Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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Yeasts and Bacteria

M Garcı

a-Garibay, L Go

mez-Ruiz and

A E Cruz-Guerrero, Universidad Auto

noma

Metropolitana, Mexico City, Apartado, Mexico

EBa

rzana, Universidad Nacional Auto

noma de

xico, Mexico City, Mexico

Introduction

0001 The single-cell protein (SCP) concept is applied to the

massive growth of microorganisms for human or

animal consumption. Single-cell protein is a generic

term for crude or refined protein whose origin is

bacteria, yeasts, molds, or algae, microorganisms

that usually contain above 40% of crude protein on

dry weight bases. Yeasts and bacteria have been

particularly important for SCP production and easily

acceptable as their biomass has been consumed by

man since ancient times in the form of fermented

foods. The production of SCP has important advan-

tages over other sources of proteins, such as its

considerably shorter doubling time, the small land

requirement, and the fact that it is not affected by

the weather conditions. Much attention was focused

on the use of petroleum derivatives as substrates for

the SCP production during the 1960s and 1970s when

the price of this reserve was low, but currently, the

production of SCP is based on renewable resources,

and its interest is also kept as a means to confer value

to waste materials. The organoleptic and functional

properties of SCP are not always competitive, and its

main drawback has been its high production costs.

However, recent advances in fermentation technol-

ogy and genetic engineering could reevaluate SCP

production.

Historical Developments and

Implementation of SCP Production

0002 The first developments in SCP production were

achieved during war times when conventional foods

were in short supply. During World War I, Saccharo-

myces cerevisiae was massively produced in Germany

from molasses to replace up to 60% of protein

imported. Similar measures were taken during World

War II for the mass production of Candida utilis

(known formerly as Torula yeast, or Torulopsis utilis)

on sulfite liquor from paper manufacturing wastes.

After the war, several plants were built in the USA and

Europe, mainly for C. utilis production.

0003 Accelerated industrial development and general

welfare expectancy led to a renewed interest in SCP

as an alternative to alleviating food shortages due

to a growing imbalance between food production

and demand by the world’s population, mainly in

developing countries. During the 1960s and 1970s,

several SCP production plants were built in the

UK, France, Italy, Russia, Japan, Taiwan, etc.

0004An important breakthrough occurred when the

production of SCP from hydrocarbons was demon-

strated by several petroleum companies during the

1950s and 1960s. During the 1970s, considerable

research efforts resulted in the use of methanol and

ethanol, derived from petroleum, as convenient sub-

strates. In the early 1970s, the cost of n-paraffin was

approximately US$80 per tonne, whereas crude

molasses had an approximate cost of US$82 per

tonne. However, concern about substrate safety and

the increase in petroleum prices shifted interest back

to the utilization of renewal sources, mainly food and

agriculture byproducts like molasses and whey, or

industrial wastes rich in starch, cellulose, and hemi-

cellulose. The major SCP projects based on petroleum

derivatives as substrates were abandoned in the

1980s.

0005In recent times, among the European Communist

countries, Russia has had the greatest capacity for

SCP production, with at least 86 plants in operation,

using different substrates. Several other countries in

Eastern Europe, like the former Czechoslovakia, had

similar capacities. The Russian delegation reported

the production of 1 million tonnes of SCP per year

in 1983 at an international symposium on SCP, and

estimated that they would be producing 9 million

tonnes by the end of the 1990s.

0006To date, an enormous number of reports about SCP

production have appeared in the scientific literature.

Two main approaches have been followed: the util-

ization of convenient substrates and the use of waste

materials where SCP production brings about pollu-

tion control.

0007Many industrial process have been developed

world-wide; the most important are shown in Tables

1 and 2. The countries with important industrial

outputs are the USA, UK, France, Russia, and

Cuba.

Suitable Organisms and Substrates

0008The most frequently used yeasts are as follows:

Saccharomyces cerevisiae (and related synonymous

species such as S. carlsbergensis, S. uvarum, etc.),

Kluyveromyces marxianus (including synonymous

subspecies and asexual forms such as K. fragilis,

K. lactis, K. bulgaricus, Candida kefyr and C.

pseudotropicalis), C. utilis (and its sexual form

Hansenula jadinii) and Yarrowia lipolytica (formerly

SINGLE-CELL PROTEIN/Yeasts and Bacteria 5277

known as Candida lipolytica and Saccharomycopsis

lipolytica).

