Mycoprotein as Food

The microbial food product of the early 21st century does not base its market success solely on account of its protein content, as vegetable proteins sources are abundant in the market at competitive prices. The resemblance in texture to that of currently appreciated foods, and a bland taste and light color, which renders it susceptible to the addition of flavoring and coloring agents, are also a prerequisite. The filamentous nature of the organism, a feature, which was considered as a technical difficulty for production at first, was foreseen as an advantage in the case of Quorn, rendering the final product a resemblance to animal or fish meat. Rarer features, but by no means less important ones, such as those favoring health in normal humans, or indeed rendering beneficial effects in patients with high blood cholesterol levels or diabetes are certainly important, as will be discussed below.

2.1.1 Composition

Although fungal biomass can be considered principally as a source of protein, it also contains nucleic acids, carbohydrate cell wall material, lipids, minerals, and vitamins. These contributions are generally considered of little relevance, with

Table 1 Yeasts and filamentous fungi species accepted for production of protein compounds and food ingredients for the food industry

Yeasts S. cerevisiae (baker's yeast)

S. cerevisiae (brewer's yeast) C. utilis (Torula yeast)

K. marxianus (formerly K. fragilis, S. fragilis) K. marxianus var. lactis (K. lactis, formerly S. lactis) C. pseudotropicalis P. pastoris P. rhodozyma

Filamentous fungi F. venenatum (formerly F. graminearum) (Quorn products)

P. variotii (Pekilo process, discontinued) A. niger; A. oryzae R. niveus; R. oryzae Mucor spp. Streptomyces spp. P. roquefortii

Data obtained from Halasz and Lasztity (1991), Peppler (1983) and Centre for Food Safety and Applied Nutrition (CFSAN) and US Food and Drug Administration (FDA) (http://www.cfsan.fda.gov).

Table 2 Daily requirements (g) of essential amino acids for the human adult

Essential amino acids

FAO recommendation

Minimum

Phenylalanine

2.2

1.1

Methionine

2.2

1.1

Leucine

2.2

1.1

Valine

1.6

0.8

Lysine

1.6

0.8

Isoleucine

1.4

0.7

Threonine

1.0

0.5

Tryptophan

0.5

0.25

Total

12.7

6.35

Data retrieved from FAO (http://www.fao.org).

Data retrieved from FAO (http://www.fao.org).

the exception of nucleic acids, which account for 10-15% (w/w) of the total nitrogen. Approximately 80% of total fungal nitrogen is composed of essential amino acids required for human growth and nutrition (Table 2). With reference to egg albumin, which is considered a perfectly balanced source of essential amino acids for human nutrition, mycoprotein presents a similar composition, although it is lower in sulfur containing amino acids. On the other hand, it is relatively rich in lysine and threonine if compared to other traditional protein sources of agricultural origin, such as wheat. A comparison with a wide range of protein sources is provided in Table 3.

2.1.2 Protein Value

The amino acid composition is only a theoretical indicator of the protein value of foods, since the degree of digestion and absorption of any one substance is determined by its susceptibility to be degraded and absorbed. The presence of inhibitors and multiple other factors in foods also modify their nutritional value. On the other hand, the digestive system of the organism in question is a key determinant in the digestibility of various protein sources. Thus, empirical tests are usually required for feed evaluation, and the parameters used are: the total quantity of microbial nitrogen ingested (I), the nitrogen of faeces (F), and nitrogen in urine (U). From these parameters, Digestibility, Biological Value (BV), and Protein efficiency (PE) can be calculated. Thus, Digestibility (D) is the percentage of the total nitrogen consumed, which is absorbed from the digestive tract.

Biological Value (BV) is the percentage of the total nitrogen assimilated, which is retained by the organism, taking into account the simultaneous loss of endogenous nitrogen through excretion in urine.

Protein Efficiency (PE) is the proportion of nitrogen retained when the protein under test is fed and compared with that retained when a reference protein, such as egg albumin, is fed.

Protein from yeast biomass such as Candida utilis has presented high digestibility values (81%), and BVs between 32 and 48%. Both parameters can be substantially increased when supplementation with 0.5% methionine is implemented (w/w, 90% in both cases). The BV of yeast has been estimated to be of 0.9, and methionine supplementation increases it to a value of 2.3 (Riviere 1977).

Quorn mycoprotein presents very high PE values, reaching 75% with respect to egg albumin. In experimental tests where mycoprotein was supplemented with 0.2% methionine, this value rose to 100%. Quorn products are not supplemented with methionine, but egg protein as explained below. Thus, mycoprotein could be used as a total replacement for the human diet (Trinci 1992,1994; US Patent 5935841 1999; WO Patent 9117669 1991).

