Quick answer: High-acid foods — most fruit, jams, pickles, chutneys, anything properly acidified — are safe in a boiling water bath. Low-acid foods — plain vegetables, beans, corn, meat, stock — are only safe in a pressure canner, because boiling water cannot reach the temperature needed to destroy botulism spores. That distinction is the whole of home preserving safety. Two things make this harder in New Zealand than it should be: there are no official New Zealand home preserving guidelines, and the dominant local supplier still recommends open-kettle canning and jar inversion, both of which were discarded overseas decades ago on good evidence.
More people are bottling again, and most of the enthusiasm is well placed. A shelf of jars is a glut you didn’t waste, it needs no power, and it is the cheapest food storage there is. But it is also the one kitchen skill where getting it wrong can genuinely hurt someone, and New Zealand’s information environment is worse than most. This is the honest version.
The gap nobody mentions
Australia and New Zealand have joint official food safety guidelines, but they cover commercial food handling only. Neither government has produced home preserving guidance, and neither has funded the kind of research the United States Department of Agriculture has run for decades — a gap set out in detail in this survey of home preserving resources in New Zealand. There is no New Zealand equivalent of the USDA’s tested recipes and processing times.
That absence has consequences. In the vacuum, advice circulates from cookbooks, grandmothers and suppliers, some of it decades out of date. Most visibly, Perfit — the dominant preserving brand in New Zealand — instructs a method with no heat processing step at all. Its own How to Preserve guide directs you to fill the jar with boiling syrup, screw the band down tightly and turn the jar upside down to cool. There is no water bath, no pressure canner, and no distinction drawn anywhere between high-acid and low-acid foods. Those are the two methods — open kettle and inversion — that North America abandoned decades ago, on the evidence that they allow pathogens to survive in unprocessed jars. If your instructions came off the box, they may not be current.
The practical fix is straightforward: use tested processing times from a research-backed source — the USDA’s National Center for Home Food Preservation is the standard reference and is free online — and adjust nothing except for altitude.
The one rule that matters: acid decides the method
Clostridium botulinum spores are common in soil and harmless to eat. The problem is what they do in a sealed jar: no oxygen, low acid, room temperature, and they germinate and produce one of the most potent toxins known. You cannot see it, smell it or taste it.
Acid stops them. So does heat — but only serious heat. Boiling water tops out at 100°C, which is not enough to destroy the spores. A pressure canner reaches roughly 116°C, which is.
Water bath: high-acid foods only
Safe in a boiling water bath, because the acidity prevents germination:
- Most fruit — apples, plums, peaches, feijoas, berries
- Jams, jellies and fruit butters
- Pickles and chutneys made to a tested recipe with enough vinegar
- Tomatoes with added acid — modern varieties sit near the borderline, so tested recipes add lemon juice or citric acid as a matter of course
Pressure canner: everything else
Low-acid, and only safe under pressure:
- Plain vegetables — beans, carrots, corn, peas, beetroot without vinegar
- Meat, poultry and fish
- Stock and bone broth
- Soups, stews and anything mixed
A pressure cooker is not a pressure canner. They are different appliances with different volumes and different heat-up and cool-down behaviour, and canning times are calculated for canners. If you want to preserve bone broth rather than freeze it, a pressure canner is the equipment, and freezing is the honest alternative if you don’t have one.
Three methods to stop using
- Open kettle canning — pouring hot food into jars and sealing without any processing. The seal is not a sterilisation. Anything that survived the pot survives in the jar.
- Jar inversion — turning filled jars upside down to force a seal. It can produce a seal that looks right over food that was never processed, which is worse than an obvious failure.
- Oven canning — dry heat transfers far too slowly and unevenly, and jars are not designed for it.
All three will appear to work almost every time. That is exactly the problem: the failure rate is low but the consequence is severe, and nothing about the jar tells you which one you have.
The New Zealand equipment problem
Preserving is genuinely expensive here. By one detailed comparison of the NZ market, Mason-style jars cost roughly ten times what they do in North America, which is why so many New Zealanders reuse commercial jars despite the higher breakage risk and the unreliable seal on lids never designed to be reused.
The local landscape, briefly: Agee was the historic New Zealand brand and production ceased in 2003, so Agee jars are now second-hand only. Perfit is the current mainstream supplier, and its jars take 85mm wide-mouth lids manufactured in North America. Electric water bath canners from both brands are no longer made and replacement elements are hard to source, though units turn up second-hand regularly. Many keen preservers end up importing jars and lids, ordering in volume to make the freight bearable.
None of this is a reason not to preserve. It is a reason to buy good jars once, look after them, and start with a large stockpot and a rack rather than a purpose-built canner.
What’s worth preserving in a NZ season
Preserve the gluts, not the staples. The things worth the jars are the ones that arrive all at once and cannot be eaten fast enough — and in most of the country that means stone fruit in January and February, tomatoes in March, feijoas and apples in autumn, and whatever the vegetable garden does all at once in late summer. Working with what’s abundant is the same logic behind eating seasonally generally: it is cheaper, better and requires no planning beyond noticing.
Things that store well as they are — pumpkins, onions, garlic, kūmara, most root crops — are usually not worth processing. Store them properly instead.
Frequently Asked Questions
Can I water bath vegetables?
Not plain ones. Beans, carrots, corn, peas and similar low-acid vegetables require a pressure canner, because boiling water only reaches 100°C and botulism spores survive that. The exception is vegetables properly acidified to a tested pickle or chutney recipe — the vinegar is what makes the water bath safe, so the quantity in the recipe is not optional.
Is there official New Zealand guidance on home preserving?
No. Australia and New Zealand have joint food safety guidelines for commercial handling only, and neither government has produced home preserving guidance or funded the underlying research. In practice, New Zealand preservers rely on USDA-tested processing times through the National Center for Home Food Preservation, which is freely available online.
Why is jar inversion considered unsafe?
Because a seal is not a sterilisation. Inverting a filled jar can pull a vacuum and produce a lid that looks and sounds correctly sealed over food that was never heat-processed at all. The seal gives false reassurance, which is more dangerous than a failure you can see. It remains in circulation in New Zealand partly because a major local supplier has continued to recommend it.
Can I reuse commercial jars and lids?
Many New Zealanders do, driven by the cost of proper jars, but it carries real trade-offs. Commercial jars are not tempered for repeated thermal cycling and break more often, and single-use commercial lids are not designed to reseal reliably. If you are doing this, keep it to high-acid water bath work, check every seal, and never rely on a reused lid for anything low-acid.
How do I know if a jar has gone bad?
Bulging lids, leaking, spurting liquid on opening, mould, or an off smell all mean discard. But the serious caution is that botulism toxin can be present with none of those signs — it produces no visible, smellable or tasteable change. That is precisely why the method matters more than the inspection: you cannot check your way to safety after the fact.
The Bottom Line
Preserving is worth doing and the safety rules are simpler than they look: acid foods in a water bath, everything else under pressure, tested times from a research-backed source, and no shortcuts on the two methods that produce convincing seals over unprocessed food. The New Zealand complication is that we have no national guidance and some local advice is decades behind, so where your instructions came from matters as much as what they say.
This article is general information, not food safety advice. Home preserving carries real risk when done incorrectly. Always follow tested processing times from a research-backed source, and if you are in any doubt about a jar, discard it. Information current as of September 2026.