0009 Some yeasts have been widely used in the manufac-

ture of human foods: S. cerevisiae, K. marxianus,and

C. utilis have the generally recognized as safe (GRAS)

status for human consumption by the Food and Drugs

Administration in the USA. S. cerevisiae is also avail-

able as spent yeast from breweries and alcohol indus-

tries; Figure 1 shows an electron micrograph of this

species. K. marxianus and related species are widely

used, due to their capacity to assimilate lactose, the

carbohydrate present in cheese whey, but it can also

grow on inulin, a fructose polymer found in some

plants, and other simple sugars such as glucose,

fructose, and sucrose; therefore, sometimes, it is also

grown in molasses. Since it is able to grow at tem-

peratures as high as 45



C, it has been used to produce

biomass in tropical areas. Figure 2 shows an electron

micrograph of cells of K. marxianus. C. utilis is used

for a wide variety of substrates such as sucrose, etha-

nol, and sulfite-spent liquor. It can also grow on wood

hydrolysates because of its ability to assimilate

pentoses. Starchy solids or water streams from potato

and maize industries require previous hydrolysis for

yeast growth, as in the case of C. utilis, or as in the

Symba process, which utilizes amylolytic yeast (Sac-

charomycopsis fibuligera). Yeasts able to assimilate

tbl0001 Table 1 Main yeast SCP industrial and pilot developments

Substrate/yeast Process/product Country

Sulfite liquor

Candida utilis St. Regis Paper USA

Candida utilis Boise Cascade USA

Candida utilis State industry Russia

Candida utilis Attisholz Switzerland

Hydrocarbons

Ya rr owia lip oly t ic a British Petroleum/

Toprina

Candida tropicalis British Petroleum/

Toprina

France

Candida tropicalis Italprotein Italy

Ya rr owia lip oly t ic a Liquichimica/

Liquipron

Italy

Ya rr owia lip oly t ic a State industry Russia

Ya rr owia lip oly t ic a Swedt Germany

Ya rr owia lip oly t ic a Roniprot Romania

Ethanol

Candida utilis Amoco/Torutein USA

Candida sp. Mitsubishi Japan

Methanol

Pichia sp. Mitsubishi Japan

Pichia pastoris Philipps Petroleum/

Provesteen

USA

Starch

Saccharomycopsis fibuligera

and Candida utilis

Symba Sweden

Saccharomyces cerevisiae Brewers Several

Molasses

Candida utilis Several industrial

processes

Cuba

Candida utilis Several industrial

processes

Taiwan

Candida utilis CINVESTAV IPN Mexico

Liquid sucrose

Hansenula jadinii Philipps Petroleum/

Provesta

USA

Whey

Kluyveromyces marxianus Wheast–Knudsen USA

Kluyveromyces marxianus Amber Lab–

Universal Foods

USA

K. marxianus, K. lactis, and

Candida pintolopesii

Fromageries Bel France

Kluyveromyces marxianus S.A.V. France

Candida intermedia Vienna Austria

Candida utilis Waldhof USA

Confectionery effluent

Candida utilis Bassett UK

tbl0002Table 2 Main bacterial SCP industrial and pilot developments

Substrate/bacteria Process/product Country

Methane

Methylococcus capsulatus Shell Oil UK

Methanol

Methylophilus methylotrophus ICI/Pruteen UK

Methylomonas clara Hoechst–Uhde/

Probion

Germany

Ethanol

Acinetobacter calcoaceticus Exxon–Nestle Switzerland

Cellulose

Cellulomonas sp. and

Alcaligenes sp.

Louisiana State

University

USA

Whey

Lactobacillus bulgaricus and

Candida krusei

Kiel Germany

4 ku 11,000

1µm

08 20 SEI

fig0001Figure 1 Saccharomyces cerevisiae observed under a scanning

electron microscope.

5278 SINGLE-CELL PROTEIN/Yeasts and Bacteria

hydrocarbons include Y. lipolytica, C. tropicalis,

C. rugosa, and C. guilliermondii, which usually can

also grow on lipids. Methanol is the preferred alcohol

for use as a substrate by Pichia species (P. pastoris,

P. methanolica, etc.), Hansenula polymorpha, Hanse-

nula capsulata, and Candida boidinii. The most im-

portant processes developed for yeast SCP production

are shown in Table 1.

0010 Bacteria have been used mostly for the production

of animal feed. The most commercially important

species utilize methane and/or methanol as substrates,

as shown in Table 2. Methanol is usually preferred

over methane because it is water-soluble and less

explosive. The Ministry of Agriculture, Fisheries and

Food in the UK allows the use of ICI’s (Imperial

Chemical Industries) product ‘Pruteen’ in animal

feed. For the production of bacterial SCP from

ethanol, either Acinetobacter calcoaceticus or Alcali-

genes sp. have been used in laboratory or pilot-

plant studies.

0011 Other petroleum derivatives such as paraffins have

been considered only to a minor degree for the pro-

duction of SCP using bacteria; some studies have been

carried out with Acinetobacter calcoaceticus.

0012 To produce bacterial SCP from whey, several lactic

acid and propionic bacteria have been investigated,

frequently in mixed cultures with yeasts as in the

Kiel process. In this case, Lactobacillus delbrueckii

ssp. bulgaricus grows using the lactose, converting

it to lactic acid; then, Candida krusei, which is unable

to ferment lactose, uses the lactic acid as a carbon

source. Both fermentations can be performed simul-

taneously, controlling the pH by adding ammonia,

which is used as a nitrogen source by the yeast.

0013 Lactic acid bacteria proposed for the fermentation

of whey include species such as Lactobacillus del-

brueckii subsp. delbrueckii, L. delbrueckii subsp. bul-

garicus, Lactobacillus casei ssp. casei, Leuconostoc

sp., etc. Fermentation of this substrate with rumen

bacteria has also been proposed to produce biomass

concentrated by ultrafiltration for use as feed.

