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

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have been used to discriminate physiologically

between Shigella spp.

0026 Serological testing, using polyvalent antiserum, is

used to identify the Shigella group (A–D). A note of

caution should be addressed, concerning EIEC. This

pathogen causes the same disease, bacillary dysentery,

as do the shigellae. In some cases, some EIEC strains

share homology with antigenic structures of some

Shigella serotypes. Several serotypes of S. dysenteriae,

S. flexneri, and S. boydii have reciprocal cross-

reactivity with E. coli O antigens of the Alkalescens-

Dispar bioserogroup or EIEC.

0027 DNA-based methods Initially, DNA probes were

developed to detect Shigella spp. from clinical

samples, a far easier task than from foods. In the

former, shigellae are shed in large numbers (10

–10

per gram of stool) in patients in the acute phase of

infection, and 10

–10

1

can be found in convales-

cent individuals. Identification and detection of shi-

gellae from stool samples is relatively easy, owing to

the high number of organisms in a relatively homoge-

neous sample. The analysis of foods is obviously

much more complicated. The diversity of foods,

such as composition (pH, fat content), microbial con-

tent of the natural flora, and the length of time from

collecting food samples, as in the case of a shigellosis

outbreak, to the laboratory are factors leading to a

diminishing chance of isolating or detecting this

pathogen.

0028 The application of the polymerase chain reaction

(PCR), as with DNA probes, was initially developed

to detect Shigella from clinical samples. PCR primers

are selected that target specific virulence genes. For

detecting shigellae, primers to amplify a segment of

the ipaH gene have an advantage over other virulence

genes. The ipaH gene is present in multiple copies in

Shigella, including the bacterial chromosome. A dis-

tinct advantage of this set of primers is that in cases

where the large virulence plasmid is no longer pre-

sent, a product can still be amplified from the ipaH

genes present in the chromosome reducing the occur-

rence of a false-negative reaction. Another positive

feature of this primer set is that having multiple

copies as targets influences the sensitivity of the

PCR-based assay. However, it is not only the quantity

of template for PCR that determines the successful

PCR: the quality of the DNA is as important. DNA

contaminated with inhibitors of PCR can lead to false

negative reactions.

0029Much attention has been given to the preparation

of DNA template free of PCR inhibitors. Clearly, in

the clinical milieu, protocols can be designed for one

particular matrix; quantity of template should not be

a problem. Even though convalescent patients shed

–10

shigellae per gram stool for perhaps weeks

after the onset of disease, this number is suitable for

detection. The limit of detection ranges from 30 to 50

CFU for PCR-based assays for detecting shigellae. In

foods, as discussed above, multiple factors will influ-

ence how to prepare template. Several commercially

available kits provide procedures for the detection of

bacterial pathogens from foods. These protocols in-

volve an enrichment step in broth for 6–18 h followed

by treatment, as described by the manufacturer. Since

shigellae do not survive well in most foods, this step

may prove futile. Recently, a method has been de-

scribed in which a small volume of a food wash is

applied directly to a filter. These filters have the cap-

acity not only to bind bacterial cells, but also to lyze

the cells on contact and sequester the DNA within the

membrane. The advantage of these filters is that bac-

terial (and parasitic) cells are concentrated and lyzed

simultaneously, and washing the filters removes PCR

inhibitors. With a pure culture of bacterial cells or

shigellae seeded in different foods, the limit of detec-

tion is usually less than 100 CFU. Unlike other

methods, which may take several days to determine

if Shigella spp. are present in foods, PCR-based assays

can yield a result in less than 1 day.

0030A method that uses a combination of polymerase

chain reaction and enzyme-linked immunabsorbent

assay in lieu of identifying polymerase chain reaction

products by gel electrophoresis was found to be

suitable for processing a large number of clinical

samples. Its application to food analysis has yet to

be tested.

tbl0005 Table 5 Biochemical differentiation of Shigella species

Test S. dysenteriae S. flexneri S. boydii S. sonnei

b-galactosidase 

þ

Ornithine

decarboxylase

þ

Indole production þ/

b, c

þ/

c,d

þ/



Gas from glucose 



Acid from

Dulcitol 





Lactose 

Mannitol þþþ

Raffinose þ/

þ

Sucrose þ

Xylose þ/



S. dysenteriae type 1 are positive.

S. dysenteriae type 1 is negative; S. dysenteriae type 2 is positive.

Reaction is variable.

S. flexneri 6 is negative.

S. flexneri 6 is positive.

S. boydii serovars 13 and 14 produce acid and gas.

S. dysenteriae type 1 may be positive.

S. flexneri 6 may be positive.

S. boydii may be positive.

Positive reactions may take 24 h or longer.

