What Rhizopus actually does
Tempeh is not inoculated and then left to chance. The mould responsible — Rhizopus oligosporus, occasionally R. oryzae — is a zygomycete fungus that germinates on the surface of cooked, dehulled soybeans and then pushes hyphae, the thread-like filaments that make up its body, down into the gaps between beans and through their outer layers. Those hyphae branch, anastomose, and eventually form a continuous white mycelium that physically stitches every bean to its neighbours. The result is structural: a slab of tempeh holds together not because anything has been pressed or glued, but because a living network runs through it.
The chemistry matters as much as the architecture. Rhizopus secretes a battery of extracellular enzymes — proteases that hydrolyse bean proteins into shorter peptides and free amino acids, lipases that break down stored triglycerides, and amylases that work on any residual starch. This enzymatic activity is part of why raw soybeans, which are nutritionally locked behind complex storage proteins, become something a cook can actually work with after a day or two of fermentation. The bean's own structure is partly dismantled and reknitted into the hyphal mass.
The narrow band that keeps it on track
Temperature controls everything. Rhizopus oligosporus grows well between roughly 25 °C and 37 °C, with vigorous mycelial growth in the low-to-mid thirties. Below 25 °C the culture is sluggish; competing organisms — yeasts, bacteria — can establish themselves before the mould dominates. Above 37 °C the hyphae struggle, and the same enzymatic machinery that makes tempeh what it is begins to break the cake down rather than consolidate it. This is not a generous range.
There is a further complication: fermentation is exothermic. As Rhizopus metabolises, it generates heat, and in a thick slab or an under-ventilated wrapper the internal temperature can climb several degrees above the ambient. A home ferment at 30 °C ambient can easily run at 34–35 °C in the centre of a dense package, which is fine; that same setup pushed to 32 °C ambient may tip the interior over the edge. The standard mitigation is thin packages — most tempeh is formed in slabs no deeper than about three centimetres — and perforated wrapping that allows the metabolic heat to dissipate, alongside any carbon dioxide the culture produces. Oxygen supply matters too: Rhizopus is an obligate aerobe, and a sealed bag kills it.
Reading the ferment
A healthy ferment follows a recognisable sequence. For the first eight to twelve hours, little is visible; the culture is germinating. Then, in a window roughly between twelve and twenty hours, fine white fuzz appears and the cake begins to consolidate. By thirty to forty hours — the range varies with temperature, inoculum level, and bean geometry — the surface is dense white and the interior fully bound. The beans are no longer separable. At this point the tempeh is done.
What happens next is the main fault the process has. Rhizopus does not stop growing simply because the cake is ready. If fermentation continues, the mould sporulates: the white then grey colour change is the spores developing, and the flavour shifts from the mild, mushroomy character of young tempeh toward something more pronounced and ammonia-adjacent. The grey stage is not dangerous, but it represents a change in character. Timing the pull — refrigerating or freezing the cake to halt the ferment — is the cook's last active decision, and it determines whether the finished tempeh is delicate or assertive.
The margin between bound-but-bland and grey-and-pungent is narrower than most beginner guides suggest. It is best measured by eye and nose together, not by the clock alone.
Setting Point is an independent publication about food science and technique. It is not a shop, restaurant, delivery service or nutrition advisory.
