Why a Living Culture Needs to Breathe
Rhizopus oligosporus, the mould responsible for tempeh's dense white mycelium, is an obligate aerobe — it must have oxygen to grow. Seal a batch of inoculated beans inside an airtight bag and you create exactly the wrong conditions: CO₂ accumulates, oxygen is consumed, and the culture either stalls or shifts toward unwanted anaerobic bacteria. The mould that should be knitting cooked beans into a coherent cake cannot do its job without a steady gas exchange.
The solution is deliberate, controlled perforation. Before the inoculated beans go into their wrapping — typically a zip-lock bag, a banana leaf parcel, or a sheet of plastic wrap — holes are punched or pierced at regular intervals. Traditional Javanese production uses banana leaves, which are porous enough by nature, but modern small-batch practice almost always uses plastic, and plastic is airtight without help. A skewer, a toothpick, or a purpose-made perforating board driven through the bag creates a grid of holes, usually spaced roughly a centimetre apart on both faces.
The spacing matters. Too far apart and there are dead zones where gas exchange is poor and the mycelium grows unevenly, leaving pale patches that look like culture failure. Too close — or holes so large that the wrapping loses structural integrity — and the surface moisture evaporates too freely, the beans dry out before the mould has consolidated them, and the cake crumbles. The holes should be small enough to slow moisture loss while large enough to allow airflow. A 1–2 mm diameter, repeated densely, achieves both.
There is a secondary function. Rhizopus respiration is exothermic: the mycelium generates heat as it grows. In a well-aerated, properly thin slab — no more than three or four centimetres deep — that heat can dissipate. In a thick, under-perforated block, the core temperature climbs, the mould cooks itself from within, and the batch develops grey sporulation or a wet, ammonia-tinged failure. Perforation and slab thickness work together as a heat management system.
Punch the holes before filling if you can, check that none have sealed under pressure when you lay the parcel flat, and keep the slab level so heat rises evenly. The rest belongs to temperature and time.