Feeding shellfish to dogs
Shellfish concerns split into what cooking removes and what cooking does not remove. Lumping them together can read as "it was cooked, so it is fine" — so we keep them apart.
Removed by cooking ① — the enzyme that breaks down vitamin B1 (thiaminase)
Bivalves such as asari clams, shijimi clams, and mussels can carry thiaminase, an enzyme that breaks down vitamin B1 (thiamine). As with raw fish, this enzyme stops working once cooked.
Freezing does not stop it. Thawing frozen shellfish and serving them raw is the same as serving them raw from the start.
A small amount now and then is rarely a problem, but when raw shellfish or raw fish appear in meals day after day, vitamin B1 is known to run short.
(Oysters are treated as the bivalve that does not carry thiaminase. The bacteria point below still applies to them.)
Removed by cooking ② — bacteria
Shellfish live by drawing in seawater and what is in the sand. Served raw they can still carry bacteria, and this is prevented by cooking through to the center.
These two points — the enzyme and bacteria — are why these notes mark shellfish as "cook first".
Not removed by cooking — shellfish toxins
Bivalves can accumulate toxins from plankton in the sea. These are shellfish toxins; in Japan the ones that matter are paralytic and diarrhetic shellfish poisoning.
Shellfish toxins are heat-stable — cooking does not weaken them. This is why "it was cooked, so it is fine" does not hold.
In Japan, producing waters are monitored and shipments are halted when regulatory limits are exceeded, so shellfish over the limit do not reach the market. Using commercially sold shellfish is the surest way to avoid the toxins.
Shellfish you gather yourself, such as at low tide, sit outside this monitoring. Local shellfish-toxin advisories are published by each prefecture.
(The nature of shellfish toxins and the shipment-control system are based on public information from Japan's MAFF and MHLW.)
Where BT-Throb stands
With shellfish, cooking them through is the basic rule. And once shellfish toxins enter the picture, commercially sold shellfish are the reassuring choice.
BT-Throb's food notes are written on these two premises.