0014Propionibacteria, such as Propionibacterium

freudenreichii ssp. shermanii and P. freudenreichii

ssp. freudenreichii, produce significant amounts of

vitamin B

, giving additional attractiveness to their

utilization in the production of SCP. A process using a

mixture of P. freudenreichii ssp. freudenreichii and

K. marxianus has been proposed for the production

of SCP from whey rich in vitamin B

and sulfur

amino acids.

0015Bacteria have been explored very little with regard

to producing SCP from substrates such as starch and

cellulose. A case in point is the project developed by

Louisiana State University in USA to produce SCP

from Cellulomonas sp. and Alcaligenes sp. from cane

bagasse; this process was scaled up to pilot-plant

level.

0016Generally, yeasts have been preferred for SCP

production over bacteria, especially for human

consumption. It seems that yeasts are more accept-

able because they are more familiar to humans

through ageless foods like bread or beer. However,

bacteria have some advantages over yeasts such as a

higher protein content (Table 3), higher yields

(carbon source to protein conversion), and a faster

growth rate, though a higher nucleic acid content

(Table 4) limits their uptake in the diet. (See Lactic

Acid Bacteria; Yeasts.)

Production Processes

0017Since the SCP must have a competitive price in the

protein market, especially proteins of vegetable

origin, it is essential to guarantee efficiency along all

stages of the process. The carbon source accounts for

up to about 60% of the operation costs, so high yields

of substrate conversion are required, high productiv-

ity processes must be implemented, and the utiliza-

tion of an inexpensive but easily assimilated carbon

source has to be guaranteed. This explains the gener-

alized use of molasses, whey, or industrial residues,

depending on local availability, and the attempts to

implement fossil fuels as substrates. An important

advantage in using hydrocarbons is the yield,

obtaining 1 g of dry biomass per gram of hydrocar-

bon, compared with carbohydrates, which typically

yield around 0.5 g of dry biomass per gram of

substrate.

0018The typical process stages for the production of

SCP comprise: raw material preparation, fermenta-

tion, biomass recovery, cell disruption, and drying.

Figure 3 shows a typical process for the production

of yeast SCP.

5 ku 3000

2µ

0620 SEI

fig0002 Figure 2 Kluyveromyces marxianus observed under a scanning

electron microscope.

SINGLE-CELL PROTEIN/Yeasts and Bacteria 5279

0019 To maximize carbon assimilation, the nutrients

must be balanced. Sources of nitrogen, minor

elements (P, K, S, Mg, etc.), trace elements, and vita-

mins are adjusted according to the general compos-

ition of the carbon source. This in turn is highly

dependent on the strain used. In general, simple ni-

trogen sources such as urea, ammonia, and nitrate are

used to keep costs down. Phosphate is supplied as

phosphoric acid or as soluble phosphate salts.

0020 Fermentation variables like temperature, pH, ionic

strength, level of oxygenation, and, in the continuous

fermentations, dilution rate, have a strong influence

on cellular yield. In particular, an abundant supply of

oxygen promotes aerobic metabolism and higher

growth rates. However, due to the low solubility

of oxygen in aqueous media, the cost of aeration,

through air sparging and agitation, increases rapidly

with the scale of operation, resulting in an important

technical challenge. Assimilation of n-paraffins

requires considerably high levels of oxygenation,

and the growth of microorganisms on these water-

insoluble substrates takes place on the hydrocarbon–

water interface. The surface area becomes the limiting

factor, and heat production is about twice that using

sugars as substrates.

0021In general, when yeast biomass is produced,

alcohol accumulates due to oxygen limitation. Some

alternatives proposed for SCP from whey are the

production of alcohol as a byproduct from fermenta-

tion or the use of Kluyveromyces in a mixed culture

with Candida pintolopesii, where the latter consumes

the alcohol produced by the former. The first ap-

proach has been followed by Amber Laboratories

(Universal Foods, USA), and the second approach

has been adopted by the Fromageries Bel in France;

Candida valida has also been demonstrated to

prevent ethanol accumulation when it is grown

tbl0004 Table 4 Nucleic acid contents of SCP products (g per 100 g of

biomass in dry weight basis)

Kluyveromyces marxianus 2.7–10

Saccharomyces cerevisiae 2.5–15

Candida utilis 6.2–10

Methylophilus methylotrophus 16

Methylomonas clara 10–15

Isolated protein from K. marxianus 1.4–5.7

tbl0003 Table 3 Nutritional parameters of SCP products

Kluyveromyces

marxianus

Saccharomyces

cerevisiae

Candida utilis Methylophilus

methylotrophus

Methylomonas clara Soya meal

Protein (grams per 100 g dry weight)

Crude (N  6.25) 43–58 48 42–57 72–88 80–85 44–50

True protein 40–42 36 47 64 69–73 48

Essential amino acids (grams per 16 g of N)

