You need to take an online course. I don't know of any book that will provide you with all of your answers.
I recommend you take the free online class at the Univ. of Georgia.
There is also a paid course at the Univ. of Idaho. It was $35 plus cost of publications. It allows you a weekly chat to get answers to questions, plus a forum for questions.

Bottled lemon juice must be used since it has a ph level that must be met in order to be sold. Fresh lemons vary and cannot be relied upon for safety in a food.
The lemon juice is added at the proper time as it will also aid in getting a gel as it is cooking. Some times it is only for flavor, but most recipes for jam have it to aid in the gel formation. You must have the right amount of acid, fruit, pectin, and sugar to get a proper gel in a regular jam/jelly.
No, ph strips and even meters will not assure safety at home. A lab tested food takes many months and also between $10,000 and $50,000 to test one food ! Even a bit off and your safety is at risk, so please do not try that at home. Foods can and do change ph level after they are processed.
Not worth the risk. We NEVER advise trying to test your own foods.
If you want low or no sugar jams then get Pomona's Universal pectin. You should be able to do a search and find the post discussions on it.
of course, applesauce and plain fruits do not need sugar to preserve them, it is only for flavor and sugar helps to retain firmness in fruits.

but I hate being told to do something without knowing why.

Don't we all! Unfortunately it isn't possible to have an in-depth working knowledge of everything. Is there a book I can read that will explain all the rules underlying physics to me? I doubt it but if you have any "teaching high school English" questions I might be able to help you there as I don't think it has changed much since I retired.

So in some cases, this being one, most of us, when dealing with an unknown topic, tend to defer to those who ARE experts in their field. ;) As Linda Lou said, take one of the many classes available and you might also want to browse through the many publications and studies available at NCHFP - many are online and others are available to order. But please don't expect even them to turn you into a Ph.D in Food Sciences. ;)

Dave

As Dave mentioned, the NCHFP has placed online quite a number of studies and abstracts which can be helpful in understanding some of the issues and basic principles.

You can learn a great deal through self-study and taking the online course. All of us, though, have to be careful not to learn just enough to make ourselves dangerous. The whole issue of food preservation gets very complicated.

There are also a number of other professional food safety groups and forums online which are well worth perusing. It was from one of those groups that I learned of the possibility for low-acid "islands" to exist in high-acid foods. For example, if you have large pieces of garlic or pepper in a salsa, while the salsa overall is high-acid due to the vinegar, if the acid has not sufficiently penetrated there can hypothetically be risk with some portion of the mixture. This also is an indicator of why pH meters or strips are an imperfect measure of safety in the home.

In a lab or commercial setting the solids and liquids in a mixture would be separated out, weighed, and then tested separately.

pH, of course, is separate from density, which may limit heat penetration in the product. Lab testing is designed to achieve optimal safety AND optimal quality.

To give another example of the complications, take pumpkin butter. For years canned pumpkin butter recipes were provided in the Ball Blue Book and other authoritative guides. They have since been pulled because additional testing demonstrated the water content and pH of the pumpkin varied so much from source to source that testers were no longer confident in the safety of that product for ALL home processors.

Carol

For canned acidified foods (pickles, chutney, relish) it is important to test the acidity before and after bottling with a pH meter (about $200), never trust litmus paper, but more so that proper pasteurization has been achieved through approved processes. It did not cost $10-15K to have my bread and butter pickles tested at the NC State Univ (NCSU) Food Science Lab (Cooperative Extension Office), it was only $100. I had it tested because I want to sell it. I have to register my "acidified food manufacturing facility" with the FDA, pass an FDA class in Better Process Control School for Acid and Acidified Foods, get approval of my method for each product from the FDA CFSAN (with the NCSU findings attached) called a Scheduled Process, register my facility with the Government, and have my kitchen re-inspected by the NC Dept of Ag, can never ever deviate from the scheduled process (methods and exact ingredients), have a written HACCP plan with complete records of each batch, before I can even sell one jar, and it is just me, a little old lady who offers her jam and pickles at the local farmers market and makes them in her canning kitchen in the outbuilding of her tiny farm. If you are not going to sell your stuff but want to be safe, you must follow approved recipes and current methods. If you want to change this or that, you can have your food tested at your State's approved site for $80 to $150, or pay a food science lab (several businesses do just food testing) which starts at fees of $2K and does go up to $15K as LindaLou says), or ask here about tweaking a specific recipe (do a search on Habanero Gold or Annie's Salsa, for example, with tons of questions about a product and tweaks and you'll see what I mean).