þ/, variable reaction.

SHIGELLA

5267

Treatment and Prevention

003 1 Shigellosis is a self-limiting infection in normally

healthy patients, and full recovery usually occurs

without the use of antibiotics. In cases where anti-

microbials are administered, the antibiotic of choice

is trimethoprim–sulfamethoxazole. Ampicillin and

tetracycline are alternative antibiotics to use,

but multiple drug resistance among isolates of Shi-

gella is becoming more common. Clinical isolates

resistant to sulfonamides, ampicillin, trimethoprim–

sulfamethoxazole, tetracycline, chloramphenicol,

and streptomycin have been found. Consequently,

drug-resistant organisms emerge from extensive

use of antibiotics, and therefore, many practitioners

believe that antimicrobial therapy for shigellosis

should be reserved only for the most severely ill

patients. However, there are persuasive public health

arguments for the antibiotic management of shi-

gellosis. Effective antibiotic treatment limits the

duration of the disease and shortens the period

of fecal excretion of shigellae, thus limiting the

carrier state. Prompt replacement of liquids and elec-

trolytes is important to restoring the health of the

patient.

0032 Since an infected person or asymptomatic carrier

can be a focal point for person-to-person and food

and waterborne spread, antibiotic treatment of these

individuals can have an immediate effect on public

health by containing the spread of shigellae. In indus-

trialized countries, Shigella are one of the leading

causes of outbreaks of diarrhea in daycare centers

and have a secondary attack rate of 10–30%. As

noted, antibiotics are not a substitute for good hy-

gienic practices to contain secondary spread of shi-

gellosis. As promulgated by the WHO, the single

most effective means of preventing secondary trans-

mission is handwashing. Food handling and prepar-

ation are important processes that also deserve

attention, and persons with diarrhea should be

excluded from handling food.

0033 Despite many years of intensive effort, an effective

vaccine against shigellosis still has not been de-

veloped. Attenuated, oral vaccine strains of S. flexneri

are currently being tested. However, one major draw-

back still to be resolved is the transient fever and mild

diarrhea associated with administration of the vac-

cine. A persistent problem impeding the development

of a safe Shigella vaccine is designing a strain that can

induce a protective immune response without produ-

cing unacceptable side-effects.

Summary

0034Foodborne outbreaks caused by Shigella species

remain a concern to public health. Shigella can spread

quickly because of the low infectious dose required to

elicit clinical symptoms. This pathogen is not indigen-

ous to any particular food; its host is either humans or

subhuman primates. Outbreaks of shigellosis, from a

relatively small number of infected people in a con-

fined environ or to a multitude of clinically ill people,

can occur by the ingestion of contaminated foods. A

primary source of contamination is food handlers

with poor personal hygiene. The disease is self-

limiting, but treatment with antibiotics can limit the

extent and possibly the spread of the illness. Cur-

rently, there is no vaccine available for shigellosis.

See also: Biosensors; Food Poisoning: Classification;

Hazard Analysis Critical Control Point; Salmonella:

Salmonellosis

Further Reading

Bacteriological Analytical Manual (1998) Shigella. Chapter

6, pp. 6.01–6.06. Gaithersburg, MD: AOAC Inter-

national (http://www.cfsan.fda.gov/*ebam/bam-

toc.html).

Blocker A, Jouihri N, Larquet E et al. (2001) Structure and

composition of the Shigella flexneri ‘needle complex’,a

part of its type III secretion. Molecular Microbiology 39:

652–663.

Buchrieser C, Glaser P, Rusniok C et al. (2000) The viru-

lence plasmid pWR100 and the repertoire of proteins

secreted by the type III secretion apparatus of. Shigella

flexneri. Molecular Microbiology 38: 760–771.

Dorman CJ and Porter ME (1998) The Shigella virulence

gene regulatory cascade: a paradigm of bacterial gene

control mechanisms. Molecular Microbiology 29: 677–

684.

Dupont HL (1995) Shigella species (bacillary dysentery). In:

Mandell GL, Bennet JE and Dolin R (eds) Principles and

Practice of Infectious Diseases, pp. 2033–2039. New

York: Churchill Livingstone.

Ewing WH (1986) Edwards and Ewing’s Identification of

Enterobacteriaceae, 4th edn., pp. 146–147. New York:

Elsevier.

Sansonetti PJ (2000) Rupture, invasion and inflammatory

destruction of the intestinal barrier by Shigella, making

sense of prokaryote–eukaryote cross-talks. FEMS

Microbiology Reviews 25: 3–14.

