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Production

Tempeh begins with whole soybeans, which are softened by soaking, and dehulled, then partly cooked. Specialty tempehs may be made from other types of beans, wheat, or may include a mixture of beans and whole grains.

The principle step in making tempeh is the fermentation of soybeans which undergo inoculation with Rhizopus spp. moulds, a type of filamentous fungus most widely used for the production of tempeh. For example, a fermentation starter containing the spores of fungus Rhizopus oligosporus or Rhizopus oryzae can be mixed in. Afterwards, the beans are spread into a thin layer and are allowed to ferment for 24 to 36 hours at a temperature around 30°C (86°F). The soybeans have to cool down to allow spore germination and abundant growth of mycelium. Later, the temperature of the beans will naturally rise and rapid mould growth happens for around 4 hours. As mould growth declines, the soybeans should be bound into a solid mass by the mycelium. In good tempeh, the beans are knitted together by a mat of white mycelium. Typically, tempeh is harvested after 48 hours of fermentation with its distinguishable whitish colour, firm texture, and nutty flavour. Extended fermentation time results in an increase in pH and colour darkening in the tempeh.

During the fermentation process, optimal time of fermentation, temperature, oxygen, humidity, and pH levels are required to encourage the growth of the Rhizopus mould, while discouraging the growth of undesired microorganisms. The pH level should be kept around 3-5 by adding a mild acidulant such as vinegar, lactic acid, or acetic acid, thereby favouring mould growth and restricting the growth of spoilage microorganisms. Oxygen is required for Rhizopus spp. growth, but should be maintained at low levels to prevent fungal spore production. Under conditions of lower temperature, or higher ventilation, gray or black patches of spores may form on the surface—this is not harmful, and should not affect the flavor or quality of the tempeh. This sporulation is normal on fully mature tempeh. A mild ammonia smell may accompany good tempeh as it ferments, but it should not be overpowering.

Traditional tempeh is often produced in Indonesia using Hibiscus tiliaceus leaves. The undersides of the leaves are covered in downy hairs (known technically as trichomes) to which the mold Rhizopus oligosporus can be found adhering in the wild. Soybeans are pressed into the leaf, and stored. In particular, the tempeh undergoes salt-free aerobic fermentation.

Process of Making Tempeh

Tempeh is made from boiled and dehulled soybeans, which undergo inoculation with Rhizopus spp. moulds, which is a type of filamentous fungus most widely used for the production of tempeh. The principle step in making tempeh is the fermentation of soybeans by mixing in the Rhizopus starter culture. During the fermentation process, optimal time, temperature, oxygen, humidity, and pH levels are required to encourage the growth of the Rhizopus mould, while discouraging the growth of undesired microorganisms. The soybeans are cooled down to an ideal temperature of 37°C to allow spore germination and abundant growth of mycelium. The pH level should be kept around 3-5 by adding an acidulant such as lactic acid or acetic acid, thereby favouring mould growth and restricting the growth of spoilage microorganisms. Oxygen is required for Rhizopus spp. growth, but should be maintained at low levels to prevent fungal spore production. Later, the temperature of the beans will naturally rise and rapid mould growth happens for around 4 hours. As mould growth declines, the soybeans should be bound into a solid mass by the mycelium. Typically, tempeh is harvested after 48 hours of fermentation with its distinguishable whitish colour, firm texture, and nutty flavour due to the mould mycelium knitted among the soybeans. Extended fermentation time results in an increase in pH and colour darkening in the tempeh.

How to Determine the Quality of Tempeh Product

Once tempeh is produced, tempeh is divided into three different categories, which are good, unfinished, and inedible tempeh, based on its quality. Good tempeh includes beans that are bound into a firm, compact cake by a dense, uniform, white mycelium, which should permeate the entire cake. Furthermore, the beans should be barely visible. The odour of good tempeh should be pleasant, clean, subtly sweet or resembles the aroma of mushroom. The entire tempeh should be lifted up as a single, cohesive cake without crumbling when shaken gently. Unfinished tempeh has beans that are bound together loosely by a sparse white mycelium, hence it crumbles easily. Unfinished tempeh should be incubated longer unless it has been incubated more than 8 hours past the recommended time. If it has been incubated for enough time and still could not become a good tempeh, it should be discarded. Inedible tempeh has beans with foul odour, which resembles odour of strong ammonia or alcohol, indicating the development of undesirable bacteria due to excess moisture or overheating. Inedible tempeh cake is wet, slimy, and mushy with a collapsed structure. Its colour is tan to brown and mould develops in sparse patches.