If you are making fruit jam and applesauce, just follow the approved recipes, as they are canned food with a very low risk for botulism, but, low-sugar jams have a higher risk for bacteria and mold issues. I did not have to have my jams tested to sell them, other than to have my kitchen inspected and canning process questioned by the inspector.

Ask "why" here for specific questions,as you did with lemon juice, and peruse the previous posts for lots of information, along with the other options posted in reply. I doubt if anyone could put in a book the why of methods, but I gleaned a lot of information from my week-long class for Acidified Foods since it is mostly for big businesses and "real" food manufacturing facilities. Not discussed here is also the issue of water activity which was also tested at the Lab I sent my acidified product to.

If you are a physicist who wants to know why - ask away. There are many very knowledgeable and highly respected experts on this forum who may just give you the scientific reasoning you want, or not. You might also write to your State's Cooperative Extension Food Safety Office that might give you the in depth answers or where to look, or ask about classes they offer in Food Science. I did, just because I had to know "why" too, but I am far from being an expert on any part of it and am past wanting another profession, since most of it was over my head.

Nancy

If you are looking for amber peach jam, you could try this recipe. I subbed amaretto for the rum. It turned out an amber color. As a matter of fact, I left it a little runny because I didn't want to cook it any longer and make it darker. I put it in the fridge overnight instead of leaving it on the counter. My kitchen is pretty warm this time of year and I wasn't comfortable with leaving it out.

Peach or Nectarine Jam with Rum and Brown Sugar
from The Good Stuff Cookbook by Helen Witty

As described the jam is a rich deep gold in color. If you'd like a deeper color and a more pronounced brown sugar flavor, use dark brown sugar instead of light brown. For more rumminess, the liquor may be increased by 2 tablespoons.

6 cups coarsely-chopped peeled firm-ripe peaches or nectarines (about 4 lbs)
2 cups light brown sugar (packed)
6 tablespoons strained fresh lemon juice
3/4 cup dark rum, preferably jamaican
2 cups granulated sugar

In a large bowl, combine peaches with the brown sugar, lemon juice and about half of the rum, stirring to mix. Cover and let stand at room temperature six hours or overnight.

Wash jars and lids in hot, soapy water. Sterilize jars for 10 minutes in a boiling-water bath, then leave in hot water until ready to fill. Prepare lids according to manufacturer's directions.

Pour the fruit mixture into a large saucepan or dutch oven. Bring to a boil over medium-high heat. Cover the pan, reduce heat and cook the mixture until the peach chunks begin to look translucent, 15 to 20 minutes; stir occasionally to prevent sticking. If the jam becomes too thick and threatens to scorch before the fruit is done, add 2 to 3 tablespoons of water. Add the granulated sugar, increase heat to medium-high and cook rapidly, stirring almost constantly, until a spoonful placed on a chilled saucer and refrigerated for a few minutes wrinkles instead of runs when the saucer is tilted. (Take jam off the heat while doing this. If using a candy thermometer, this should happen at about 220 degrees.) Add remaining rum and stir the jam (it will boil up when you add the rum) for 2 minutes over the heat.

Ladle boiling-hot jam into hot, prepared jars, leaving 1/4 inch headspace. Top with lids and process for 15 minutes in a boiling-water bath. Cool jars completely on a dish towel before labeling and storing.

how can i lower the water activity in my homemade salsa that i can i am trying to sell it but the heath department says that i have to keep it 0.85 i did the test and its at the 0.99 is there any way i can lower it my ph level is ok thanks hope u can help.

I am not a food scientist, but try using dried onions and things, not fresh . That is my suggestion.