BIOLOGY Volume 2 - A Guide to General Biology - 2004

12. MICROBIOLOGY AND BIOTECHNOLOGY

12.12. Food and Drink

12.12.3. Single-Cell Protein

A relatively novel nutrient source is "Single-Cell Protein" (SCP), production of which began in the late 1960s. The term refers to protein derived from the large-scale CULTIVATION OF MICROORGANISMS such as Bacteria, Algae, Yeasts, and other Fungi. This protein is suitable for human consumption and can also be used as animal feed. It serves as a valuable source of minerals, Vitamins, fats, and CARBOHYDRATES. Theoretically, this could free up a variety of protein-rich products for human needs—such as soybean meal and grain—that are currently used as livestock feed. However, in the West, agricultural produce is in surplus, and this is unlikely to happen anytime on a large scale. It is well known that a global protein shortage exists, but this affects primarily developing countries that lack the resources to develop such biotechnologies.

The advantages of using microorganisms as a food source are outlined below.

1. Microorganisms require far less space than traditional crops and livestock.

2. Microorganisms grow much faster.

3. Microorganisms can utilize a wide range of cheap feedstocks, as well as agricultural and industrial wastes, such as petroleum products, methane, methanol, ethanol, sugar, molasses, cheese whey, and paper waste. This offers the additional benefit of facilitating the recycling of various Materials and waste disposal.

4. The exploitation of microorganisms involves fewer ethical and animal rights concerns.

5. Introduction/32.html">Genetic Engineering techniques are much easier to apply to microorganisms.

6. Microorganisms have a relatively high protein content.

7. Microorganisms are independent of climatic conditions and do not require large areas of land.

One of the earliest major products was "Pruteen", a single-cell protein whose production serves as an excellent example of sound biochemical engineering designed for a continuous Fermentation process. Pruteen was marketed as an animal feed Supplement. Initial high hopes for single-cell protein were not fully realized due to shifting economic and political factors in its production, leading to the following challenges.

1. Soaring oil prices in the late 1970s significantly drove up production costs, as the manufacturing process, which involves methanol production, is highly energy-intensive.

2. In developed countries such as the USA and Europe—where single-cell protein was intended to be produced—agricultural overproduction became commonplace. This particularly affected competing protein-rich products such as grain, soybeans, and dairy.

3. Developing countries facing protein shortages lacked the investment capital and technological expertise required to set up their own production of feed protein.

4. The production of competing animal feed supplements, such as soybeans, fishmeal, and corn gluten, increased while their costs dropped. Corn gluten itself is a byproduct of biofuel biotechnology.

Pruteen was produced by ICI (Imperial Chemical Industries). Following the discontinuation of its commercial use, the plant was dismantled; however, ICI utilized the technology developed in collaboration with Rank, Hovis, and McDougall to produce protein from the fungus Fusarium. This mycoprotein (myco- meaning fungus-related) is unusual in being directly utilized for human consumption. It is sold under the brand name "Quorn", and its production is described later in this section.

Pruteen

In 1980, ICI commissioned a massive fermenter (a 1500 m³ vessel, 60 m high) in Billingham, northeast England. The project cost the company £40 million, with a production capacity of 70,000 tonnes of single-cell protein annually. The bacterium Methylophilus methylotrophus was used for cultivation, relying on cheap and readily available methanol as a source of carbon and energy. The process took place under aerobic conditions. The overall chemical equation for the process outlines the required raw materials and resulting products:

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Carbon dioxide, generated as a waste product, was compressed into cylinders and sold commercially.

The Selection of a suitable bacterium was guided by several key criteria: it needed a high growth rate, The ability to utilize a relatively cheap carbon and energy source, thermotolerance (since the fermentation process generates considerable heat), non-pathogenicity to other organisms, and a high protein content per unit of dry mass.

The chosen bacterium, M. methylotrophus, had a generation time (doubling time) of 2–5 hours. Nitrogen was supplied in the form of ammonium salts, and additional mineral supplements—phosphorus, calcium, and potassium—were also required. Methanol was derived from natural gas at the same Billingham industrial site and was free of harmful byproducts. Temperature was carefully monitored and maintained between 30–40 °C, with the pH kept at 6.7. The fermenter could operate continuously for several months.