Toxicity testing of Quorn mycoprotein has shown that the product can be consumed as the sole source of protein on a continued basis, without any adverse effects. Given the unconventional nature of this product, the tests undertaken for

Table 3 Essential amino acid content (g per 100 g edible portion) of mycoprotein (Quorn), baker's yeast, egg (whole raw fresh), beef (ground, regular baked-medium), soybeans (mature seeds, raw) and wheat (Durum)

Amino acids

Myco protein

Baker's yeast

Egg

Beef

Soybeans

Wheat

Histidine

0.39

0.22

0.30

0.78

0.98

0.32

Isoleucine

0.57

0.48

0.68

1.02

1.77

0.53

Leucine

0.95

0.67

1.10

1.80

2.97

0.93

Lysine

0.91

0.69

0.90

1.89

2.43

0.30

Methionine

0.23

0.17

0.39

0.58

0.49

0.22

Phenylalanine

0.54

0.41

0.66

0.89

1.90

0.68

Tryptophan

0.18

0.11

0.15

0.26

0.53

0.18

Threonine

0.61

0.44

0.60

0.99

1.58

0.37

Valine

0.60

0.51

0.76

1.11

1.82

0.59

Data from US Department of Agriculture, Agricultural Research Service. USDA Nutrient Database for Standard Reference, Release 14 July 2001. Nutrient Data Laboratory (http://www.nal.usda.gov/fnic/foodcomp) and P. Collins, Marlow Foods, UK.

Data from US Department of Agriculture, Agricultural Research Service. USDA Nutrient Database for Standard Reference, Release 14 July 2001. Nutrient Data Laboratory (http://www.nal.usda.gov/fnic/foodcomp) and P. Collins, Marlow Foods, UK.

its approval were especially thorough, lasting ten years. Human trials involved 2500 people with no adverse effects. The mycoprotein product is approved for consumption in the European Union. The U.S. Food and Drug Administration is currently in the process of publishing an official response to the Full Food Additive Petition submitted by the manufacturers. This response will be made public in 2002 (FDA, http://www.fda.gov, GRAS Notice No. GRN 000091). Products containing mycoprotein have been on sale in the United States since January 2002.

2.1.3 Nucleic Acid Content

Nucleic acids are a necessary component of all cells, and are found in relatively high levels in rapidly dividing cells. Thus, the nucleic acid content of yeast (around 10% of dry weight) is approximately five times greater than in the average mammalian organ. When nucleic acids are ingested, they are first attacked in the duodenum by pancreatic nuclease. The resulting nucleotides are then attacked by nucleotidases in the intestine, resulting in nucleosides and phosphate. These in turn are further degraded to purine and pyrimidine bases. The degradation of purine bases in man results in the production of uric acid. Accumulation of uric acid beyond the excretion capacity of the kidney results in the formation of crystalline deposits in the joints and soft tissues, leading to goutlike manifestations and calculi in the urinary tract. Pyrimidines are degraded to orotic acid, the accumulation of which results in liver damage. The administration of foods of microbial origin is limited by the amount of nucleic acid contained within. The administration of 130 g of yeast daily for one week results in uric acid levels ranging between 4.8 and 8.3mg/100ml in human volunteers. Normal plasma levels of uric acid range between 2 and 7mg/100ml in males (Riviere 1977).

Quorn mycoprotein is obtained from a filamentous fungus which proliferates at slower rate than yeast (Trinci 1994; Ugalde and Castrillo 2002). Thus, the starting nucleic acid content subject to removal is also slightly lower (8-9%). The RNA content reduction of mycoprotein is further effected by a heat shock treatment that will be described below [see section "Mycoprotein production (Quorn products")]. RNA levels are thus reduced well below the levels which limit consumption to 100 g per day per person (2% of dry weight), although this treatment also results in important losses in dry weight (Trinci 1992).

2.1.4 Texture

Another favorable feature which differentiates Quorn products is the advantage taken from the filamentous nature of the microorganism in product design. Fusarium venenatum A3/5 filaments are aligned in parallel by a specially designed mechanical process which renders the product a texture very similar to that of meat fibers once set in a light matrix of egg white protein and heated. The final product has a bland taste, light color which renders it susceptible to the addition of flavoring and coloring agents (Anderson et al. 1975).

2.1.5 Additional Functionalities

In addition to the nutritional effects, consumption of mycoprotein under both controlled and free-living studies has been shown to beneficially reduce total and low density lipoprotein (LDL) serum cholesterol levels. Studies by Homma et al. (1995), Turnbull et al. (1992), Udall et al. (1984) incorporating realistic amounts of the product concluded that mycoprotein consumption has a beneficial effect in serum lipid variables. Post-meal glycemia has been shown to be reduced after consumption of mycoprotein, by 13% with respect to controls. On the other hand, insulinemia is reduced by 19% thirty minutes after ingestion (Turnbull and Ward 1995). Finally, a mycoprotein lunch has a significant effect on the sensation of satiety, in ways that would help control the appetite of dieting patients, as proposed by Burley et al. (1993), Turnbull et al. (1993).

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