BYTESLIP ← Back
Knowledge Base // Tutorial 100

Pressure-Canning Aku (Skipjack Tuna), and Why a Water Bath Won’t Do

Difficulty: Hard 2 days (overnight chill) · ~4 h canning day Pressure canner

Aku is a local favorite, and a good run of fish can mean more than you can eat fresh. Canning it lets you keep it on the shelf, but fish is the kind of food where a shortcut can kill: it’s low-acid, and sealed jars of low-acid food are exactly where botulism toxin can form. This guide follows the USDA tuna process published by the National Center for Home Food Preservation (NCHFP) on every number, adds Hawaiʻi Island elevation examples, and explains why the boiling-water canner you might use for jam is the wrong tool.

Safety // Read First

Warnings

CriticalFish must be pressure-canned. CDC: pressure canning is the only recommended method for low-acid foods; a boiling-water canner “will not protect against botulism,” and electric multi-cookers shouldn’t be used even if they have a “canning” button. Follow the tested time and pressure exactly, vent the canner for 10 minutes, and if pressure drops below the target, start the full timing over. You can’t see, smell or taste botulinum toxin, so never taste a suspect jar. Botulism is an emergency: anyone with symptoms like double or blurry vision, drooping eyelids, slurred speech, difficulty swallowing or breathing, or muscle weakness should get to a doctor or emergency room immediately.

This is a home kitchen guide, not professional food-safety advice. Use it only with the exact USDA/NCHFP tuna process: fish, jar sizes, pack and times can’t be swapped. Old family recipes, open-kettle canning, oven canning and “water bath for longer” methods are not tested safe methods. When in doubt, throw it out.

Checklist // Before You Start

What You Need

Science // Why Not a Water Bath

Why a water bath won’t do

Botulinum spores are on most fresh food. They’re harmless in open air, but NCHFP explains that in a moist, low-acid food, between 40 and 120°F, with less than 2% oxygen (a sealed jar), they can grow and may produce a deadly toxin within 3 to 4 days.

Low-acid means a pH above 4.6, and that includes all seafood. The spores are very hard to destroy at boiling-water temperatures, so USDA says low-acid foods must reach 240 to 250°F, which takes a pressure canner at 10 to 15 PSI. NCHFP notes that processing low-acid food in a boiling-water canner would take 7 to 11 hours, and the USDA tuna process is given for pressure canners only.

Not recommended either, per NCHFP: open-kettle canning, and processing jars in an oven, microwave or dishwasher.

Numbers // Your Pressure

Find your pressure for your elevation

The tuna process is 100 minutes for pints and half-pints. Only the pressure changes with elevation:

Dial-gauge canner        Weighted-gauge canner
0-2,000 ft      11 lb     0-1,000 ft      10 lb
2,001-4,000 ft  12 lb     above 1,000 ft  15 lb
4,001-6,000 ft  13 lb
6,001-8,000 ft  14 lb

Look up your address with the USGS National Map spot elevation tool (NCHFP’s suggestion) or this one-liner with your own longitude and latitude:

curl -s "https://epqs.nationalmap.gov/v1/json?x=-155.24&y=19.43&wkid=4326&units=Feet&includeDate=false"

Examples from the USGS service: a point near Volcano village (19.43, −155.24) came back at about 3,796 ft, so 12 lb on a dial gauge or 15 lb on a weighted gauge. A point in Waimea (20.02, −155.67) came back at about 2,664 ft: also 12 lb dial or 15 lb weighted. Near sea level (under 1,000 ft) it’s 11 lb dial or 10 lb weighted. Use your own address, not these examples.

Walkthrough // Steps 01–10

Check canner → ice → precook → chill → clean → pack → vent → process → cool → check

  1. Step 1: Check the canner before you buy fish

    Clean the lid gasket and parts per the manufacturer’s directions and make sure the vent pipe is clear. NCHFP says to check dial gauges for accuracy every year: a gauge that reads high causes under-processing. Replace a gauge that’s off by more than 2 lb. Many Cooperative Extension offices check gauges; in Hawaiʻi, ask your local UH CTAHR Extension office, or contact the canner’s manufacturer.

  2. Step 2: Keep the aku cold, every minute

    NCHFP: keep tuna on ice until ready to can. This matters for a second reason besides bacteria: aku is one of the fish the Hawaiʻi Department of Health links to scombroid poisoning. When these fish warm up, bacteria make histamine. FDA says toxic levels can form within 6 to 12 hours without ice, and the toxins are not destroyed by cooking or canning. FDA specifically warns about catch left lying on a warm dock or beach.

  3. Step 3: Clean and precook the fish

    Remove the guts, wash the fish well in cold water and let the blood drain from the belly cavity. Put it belly down on a rack in a large baking pan, cut in half crosswise if needed. Precook by baking at 250°F for 2½ to 4 hours (depending on size) or 350°F for 1 hour, or steam it for 2 to 4 hours. With a thermometer, cook to 165 to 175°F inside.