The production of Pruteen is a classic example of continuous culture. The Basic principles of its production are the same as those described in Section 12.10.5. The fermenter was tall and narrow, which facilitated aeration and cooling. It featured a unique air supply system for medium aeration. Compressed air was introduced from the bottom of the fermenter, and the rising bubbles agitated the contents (more effectively than spargers). The output was 6,000 tonnes of protein per month. From 2 tonnes of methanol, 1 tonne of dried Pruteen was produced, containing 72% protein and 8% moisture. It was enriched with Essential Amino Acids and vitamins, making it twice as nutritious as soybean meal. Pruteen was used as animal feed.

Since the cultivation was continuous, the bacteria were harvested constantly as soon as they reached their maximum growth rate (exponential growth). A portion of the contents was removed from the fermenter, treated with a flocculating agent to promote bacterial clumping, and then centrifuged. The separated Cells were spray-dried, and the useful Components of the supernatant were returned to the fermenter. After drying, the bacterial cells were milled to improve digestibility, and the pH and mineral composition were adjusted before packaging.

Mycoprotein

Another potential source of single-cell protein is fungi. Yeasts, like bacteria, can be used by both humans and animals (see the "Yeast Extract" section below). Molds are also utilized. They are characterized by a typical fungal body (described in Section 2.5.2) consisting of a network of fine, thread-like hyphae called a mycelium. A prime example of mold utilization is the production of mycoprotein. In 1985, Ranks Hovis McDougall (RHM), in a joint venture with ICI, formed a company named Marlow Foods to produce mycoprotein under the brand name "Quorn". The fungus Fusarium graminearum was used for this purpose. It was originally isolated in the early 1960s from a soil sample near Marlow in Buckinghamshire, hence the company's name. Research into the protein product began in 1964, and it received approval as safe for human consumption and clearance for commercial production in 1986. Initially, Quorn was used in savory pies sold by J. Sainsbury. Although it was first used as an ingredient in manufactured foods such as pies and curries, Quorn pieces for home use went on sale in 1990, followed by minced Quorn in 1992. Quorn had a delicate flavor and pleasant aroma, proving to be a commercial success.

The doubling time of the fungus in culture is 5.5 hours, meaning it grows more slowly than bacteria. As a source of carbon and energy, the fungus utilizes glucose obtained from any sufficiently cheap starch-containing raw material, such as maize, wheat, rice, potatoes, or molasses. The fungus produces 0.5 kg of dry biomass per kilogram of sugar consumed. An advantage of fungi is their ability to grow at fairly acidic pH levels, which inhibits bacterial growth and thus reduces the risk of culture contamination. Fusarium grows at 30 °C under continuous culture conditions. As in bacterial cultivation, ammonium salts serve as the nitrogen source, and mineral salts are added to the medium to sustain growth. As in all the processes described above, aeration and cooling of the medium are required, and all materials used must be sterilized. Agitation of the medium is achieved using a specialized aeration mechanism. Mechanical stirrers are excluded in this case because the hyphae would entangle around them, disrupting the uniform distribution of hyphae within the fermenter. The mechanism is called an "air-lift loop" because the culture continuously circulates, making one complete cycle every 2 minutes driven by air blown through a vertical, elongated 40-meter-high loop. The fermenter volume is approximately 40,000 dm³. Since this is a continuous process, product is constantly harvested from the fermenter and fresh medium is added.

Eukaryotic cells are characterized by a higher nucleic acid content compared to Prokaryotic Cells, so mycoprotein contains a significant amount of Nucleic Acids (5–15% of dry mass). These are predominantly RNA, and it is desirable to reduce its content, as human consumption exceeding 2 g per day can lead to Kidney stones or Gout. To remove RNA, the culture is heated at 64 °C for 20–30 minutes in a special vessel through which hot steam is passed. This heat Treatment inactivates fungal proteinases (thus preventing protein breakdown) while leaving ribonucleases unaffected, which helps degrade the RNA. The RNA content is reduced to 1%, which is well below the World Health Organization's recommended lower limit of 2%.

