Making tempeh requires experimentation and after a bit of learning curve, you will find your own go-to method.
How to differentiate a good tempeh from a bad one?
So you may ask, how do I know if I have made a good or bad tempeh? The smell is certainly one of the easiest ways to distinguish between normal tempeh and one that has gone bad, followed by it’s texture then color variation, in that order.
As a rule of thumb, a fresh block of tempeh should have an earthy or a nutty smell, similar smell to edible mushrooms. Texture wise, a good block of tempeh should be firmly packed. A dense cottony mycelium layer grows during the fermentation and binds the soybean together to form a compact cake. A healthy mycelium layer is commonly white to greyish in color. Some gray & black spots can be expected at times, it is not a sign of bad tempeh and is still safe to be consumed.
Similarly, a rotten smell is the first thing you will notice, which tells you that something is OFF. The rotten smell usually equals to strong odor of pure alcohol or strong ammonia. Once you smell that, the whole block should not be consumed.
Rotten tempeh’s texture usually either crumbly or a bit slimy/mushy when pressed. Rotten tempeh may also be colonized by other harmful bacteria/mold in which case, some greenish fuzzy mold may appear amongst the white mycelium layer. When you see this, the tempeh should be discarded as it is not safe for consumption.
Here are the summary of what we’ve discussed so far: How to easily tell apart good from rotten tempeh.
| Good/Fresh Tempeh | Bad/Rotten Tempeh | |
| Smell | Earthy/nutty smell (similar to edible mushroom) | Alcohol odor/strong ammonia |
| Texture | Firmly packed, full mycelium outer layer and in between beans | mushy/slimy, crumbly, uneven mycelium cover |
| Color | Consistent white/greyish layer | Greenish fuzzy mold spots |
Tempeh failure case study and their possible causes:
There can be myriad of reasons why you may end up with bad tempeh during your first few trials. The problems could arise in the selections of the quality beans, the starters and/or the incubators. The issue could also be in the ratio raw ingredient mix or in the de-hulling/cooking method. The duration of fermentation & the stability of the environment during the fermentation may also be a factor.
We have analyzed all these possible factors and narrowed down to these three failed tempeh case studies and their possible causes. Hopefully it will help flattened the learning curve and you get to enjoy a good home made tempeh in no time.
- WET/SLIMY TEMPEH
The main possible cause of wet tempeh is due to the temperature rises too high during fermentation process. The ideal temperature for successful tempeh fermentation is between 29-32ºC (85° to 91ºF). Insufficient ventilation thus resulting in overly high humidity may also play a role in creating wet tempeh.
Another common mistake that often overlooked, yet detrimental to overall success, is the drying process. Before you apply the starter, it is crucial that the cooked beans are thoroughly dried. Otherwise it creates an environment where harmful bacteria thrive before the rhizopus molds have a chance to colonize.
So How to prevent wet tempeh?
From our experience, it is better to go with lower end of temperature range if you don’t have incubators with thermostat to set the temperature to the desired range. This is to allow for the heat created by the tempeh mix itself during the fermentation process. Take an extra step in the drying process to make sure the beans are completely dried (See our step by step instruction here). We find that placing a moist tea towel over the chosen container provides a nice balance of ventilation & moisture that a good tempeh needs.
- Crumbly Tempeh
Crumbly tempeh is usually associated with one of these starter problems:
- The starter you use no longer active/over the use by date.
- You apply too little starter.
- The starter is unevenly spread
- Rotten Smells Tempeh
Wet tempeh usually also accompanied with rotten smells such as strong alcohol or ammonia odor. So you can follow the prevention for wet tempeh. On top of those, rotten smells tempeh can be resulted from over fermentation. Standard fermentation period is between 24-36 hours. The ideal period will cary for different localities, it depends on the temperature and humidity. You can minimize the likelihood of over fermentation by checking every 12 hours.
Here are the summary of the three tempeh failure case studies:
| Possible Causes | Possible Solutions | |
| Wet/Slimy Tempeh | – Temperature too high – insufficient ventilation – raw ingredient too wet | – Start with lower temperature – Use breathable cover – Take extra measure to dry beans |
| Crumbly Tempeh | – Inactive starter – Not enough starter – Uneven mix | – Choose quality active starter – Check measurements – Mix the starter for at least 1 min |
| Rotten Smells Tempeh | – Same as wet/slimy causes – Over fermentation | – Same as wet/slimy solutions – Frequent checking |