  4. Step 4: Refrigerate overnight

    Chill the cooked fish in the refrigerator overnight. This firms the meat so it can be cleaned neatly.

  5. Step 5: Clean down to light meat

    Peel off the skin with a knife, removing blood vessels and any discolored flesh. Cut the meat off the bones, then cut out and discard all bones, fin bases and dark flesh. NCHFP notes dark flesh has a strong flavor that affects the lighter meat, and many people don’t can it.

  6. Step 6: Pack the jars

    Quarter the meat and cut it crosswise into lengths that fit half-pint or pint jars. Fill the jars, pressing gently to make a solid pack. Add water or oil, leaving 1-inch headspace. If you like, add ½ tsp salt per half-pint or 1 tsp per pint. Put on the lids.

  7. Step 7: Load the canner and vent for 10 minutes

    Put the rack and 2 to 3 inches of water in the canner (or the amount your canner’s directions specify). Warm the water no hotter than 140°F, NCHFP’s figure for raw-packed food, since these jars go in cold. Set the jars on the rack with a jar lifter, keeping them upright. Fasten the lid, leave the weight off (or the petcock open), and heat on high until steam flows freely in a funnel shape. Let it vent for a full 10 minutes, then put on the weight or close the petcock. It should pressurize in 3 to 5 minutes.

    Venting matters: NCHFP explains that air trapped in a canner lowers the temperature at a given pressure and causes under-processing.

  8. Step 8: Process 100 minutes at your pressure

    Start timing when the dial reaches your pressure, or when the weighted gauge starts to jiggle or rock as the manufacturer describes. Process 100 minutes. Adjust the heat to hold the pressure steady at or slightly above the target. If the pressure drops below the target at any point, bring it back up and start the 100 minutes over from zero.

  9. Step 9: Let it cool on its own

    Turn off the heat and let the canner depressurize naturally. Don’t run water over it and don’t open the vent early; NCHFP warns that forced cooling can result in unsafe food or spoilage. When the vent lock drops (or after the time your older canner’s manual gives), remove the weight, wait 10 minutes, then open the lid tilted away from your face. Lift the jars out upright, set them 1 inch apart on a towel, and leave them undisturbed for 12 to 24 hours. Don’t retighten the lids.

  10. Step 10: Check seals, label and store

    Take off the bands and test each lid: press the center (it shouldn’t spring back), tap it (a sealed lid rings), or look across it (it should curve down slightly). A jar that didn’t seal can be reprocessed within 24 hours with a new lid and the full time, or refrigerated and eaten within several days. Label sealed jars with the date. Store them in a clean, cool, dark place: below 95°F for safety, 50 to 70°F for best quality.

Safety // Before You Eat It

Before you open a jar

CDC says to throw out any home-canned jar that is leaking, bulging or swollen, looks damaged or cracked, spurts liquid or foam when opened, or has food that’s discolored, moldy or smells bad. Don’t taste it.

CDC also advises boiling all home-canned low-acid foods in a saucepan before eating: 10 minutes at altitudes below 1,000 feet, adding 1 minute for each additional 1,000 feet.

Disposing of a suspect jar, per NCHFP: if it’s still sealed, put it in a heavy garbage bag, close it, and put it in the trash. If it’s unsealed, open or leaking, wear gloves and detoxify it first: lay the jars and lids on their sides in a big pot, cover with at least 1 inch of water, cover the pot, bring to a boil and boil 30 minutes, then cool and discard. Clean any spills with a fresh solution of 1 part unscented household bleach to 5 parts water, left on for 30 minutes.

Harmless surprise: glass-like crystals in canned tuna are usually magnesium ammonium phosphate. NCHFP says they usually dissolve when heated and are safe to eat.

Pro Tips

Tips

Kitchen Questions // FAQ

FAQ

Does this USDA “tuna” process apply to aku?

Aku is skipjack tuna (NOAA calls it Pacific skipjack tuna; Hawaiʻi Seafood lists its Hawaiian name as aku). NCHFP’s process is titled simply “Tuna” and doesn’t list species. If you have doubts about a particular fish, ask your CTAHR Extension office before canning.

Can I use my Instant Pot’s canning button?

No. CDC says not to use electric multi-cooker appliances for low-acid foods, even if they have a “canning” or “steam canning” button.

Can I use quart jars?

The tested process only covers pints and half-pints. There’s no tested time for quarts, so don’t use them.

Can I can raw aku?

NCHFP says tuna may be canned precooked or raw, but the procedure it gives is the precooked one used here. Follow it as written.

What about smoked aku?

Smoked fish has its own directions; NCHFP notes, for example, that it needs more water in the canner. Don’t use this process for smoked fish.

My canner lost pressure for a moment. Is it ruined?

Bring it back to pressure and start the full 100 minutes again from zero. NCHFP says this is important for safety, even though it may affect quality.

Can I process longer in a water bath instead?

No. The USDA tuna process is for pressure canners only, and CDC says a boiling-water canner won’t protect against botulism for low-acid food.

References // Sources

Sources

← All Tutorials Food tutorials Sauerkraut by the Numbers