I found this short paragraph from the, “Complete Home Canning Guide”, interesting. See my reaction can be found after the duplicate.
If Clostridium botulinum bacteria survive and grow inside a sealed jar of food, they can produce a poisonous toxin. Even a taste of food containing this toxin can be fatal. Boiling food 10 minutes at altitudes below 1,000 feet destroys this poison when it is present. For altitudes at and above 1,000 ft, add one additional minute per 1,000 feet additional elevation. Caution: To prevent the risk of botulism, low-acid and tomato foods not canned according to the recommendations should be boiled as above, even if you detect no signs of spoilage. All low-acid food canned according to the approved recommendations may be eaten without boiling them when you are sure of all the following:
1) Food was processed in a pressure canner
2) Gauge of the pressure canner was accurate
3) Up-to-date researched process times and pressures were used for the size of jar, style of pack, and kind of food being canned.
4) The process time and pressure recommended for sterilizing the food at your altitude was followed.
5) Jar lid is firmly sealed and concave
6) Nothing has leaked from the jar
7) No liquid spurts out when the jar is opened.
8) No unnatural or “off” odors can be detected.
Numbers one to four can only be known and assure if:
a) The manufacturer is willing and will honestly include each product processing information. Printed legibly as part of label or on a separate sheet. The separate information sheet is included as part of the product package and could be read by customer before purchase.
b) Regulatory authorities is regularly monitoring the manufacturer’s activities and assuring its conformity to set standards.
c) The company has an advanced and efficient QA and RD division.
Items 5 and 6 could be easily inspected by buyers. Anyone cannot go wrong if he look closely and diligently.
The lasts number 7 and 8 could only be detected upon opening the jar which requires paying for the item first. Unlucky if the liquid spurts and unnatural color and odors are detected. Still consider yourself lucky if you have not tasted it first.
Regarding the relationship between altitudes and boiling times, I have never studied it during college and have never applied it in actual. I think it needs a deeper digging.