Isoleucine 4.0–5.1 4.6–5.5 4.3–5.3 5.2–5.4 3.6 5.4

Leucine 7.0–8.1 7.0–8.1 7.0 8.2–8.4 6.6 7.7

Phenylalanine 3.4–5.1 4.1–4.5 3.7–4.3 4.3–6.5 5.1 5.1

Tyrosine 2.5–4.6 4.9 3.3 3.5–3.8 5.1 2.7

Threonine 4.1–5.8 4.8–5.2 4.7–5.5 5.7–6.5 4.8 4.0

Tryptophan 0.9–1.7 1.0–1.2 1.2 1.1–1.6 1.5 1.5

Valine 5.4–5.9 5.3–6.7 5.3–6.3 6.3–6.5 4.8 5.0

Arginine 4.8–7.4 5.0–5.3 5.4–7.2 4.3–5.6 3.4 7.7

Histidine 1.9–4.0 3.1–4.0 1.9–2.1 2.2–2.3 2.8 2.4

Lysine 6.9–11.1 7.7–8.4 6.7–7.2 4.1–7.3 6.2 6.5

Cystine 1.7–1.9 1.6 0.6–0.7 0.8 1.4

Methionine 1.3–1.6 1.6–2.5 1.0–1.2 1.4–3.0 2.5 1.4

PER 1.8 2.0 1.7 1.4–2.2

NPU 67 84 64

Vitamins ( mgperg)

Thiamin 24–26 104–250 8–9.5 5 9.0

Riboflavin 36–51 25–80 44–45 40 3.6

Pyridoxine 14 23–40 79–83 2 6.8

Nicotinic acid 136–280 300–627 450–550 57 24.0

Folic acid 6 19–30 4–21 15 4.1

Pantothenic acid 67 72–86 94–189 11 21.0

Biotin 2 1 0.4–0.8 3

Vitamin B

0.015–0.05 0.0001 0

PER, protein efficiency ratio; NPU, net protein utilization.

5280 SINGLE-CELL PROTEIN/Yeasts and Bacteria

mixed with Candida kefyr (the asexual form of K.

marxianus), increasing the efficiency of whey conver-

sion into biomass up to 20%. A limited oxygen

supply, when hydrocarbons are used as substrates,

has led to the production of some metabolites such

as mono and dicarboxylic acids and ketones.

0022 To sustain high oxygen transfer rates, large air

volumes have to be supplied with high agitation

rates. Alternative fermenter designs include the air-

lift type, which leads to maximum oxygenation with

minimum power requirements, diminishing aeration

costs considerably. In fact, the largest fermenter ever

operated is an air-lift (3000 m

) for the aerobic pro-

duction of Methylophilus methylotrophus by ICI.

Figure 4 shows a simplified diagram of the process

developed by ICI, which is the most successful process

developed for bacterial SCP production. Currently,

the high-cell-density fermentation designs pioneered

by Provesta (a company belonging to Philipps Petrol-

eum) allows them to obtain up to 160 g l

1

of yeast

biomass, while traditional fermentation techniques

yield at most 30 g l

1

. These fermenters have attached

very efficient systems for heat removal and oxygen

transfer.

0023 Once obtained, the microbial biomass is recovered

by filtration or centrifugation. The resulting cell

suspension can be either spray-dried, or the cells

can be broken to obtain extracts, hydrolysates, or

autolysates. Finally, the protein can be concentrated,

isolated, or texturized. In the Philipps Petroleum pro-

cess, owing to the high cell density, the spent medium

is fed directly to the spray-drier without prior concen-

tration.

Nutritional Value

0024The main nutritional contribution of SCP either for

human food or animal feed is its high protein content.

Bacteria have a protein concentration ranging from

50 to 85% and yeasts from 45 to 70%. Protein

quality is also quite acceptable, as compared with

vegetable proteins. Generally, SCP from any source

is limited in sulfur-containing amino acids; however,

bacterial proteins tend to have better balances of

essential amino acids than yeasts and other micro-

organisms, particularly with respect to sulfur amino

acids. Some parameters of protein quality are given in

Table 3. When methionine is added to SCP, the pro-

tein quality increases considerably and reaches values

similar to those of casein. (See Amino Acids: Proper-

ties and Occurrence; Metabolism; Protein: Quality.)

0025SCP is also an important source of vitamins;

actually, brewer’s yeast has long been used as a vita-

min supplement. Vitamin contents are also listed in

Table 3.

Toxicological Aspects

0026Safety of both the microorganism and the substrate

is an important consideration. Microorganisms used

for SCP production have to be subjected to exten-

sive toxicological clearance. Those microorganisms

Ammonia

Centrifugation

Spray dryer

Grinding

Phosphoric acid

Flocculation

Air-lift

fermenter

Salts

Sterilization

Methanol

fig0004Figure 4 Industrial process developed in the UK by ICI for the

production of Pruteen using Methylophilus methylotrophus grown

in methanol.

Molasses

or whey

Substrate

preparation

Sterilization

Fermentation

Centrifugation

Cell disruption

Spray dryer

Nitrogen source

Salts

fig0003 Figure 3 Typical process for the production of yeast SCP.

SINGLE-CELL PROTEIN/Yeasts and Bacteria 5281

normally present in fermented foods, such as Sacchar-

omyces cerevisiae, are safe for human consumption.

The main concern has been linked to the use of pet-

roleum derivatives in which residual alkanes must be

removed with solvents. However, some residual

hydrocarbons may still be present, and several reports

have stated the presence of unusually high amounts of

odd-chain fatty acids and paraffins in tissues from

animals fed with SCP from alkanes. These fatty

acids, particularly unsaturated C17, have been

suspected of having toxic effects, even though no

evidence has been reported.