Fungal mycelium is easier to separate from the culture medium than bacterial cells. Centrifugation is not required; filtration is sufficient. After filtration and drying, a thin, pliable sheet of Quorn remains. At this stage, its appearance and texture resemble raw dough. Vegetable seasonings and a small amount of egg albumen are added to it. It is then sliced, cubed, or minced before being packaged for sale. The fungus is naturally fibrous (one of the reasons it was chosen), making it easy to give it a meat-like texture.

When it comes to human consumption, economic factors are not the only ones that matter; a range of other considerations are equally important. Very strict safety regulations must be met, and the Nutritional Value of the product must be sufficiently high. Long-term studies spanning over 10 years revealed no long-term adverse effects. The studies involved at least four generations across eleven animal species, including rats, pigs, and cows. Before the product went on the market, trials were also conducted with human volunteers who agreed to taste the mycoprotein. The resulting product proved to be healthier in some respects than meat. Table 12.5 compares its composition with that of some typical animal-derived Proteins. It is Cholesterol-free and, unlike meat, high in fiber. It is low in fat and calories (energy); it also features a favorable ratio of polyunsaturated to saturated Fatty acids (see Section 8.7). It is a good source of Vitamin B12 and zinc, which are often lacking in vegetarian diets.

When adopting a novel food, psychological barriers must also be overcome. The presentation of the new product, including packaging and advertising, is crucial (Fig. 12.21). Aroma, color, taste, and texture must be carefully engineered. Initially, it was decided to market Quorn as a meat alternative. It could easily be spun into fibers that perfectly mimic meat Structure and flavored to taste like chicken or even beef.

Table 12.5. Typical composition of Quorn mycoprotein compared with traditional animal proteins

Fig. 12.21. Food products containing Quorn.

12.11. In advertisements, the producer of Quorn often refers to it as a "natural tiny plant" or a "tiny fungus relative". Explain why.

Yeast Extract

The yeast left over from brewing beer can be put to various uses. One example is whisky production, which, like brewing, relies on the fermentation of sugars produced in germinated barley grains. However, whisky is distilled from the fermented mash, a process that requires boiling, which kills the yeast. Spent yeast from beer brewing can be used as a replacement.

Yeast cells are rich in B-group vitamins, particularly niacin (B₃), riboflavin (B₂), thiamine (B₁), Folic acid, and B₁₂. Yeast can be dried to produce vitamin-rich tablets or spreads such as Marmite. To make the latter, yeast is heated to 50 °C in large vats, and salt is added to promote autolysis (Section 5.10.6). Autolysis is self-Digestion brought about by Enzymes within dying cells. The mixture is filtered and centrifuged to remove cell walls, and then concentrated to the consistency of a thick paste. Vegetable extract is added to Marmite.

An alternative to autolysis is Hydrolysis using Hydrochloric acid. The hydrolysate is then neutralized with sodium hydroxide. The product is added to foods such as crisps, hamburgers, soups, sauces, and gravies as a savory, meat-flavored seasoning.

Single-Cell Protein from Photosynthetic Organisms

Both photosynthetic bacteria, such as cyanobacteria, and algae are used to produce single-cell protein. An example of a cyanobacterium is Spirulina. The Aztecs used to make cakes from it, and it forms a staple diet for flamingos living on the shores of East African lakes. Spirulina is cultivated on a small scale in Mexico and Hawaii for sale in health food stores. It is very high in protein and grows rapidly, and can be harvested from The surface of ponds or lakes. An example of a unicellular alga is Chlorella, which is sold in dried form as a health food in Japan and Taiwan.

The nutritional requirements and growth conditions for photosynthetic organisms were discussed in Sections 12.1.1 and 12.1.2. The fact that they require light and, for the most part, use carbon dioxide as a carbon source sharply distinguishes these organisms from fungi and non-photosynthetic bacteria. They are usually grown under non-sterile conditions in warm, open ponds, typically as mixed cultures (containing several different species).

Cultivating algae in wastewater achieves a dual benefit: treating the wastewater while simultaneously producing single-cell protein for animal feed. This approach has been implemented in Israel.