Packaging

Food grade wrapping paper and perforated polyethylene bags are the most suitable materials for packaging tempeh. They have demonstrated good retention of the quality of tempeh and extension of the shelf life of tempeh for three days compared to fresh tempeh. Appropriate packaging is important as it provides optimum oxygen supply and temperature for inoculation and fermentation to occur during processing. Tempeh is a perishable food and must be wrapped and placed into the refrigerator or freezer immediately after incubation or other processing steps such as blanching. In the refrigerator or freezer, stacking of tempeh should be minimized to prevent overheating and the undesirable, gradual continuation of fermentation, both of which shorten the storage life of tempeh. Even under cold temperature, tempeh continues to respire and undergo slow decomposition from microorganisms and its natural enzymes. Therefore, tempeh should be well cooled for at least two to five hours in a cooler before they undergo further packaging. Tempeh packaged in perforated polyethylene bags is usually repacked inside another labeled, non-perforated bag for distribution and sale, and for easier labeling. If the tempeh is only packaged in one perforated bag, the label must be directly attached to the perforated surface with the use of government food contact approved adhesive. They are then bulk packed in cartons and returned to the refrigerator or freezer to await shipment.

Preservation

Antimicrobial Agents

Rhizopus oligosporus culture responsible for the fermentation of tempeh from soybean produces natural, heat stable antimicrobial agents against spoilage and disease-causing microorganisms, extending the shelf life of the fermented product through microbial antagonism. The mould is capable of inhibiting the growth of other fungi such as Aspergiluus flavus and Aspergillus parasiticus by interfering with the accumulation of aflatoxin (especially aflatoxin B1), the mycotoxin of greatest concern. R. oligosporus has also been reported to produce four to five antibacterial substances during fermentation process. It produces phenolic compounds against pathogenic bacteria such as Helicobacter pylori and an antibacterial protein has been identified with activities against Bacillus species (especially against Bacillus subtilis), Staphylococcus aureus, and Steptococcus cremoris.

Non-refrigerated Fresh Tempeh

Tempeh can be sold and consumed fresh within 48 hours once removed from its incubator. It is commonly transported to the market in its incubation container (e.g. polyethylene bag, banana leaf wrapper, etc.) and placed in the shade. In areas with warmer climates, tempeh can be kept at room temperature for 1-3 days before it becomes overripe. In locations with more temperate temperatures, it can keep for 1-4 days but will usually need to be refrigerated to prevent spoilage.

Refrigeration

Fresh refrigerated tempeh should be sealed in a labeled polyethylene bag and kept in temperatures below 4˚C (40˚F). It can be kept at this temperature for 3-5 days and sometimes, even as long as a week. Storage life could be extended to 2 or 3 weeks if the tempeh is blanched or steamed prior to refrigeration due to the inactivation of enzymes and destruction of bacteria.

Freezing

Freezing is the preferred way to preserve tempeh due to its capability for wide distribution. Tempeh can be frozen whole or in slices, depending on preference. During the freezing process, whole tempeh is placed in its perforated wrapper whereas sliced tempeh is packaged in a labelled polyethylene bag prior to being sealed in a outer bag and then frozen immediately. This method will keep for months with only a small loss of texture and flavor.

Blanching

As mentioned above, blanching tempeh by steaming or parboiling helps to extend the storage life by preventing bacterial growth, stopping mould growth and inactivating enzymes. Steaming appears to have a less negative effect than parboiling in terms of texture, flavor and nutritional value. Blanching is a great method for preserving tempeh prior to refrigeration, though not as beneficial for tempeh that is to be frozen.

Dehydration

Air Tray Drying

Tempeh can be dried via the air tray drying method. Cubes of tempeh placed on steel, mesh bottom trays are dried by the circulating hot air dryer. After the product is finished, they can be cut into 1inch squares at 200˚F for 90 to 120 minutes in order to reduce moisture content to 2-4%. When placed in moisture proof Pliofilm bags, the tempeh has a shelf life of several months at room temperature. Although this is a convenient method that produces a shelf stable product without requirement of refrigeration, the process of hot air drying can cause a significant loss of nutritional content such as the soluble solids and nitrogen protein content.

Sun Drying

This preservation method is most economical out of all methods. The tempeh can be blanched prior to dehydration to preserve flavor and prolong shelf life. Tempeh is exposed to internal solar dryer temperature of 180-200˚F in this method. A disadvantage of this method is that sunlight can destroy some of the vitamin B12 of tempeh.                                                                                    

Freeze drying

This method is the most expensive out of all dehydration methods but provides the advantage of long stable shelf life at room temperature and an excellent retention of soluble nutrients (nitrogen protein and other solids). The product undergoes quick freeze at 50 ˚F and then dried at a moderate temperature inside a strong vacuum. Due to the pricy nature of the equipment, the final product price will be higher than tempeh preserved through other methods.

Spray drying

As this method is traditionally used for small particles, it is used to produce tempeh powder for products such as soups, breads, tortillas, etc. However, this method can be expensive due to the bulky nature of the equipment.

Deep frying

This method produces ready to eat tempeh products. Different types of oils such as rapeseed, soy, safflower, peanut, or coconut oil is heated to 350˚F in the deep fryer . The tempeh is deep fried until golden brown and crisp, and then cooled quickly in a sterile environment to be sealed in Pliofilm bags and stored in a cool, dry place. The shelf life of this product lasts around a week but can be extended if the tempeh is sun dried or oven dried prior to deep frying.