0027 The high content of nucleic acids present in micro-

bial cells also has some consequences. Human con-

sumption of nucleic acids in amounts higher than 2 g

per day could lead to the accumulation of uric acid,

resulting in kidney stones and/or gout onset in suscep-

tible people. The concentration of nucleic acids in the

biomass is highly variable and depends on several

factors. The first element is the nature, species, and

strain of the microorganisms; normally, bacteria are

present in higher concentrations than yeasts. Another

factor is the growth conditions: the higher the growth

rate, the higher the nucleic acids content in the cell.

The nucleic acid content also changes with the

growth phases. For instance, for K. marxianus

grown on whey, the concentration of nucleic acids

reached its peak at the middle of the exponential

phase. Table 4 shows the nucleic acids content of

several SCP products.

0028 In order to reduce the nucleic acid content, two

approaches have been followed: grow the biomass

at low rates or isolate the protein by eliminating

undesirable compounds. The later is usually applied,

to eliminate not only nucleic acids but also the cell

wall. In recent years, research has been conducted on

the use of external (added) endonucleases to hydro-

lyze nucleic acids and so obtain protein isolates free

from these polymers. Immobilized endonucleases

have been proposed particularly for this purpose.

0029 Yeasts and bacterial cell walls are difficult to digest,

leading to poor bioavailability of proteins, flatulence

and diarrhea, and allergic responses. Some cases of

skin rashes, nausea, vomiting, and other allergic reac-

tions have been reported, but they may be eliminated

by reducing cell walls and nucleic acids.

0030 Nucleic acid content in SCP for animal feed is not a

problem, because animals have the enzyme uricase,

which prevents uric acid accumulation. However, cell

wall digestibility in monogastric animals is also poor.

Cell Disruption and Protein Isolation

0031 Many processes to disrupt the cells have been

developed. A common process is autolysis, in which

the biomass is exposed to a heat shock or to chemical

compounds, such as low-molecular-weight thiols.

Yeast autolysis usually takes place when cells are

heated to 45–55



C, and it is enhanced by the pres-

ence of NaCl. Further incubation for around 24 h

induces cellular enzymes, leading to complete cell

lysis. The process also activates endogenous ribo-

nucleases that reduce nucleic acids. Lysis can be fa-

cilitated by the addition of exogenous enzymes such

as proteases, b-glucanases or lysozyme. The disad-

vantages of these techniques are the high costs in-

volved and extensive proteolysis, which reduces the

yield and functional properties of proteins. Chemical

treatments with alkalis, organic solvents, or salts,

which weaken cell walls, are also used. Alkaline treat-

ment may result in undesirable side reactions,

forming compounds such as lysinoalanine and off-

flavors.

0032Hydrolysis requires the use of hydrochloric acid

at temperatures as high as 100



C, to treat a slurry

with 65–85% solids content, followed by neutraliza-

tion with NaOH. This process has many drawbacks

such as the risk of accidents with concentrated acid,

the need for anticorrosive equipment, which is cost-

intensive, and the destruction of some amino acids

and vitamins, reducing the nutritive value of the

product.

0033Physical methods to break cell walls are the

methods most widely used. High shear rates are

achieved by means of homogenizers or colloidal

mills and have been used extensively for SCP

processes.

0034Once the cells have been broken, the protein is

extracted using water or alkaline conditions; the cell

wall debris is removed by centrifugation, and the

protein is further precipitated with acid, salt, or heat

treatment, while the nucleic acids remain in the

supernatant. Usually, microbial proteins have their

maximum solubility at pH 12, and the precipitation

occurs at pH 4.5. The protein isolate is then obtained.

Some chemical modification during protein extrac-

tion includes phosphorylation or succinylation,

which facilitates protein separation from nucleic

acids and improves its functional properties.

Utilization for Human Food

0035For food applications, besides toxicity and nutritional

quality, organoleptic acceptability and functional

properties are important considerations.

0036SCP has been used as a protein supplement in baked

goods, cookies, crackers, snacks, soups, noodles, and

special foods like geriatric or baby meals. Its use as

an extender in sausages and other meat products

has been important, mainly in Eastern European

5282 SINGLE-CELL PROTEIN/Yeasts and Bacteria

countries. Despite the justified production of SCP in

terms of the world’s protein shortage and widespread

malnutrition, a real demand for a protein is based on

its favorability in terms of functional properties like

solubility, water binding, emulsification capacity, gel-

ation, whippability, and foam stability. The successful

supplementation of existing products and the replace-

ment of traditional proteins with SCP depend on

the availability of proteins matching in functionality,

price, and organoleptic acceptability.

0037 For food applications, whole dried cells, disrupted

cells, and textured protein products are useful. From

disrupted cells, either protein concentrates or isolates

can be obtained, which are better suited for the food

industry. Moreover, SCP isolates can compete favor-

ably with soy isolates from the functional point of

view. However, isolation or concentration increases

production costs dramatically.

0038 Processes such as texturization, by spinning and

extrusion, and enzymatic or chemical modification

can improve the functionality of SCP. For instance,

protein fibers obtained by spinning can form textured

protein products such as meat extenders.

0039 Enzymatic modification includes partial proteo-

lysis to improve solubility, emulsification capability,

and whippability, or the reverse reaction known as

plastein (peptide bond formation) to improve the

nutritional value through the addition of limiting

amino acids. Promising chemical modifications in-

clude acetylation, which improves the thermal stabil-

ity, succinylation, or phosphorylation to increase the

solubility, emulsification, and foaming capacities.

