspace and probably pop or warp the lid; if you're unlucky, it might even break the container.

Is this method safe? No. How unsafe it is depends on the acidity. Low acid foods must be processed in a pressure canner. That includes the majority of vegetables and especially garlic, peppers, etc. The main reason is botulism; the anaerobic environment is perfect for the C.botulinum bacteria, but spores cannot grow in high-acid environments with pH < 4.6. If the food is high-acid or has been acidified (i.e. pickled) then you don't need to worry, but with low-acid food you must kill all the bacteria and spores, and botulism spores are extremely heat-resistant.

In fact, they are so heat resistant that you cannot even kill them reliably in boiling water. That is why you need a pressure canner, to get the temperature all the way up to 121° C / 250° F, and you need to hold it there for at least 3 minutes. That is the only way to safely can low-acid food. Simply boiling it isn't good enough, and pouring in some boiling brine definitely isn't enough.

For high-acid foods, such as jams, a hot water bath is OK. This still involves boiling the entire jar after it's been closed, but you don't need a pressure canner. Pouring boiling liquid into a jar of cold or warm food will still not get the food up to a sufficient temperature, and even if it did, you would still need to sterilize the jar itself; that's done by boiling the whole apparatus. In this case you're not worried about botulism (since it can't grow in those conditions), but you still have to kill the other kinds of household bacteria, which don't have heat-resistant spores.

Since canning is intended to preserve the food, i.e. for long-term storage, you have to be a whole lot more careful about bacteria. You can't leave any opportunity for it to grow. Lots of people can the way your wife does and don't get sick, but it is a risky proposition, and I would strongly recommended you use safer methods, especially if guests or children are involved.

Tomatoes aren't high acid, so they need the addition of vinegar or lemon juice in order to safely can with a hot water bath. Honestly, I'm not sure why you would want to make tomato sauce from canned tomatoes because for me, the whole point of canning tomatoes is because the tomatoes will otherwise go bad. But anyhow...

I recommend finding a tomato sauce recipe from a canning cookbook or recipe website. Here is a recipe I found from the Ball website which is a reputable source, and here is an article about hot water bath canning tomato sauce. Notice that the recipe calls for a high acid ingredient, which is extremely important in canning tomato sauce with a hot water bath!

Secondly, it is NOT okay to seal jars by inverting them!!!! Get yourself a book about canning, Ball sells several good ones (Blue Book and Complete Guide to Home Preserving) and learn about the canning process. The steps are as follows:

(0) Follow the recipe EXACTLY, don't add any other ingredients that may change the pH of the final product!

(1) Get the hot water boiling in a jar large enough to have a few inches both above and below the jar that you'll use. You can use a metal trivet or canning rings to act as a base for the jars so that they have water going under them and aren't touching the base of the pot directly.

(2) Start with jars that are appropriate to canning and have fresh lids (or clean, working reusable lids like Weck jars or Tattler lids). If processing for under 15 minutes the jars need to be sterile (can be made sterile in a hot water bath). Inspect Tattler seals for any nicks, and throw away the bad ones.

(3) Normal canning lids and Tattler lids need to be prepared by soaking in hot water right up until use.

(4) Add the ingredients to your jars, remove air bubbles with a spatula, wipe the rims clean, and apply the lid as per the instructions of your lid. (Regular Ball lids are slightly different from Tattler lids, and I've never used Weck jars which I think are different again.)

(5) Insert into the hot water bath with canning tongs, cover, make sure that you start the countdown once the water has returned to a full boil.

(6) Remove once the time is up. Don't tilt the jars when removing them as the seal isn't airtight yet. Check the instructions for your lids, at this point Tattler lids have to be tightened fully.

(7) Leave the jars alone to cool. After a few hours or overnight, check the seals by trying to gently pry open the can. They should be "finger tight" and not pop off. The jars that aren't properly sealed can be put into the fridge and consumed within a short time. Otherwise they're good for up to a year or whatever the recipe suggests.