However, such modifications tend to reduce the

nutritive value of the proteins. Experiences with

phosphorylated yeast proteins have demonstrated

that protein recovery can be improved, reducing nu-

cleic acids. Functional properties such as water solu-

bility, water-holding capacity, and thickening

properties can be enhanced, whereas the emulsifying

activity is better than soy protein isolate and equiva-

lent to sodium caseinate.

0040 Although dried whole cells have limited functional

properties, they are frequently used as flavor-carrying

agents and food binders. Dried yeast cells can act as

oil-in-water emulsion stabilizers. (See Emulsifiers:

Phosphates as Meat Emulsion Stabilizers; Uses in

Processed Foods.)

0041 The major market for microbial biomass is as a

flavor enhancer for meat products, soups, gravies,

barbecues, sauces, salad dressings, seasonings, and

any food with savory, cheesy, or meaty flavors (flavor

notes associated with the fifth basic flavor called

‘umami’), including pizzas, snacks, chips, etc. In

fact, yeast protein hydrolysates, autolysates, and

extracts have long been used as food flavorings.

Prospects

0042SCP has to compete with other protein sources such

as soy bean, fish meal, and milk proteins. It has been

widely demonstrated that in proteins to be used as

additives or to be incorporated into a food, the most

important factors to be considered are their func-

tional properties and price; therefore, these are the

main challenges that SCP has to face. Unfortunately,

production and isolation of protein from microbial

biomass are rather expensive because they are capital-

and energy-intensive. Its broad utilization has

been limited for economical reasons. However, auto-

lysates or hydrolysates prepared mainly from yeasts

have gained a wide acceptability as functional food

ingredients. In addition, recent biotechnological ad-

vances such as high-cell-density fermentations, more

efficient downstream operations, and the possibility

to genetically improve microorganisms could re-

evaluate SCP.

0043High-cell-density fermenters have made possible a

considerable reduction of equipment size, energy

savings, very high productivities, and cheaper down-

stream processing. For instance, direct spray-drying

from the fermenter is possible. These kinds of im-

provements could bring SCP to a competitive level.

Currently, Philipps Petroleum is producing Provesta

and Provesteen, trade marks for SCP from different

strains, using this process (see Table 1).

0044The use of fed-batch fermentations has not been

fully explored for SCP production. This technique is

widely used for the production of bakers’ yeast,

increasing yields and avoiding ethanol accumulation.

Some reports applying this strategy to yeast SCP

production have demonstrated a yield increase up to

70%. The possibility of implementing continuous

fermentation technologies to improve productivity

has been explored for the production of SCP. A com-

mercial process operated by Socie

des Alcohols du

Vexin (SAV) in France is currently using a continuous

culture for SCP production from whey.

0045Genetic engineering has focused on the possibility

of improving substrate utilization. The first modified

microorganism utilized in an industrial process

and the largest-scale application for genetic engineer-

ing is a strain of Methylophilus methylotrophus

developed by ICI in 1977. The improved strain,

grown on methanol, is able to utilize the ammonium

ion as a nitrogen source more efficiently than the

wild strain, saving 1 mol of ATP per mole of

ammonium assimilated, with an increase in efficiency

of 4–7%.

0046Considerable research based on genetic engineering

has been carried out to obtain yeasts able to utilize a

broader range of carbon sources, such as lactose,

SINGLE-CELL PROTEIN/Yeasts and Bacteria 5283

starch, cellulose, xylose, and chitin, in order to use

cheaper and more available substrates.

0047 Another interesting issue is to obtain easily and/or

genetically controlled autolytic cells, for which differ-

ent approaches have been followed, including the

selection of mutants with weaker cell walls; also, the

introduction of genes coding for lytic enzymes to

facilitate cell disruption, and genes coding for nucle-

ase activity to reduce nucleic acids content, have been

explored. Recently, a system was developed that con-

sists of a regulated promoter that controls two genes

involved in cell wall biogenesis, which led to the

possibility of triggering cell lysis of S. cerevisiae

by the addition of methionine; this system can be

extended to other yeast species. Also, an enhance-

ment of the nutritional value by modification of the

amino acid content, or obtaining proteins with better

functional properties looks promising. All these pos-

sibilities have been investigated, but practical results

will take longer to be implemented.

0048 Another approach to improve the economics of

SCP would be to produce it as a low cost byproduct

in multiproduct microbial processes, such as during

processing of food wastes to reduce the biological

oxygen demand, or as a byproduct from high-added-

value enzymes. Another possibility is the recovery of

nucleic acids from bacteria and yeast biomass to

produce 5

-nucleotides, which can be used as flavor

enhancers.

See also: Single-cell Protein: Algae; Yeasts

Further Reading

Batt CA and Sinskey AJ (1987) Single-cell protein: produc-

tion modification and utilization. In Knorr D (ed.) Food

Biotechnology, pp. 347–362. New York: Marcel Dekker.

de la Torre M and Flores-Cotera LB (1993) Proteı

nas Uni-

celulares. In: Garcı

a-Garibay M, Quintero-Ramirez R

and Lo

pez-Munguı

´a

A (eds) Biotecnologı

´a

Alimentaria,

pp. 383–397. Mexico City: Editorial Limusa.

Goldberg I (1985) Single-cell Protein. Berlin: Springer.

Guzma

n-Juarez M (1983) Yeast protein. In: Hudson BJF

(ed.) Developments in Food Proteins – 2, pp. 263–291.

London: Elsevier Applied Science.

Harrison JS (1993) Food and fodder yeasts. In: Rose AH

and Harrison JS (eds) The Yeasts, 2nd edn, vol. 5,

pp. 399–433. London: Academic Press.

Litchfield JH (1983) Single-cell proteins. Science 219:

740–746.

Litchfield JH (1991) Food supplements from microbial pro-

tein. In: Goldberg I and Williams R (eds) Biotechnology

and Food Ingredients, pp. 65–109. New York: Van

Nostrand Reinhold.

Moo-Young M and Gregory K (1986) Microbial Biomass

Proteins. London: Elsevier.

Reed G (1982) Microbial biomass, single cell protein, and

other microbial products. In: Reed G (ed.) Prescott &

Dunn’s Industrial Microbiology, 4th edn., pp. 541–592.

Westport CT: AVI.

Reed G and Nagodawithana TW (1991) Yeast Technology,

2nd edn. New York: Van Nostrand Reinhold.

Rolz C (1990) Caracterı

´sticas

Quı

´micas

y Bioquı

´micas

la Biomasa Microbiana. Archivos Latinoamericanos de

Nutricio

n 40(2): 147–193.

Rose AH (ed.) (1979) Microbial Biomass. Economic Micro-

biology, vol. 4. London: Academic Press.

Schlingmann M, Faust U and Scharf U (1984) Bacterial

proteins. In: Hudson BJF (ed.) Developments in Food

Proteins – 3, pp. 139–173. London: Elsevier Applied

Science.

Slaughter See Meat: Sources; Structure; Slaughter; Preservation; Eating Quality; Sausages and

Comminuted Products; Analysis; Nutritional Value; Hygiene; Extracts

5284 SINGLE-CELL PROTEIN/Yeasts and Bacteria

SLIMMING

Contents

Slimming Diets

Metabolic Conseqences of Slimming Diets and Weight Maintenance

Slimming Diets

D L Miller, Central Soya Company Inc., Fort Wayne,

IN, USA

Background

0001 The prevalence of being overweight or obese in Amer-

ica is staggering. An accepted measure of overweight

status and obesity is the body mass index (BMI). BMI

is weight (kg) divided by height-squared (m

). A BMI

of 30 is generally accepted as the lower limit for

obesity, whereas individuals with BMIs between 25

and 30 are considered overweight (Table 1). A recent

survey has estimated that more than 33% of Ameri-

cans meet the criteria for being obese, whereas 50%

of American adults meet the criteria for overweight/

obese.

0002 It is not surprising, then, that a large number of

adults are attempting to lose weight. Consumers

are bombarded with endless commercially available

weight-loss programs – many promising fast and easy

weight loss. Some commercial programs recommend

a low-fat diet, whereas others recommend low-

carbohydrate diets, and protein intake varies among

these programs. Still others advocate single foods,

juices, or herbal remedies (Table 2). In order to

review this topic in a concise method, weight-loss

diets will be discussed, based on their macronutrient

composition.

Concept of Macronutrient Composition

0003 Foodstuffs are generally comprised of three major

‘macro’ nutrients: protein, carbohydrate, and fat (see

Figure 1). Foods are not solely made of one macronu-

trient, but are composites of all macronutrients in

various proportions. Macronutrients vary in their

energy yield per gram: protein and carbohydrate

both yield 17 kJ (4 kcal) g

1

, whereas fat is a much

denser source of energy, yielding an average of

38 kJ (9 kcal) g

1

. Alcohol is also a dietary source of

energy, with 29 kJ (7 kcal) g

1

but alcohol will not

be discussed in this review. In terms of energy (kJ/

kcal), the typical American diet comprises 10–15%

protein, 50–55% carbohydrate, and 35–40% fat.

Energy Equation

0004In order to discuss the effects of alterating the macro-

nutrient composition of a food on body weight, it is

useful to discuss the basic concept of energy balance.

To maintain a stable body weight, the total energy

intake must equal the total energy loss or output, as

shown by the following equation:

input

 E

output

¼ E

stored

The simple interpretation of this equation, as it relates

to human obesity, is that if more energy is consumed

by an individual than is expended by that individual,

the excess energy will be stored. This model becomes

more complicated when the ingested and stored

energy is subdivided into the three macronutrients.

A normal-weight human being typically stores

585 760 kJ (140 000 kcal) of fat in adipose tissue,

100 416 kJ (24 000 kcal) of protein, mainly in muscle

mass, and 3347 kJ (800 kcal) as carbohydrate in the

form of glycogen (stored mainly in the liver, kidney,

and muscles) plus glucose that circulates in the blood.

In humans, stores of protein and carbohydrate remain

rather constant, regardless of the level of intake,

whereas theoretically, fat can be stored in an almost

unlimited capacity.

Macronutrient Composition and Weight

Loss

High-protein Diets

0005Protein makes up a small and relatively constant pro-

portion (14–18%) of total energy intake of adults.

There is much debate in the scientific community

about the role of protein intake in weight loss. Some

researchers theorize that, because dietary protein is

essential for physiological processes (growth, tissue

maintenance, enzyme production, hormonal synthe-

sis, etc.), humans and other animals consume foods to

deliver the protein needed to sustain these processes.

As it relates to weight control, if an individual

chooses high-carbohydrate and/or high-fat foods

SLIMMING/Slimming Diets 5285

with little protein content, the individual would need

to consume a larger volume of food to obtain the

necessary amount of protein.

0006 Indeed, researchers have found that high levels of

protein (> 25% of energy) have been shown to have a

suppressive effect on energy intake in humans and

animals. In humans, the consumption of high-protein

foods has been shown to acutely reduce subsequent

energy intake relative to low-protein foods. However,

the protein intake of adults in Western populations is

generally higher than the recommended dietary

levels. The US recommended daily amount (RDA)

tbl0001 Table 1 Body mass index

Weight (lb)

Height

(ft)

100105110115120125130135140145150155160165170175180185190195200205210215220225230235240245250

20 21 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49

19 20 21 22 23 24 25 26 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 43 44 45 46 47

18 19 20 21 22 23 24 25 26 27 27 28 29 30 31 32 33 34 35 36 37 37 38 39 40 41 42 43 44 45 46

18 19 19 20 21 22 23 24 25 26 27 27 28 29 30 31 32 33 34 35 36 37 37 38 39 40 41 42 43 43 44

17 18 19 20 21 21 22 23 24 25 26 27 27 28 29 30 31 32 33 33 34 35 36 37 38 39 39 40 41 42 43

17 17 18 19 20 21 22 22 23 24 25 26 27 27 28 29 30 31 32 32 33 34 35 36 37 37 38 39 40 41 42

16 17 18 19 19 20 21 22 23 23 24 25 26 27 27 28 29 30 31 31 32 33 34 35 36 36 37 38 39 40 40

16 16 17 18 19 20 20 21 22 23 23 24 25 26 27 27 28 29 30 31 31 32 33 34 34 35 36 37 38 39 39

15 16 17 17 18 19 20 21 21 22 23 24 24 25 26 27 27 28 29 30 30 31 32 33 33 34 35 36 36 37 38

15 16 16 17 18 18 19 20 21 21 22 23 24 24 25 26 27 27 28 29 30 30 31 32 32 33 34 35 35 36 37

14 15 16 17 17 18 19 19 20 21 22 22 23 24 24 25 26 27 27 28 29 29 30 31 32 33 34 34 34 35 36

14 15 15 16 17 17 18 19 20 20 21 22 22 23 24 24 25 26 26 27 28 29 29 30 31 31 32 33 33 34 35

14 14 15 16 16 17 18 18 19 20 20 21 22 22 23 24 24 25 26 26 27 28 28 29 30 31 31 32 33 33 34

13 14 15 15 16 16 17 18 18 19 20 20 21 22 22 23 24 24 25 26 26 27 28 28 29 30 30 31 31 32 33

13 13 14 15 15 16 17 17 18 19 19 20 21 21 22 22 23 24 24 25 26 26 27 28 28 29 30 31 31 32 32

12 13 14 14 15 16 16 17 17 18 19 19 20 21 21 22 22 23 24 24 25 26 26 27 27 28 29 29 30 30 31

12 13 13 14 15 15 16 16 17 18 18 19 19 20 21 21 22 23 23 24 24 25 26 26 27 27 28 29 29 30 30

Healthy; overweight; obese

tbl0002 Table 2 Various diet plans

Diet plan Type of diet Comments

Cabbage Soup/All

You can

Eat Soup Diet

Single-food, low-fat, low-carbohydrate,

very-low-energy density

Based on continued consumption of a buillion-based

cabbage and vegetable soup, which is not highly

palatable; other foods permitted include fruit, but not bananas

Dr. Atkins Diets Low-carbohydrate Ketogenic diet, which encourages free intake

of fat and protein foods with severe carbohydrate

restriction (<20 g per day). Compliance can be

checked by urine ketostix

Grapefruit Juice Diet Single-food, high-water Grapefruit juice is touted as a energy burning

catalyst with the specific foods recommended.

No evidence exists regarding

the validity of these claims

Russian Airforce Diet Low-fat, low-carbohydrate, low-energy Diet includes red meat, eggs, fruit, and salads

with no starch sidedishes

Scarsdale Diet Low-fat, low-carbohydrate, low-energy Lean meats, high-water fruits (melons and citrus),

and salads, no starch sidedishes

The Zone Diet Low-carbohydrate Touted to stimulate the ideal amount of insulin

by consuming macronutrients in the ratio of

40% carbohydrate, and 30% each of protein and fat

Liquid Diets Low-fat, portion-controlled Simple to use and may be of use for those who

have difficulty with portion control.

Monotonous food intake may be difficult to maintain

Dieting, Italian Style Portion-control, increased-fat This diet advocates determining your individual energy needs

(via table of metabolic test) and consuming a diet high in

pasta, bread, cheese, avocado and olive oil. Portion-control

is necessary on this diet

5286 SLIMMING/Slimming Diets