Smith JL (1987) Shigella as a foodborne pathogen. Journal

of Food Protection 50: 788–801.

Smith JL and Palumbo SA (1982) Microbial injury reviewed

for the sanitarian. Dairy and Food Sanitation 2: 57–63.

5268

SHIGELLA

Shrimps See Shellfish: Characteristics of Crustacea; Commercially Important Crustacea; Characteristics of

Molluscs; Commercially Important Molluscs; Contamination and Spoilage of Molluscs and Crustaceans;

Aquaculture of Commercially Important Molluscs and Crustaceans

SINGLE-CELL PROTEIN

Contents

Algae

Yeasts and Bacteria

Algae

M Garcı

a-Garibay, L Go

mez-Ruiz and

A E Cruz-Guerrero, Universidad Auto

noma

Metropolitana, Mexico City, Mexico

EBa

rzana, Universidad Nacional Auto

noma de

xico, Mexico City, Mexico

Introduction

0001 Single-cell protein (SCP) refers to crude or refined

protein of algal, bacterial, mold, or yeast origin

which is used either as animal feed or human food.

The production and utilization of microbial biomass

as a source of food proteins gained particular interest

as an alternative source for proteins of agricultural

origin due to its high content of protein. In addition to

proteins, SCP contains other nutrients such as lipids

and vitamins. Algae as a source of SCP is a term

which refers to either microscopic single-cell true

algae or prokaryotic cyanobacteria, and their growth

is based on use of carbon dioxide and light energy

(autotrophic growth). In contrast to other SCP-

producing organisms, algae are grown in many cases

by processes resembling traditional agriculture, since

they depend on large areas and sunlight radiation.

(See Mycoprotein.)

Organisms

0002 The term ‘algae’ is generically given to photosynthetic

organisms, either microscopic or macroscopic, living

largely in water habitats, growing as undifferentiated

or little-differentiated tissues. The designation is

applied to taxonomically unrelated species, and in

some cases includes groups like cyanobacteria or Eu-

glena. However, the cyanobacteria is a group in-

cluded in the Procaryotae kingdom, therefore they

are more closely related to bacteria than to ‘true’

algae. However, they are sometimes known as blue-

green algae, and for SCP purposes they are usually

considered as such. Euglena is a genus of microorgan-

isms belonging to the Protozoa kingdom; it is a

strange case of an unicellular animal with chlorophyl,

therefore it should not be considered as algae, and it is

not commonly used as a source of SCP.

0003‘True’ algae belong to the Plantae kingdom, being

the simplest plants. There are unicellular and multi-

cellular organisms, some of them reaching huge sizes.

0004Many algae have been used as food for a long time.

These include single-cell organisms as well as multi-

cellular seaweeds, which have an important position

in the diet of coastal communities. Table 1 shows the

main genera of algae used as food and their cellular

characteristics.

0005Macroscopic algae do not easily fit the SCP defin-

ition, due to their multicellular nature, and the low

protein content of the final product (6–30% on dry-

weight basis). They are mostly collected from the

sea and, when cultivated, the production resembles

farming more than a biotechnological process. The

most widely consumed seaweed is Porphyra (an alga

belonging to the Rodophyceae – the red algae), par-

ticularly P. tenera, which has a widespread distribu-

tion. It is mainly consumed in Japan, and also in the

Philippines, Wales, and New Zealand. Among the

most important species of seaweeds used as food

SINGLE-CELL PROTEIN/Algae 5269

is Ulva lactuca (sea lettuce) which is used as a salad

ingredient in western Europe. Enteromorpha is

consumed in Hawaii and the Philippines in salads or

as a flavor-enhancer for fish dishes. Caulerpa is also

consumed in the Philippines. These three later algae

belong to the Chlorophyceae – the green algae.

0006 Both true unicellular algae and cyanobacteria have

been consumed for centuries. This practice dates back

to the Aztecs in Central Mexico, long before the

discovery of the New World, when Spirulina maxima

was harvested from natural habitats for human

consumption. A similar species, S. platensis, is still

used in Lake Chad in Central Africa for the same

purpose. Both species S. maxima and S. platensis

have been classified interchangeably in either the

genus Arthrospira or the genus Spirulina. Figure

1 shows a picture of S. maxima from Lake Texcoco

in Mexico. Other ancient cultures have used microal-

gae as food but on a lesser scale. For instance, Nostoc is

consumed in Mongolia, China, Thailand, and Peru,

while Oedogonium and Spirogyra are consumed

in Burma, Thailand, and Vietnam; Chlorella is pro-

duced in Japan and Taiwan and Scenedesmus in China.

Production

0007Besides the traditional harvesting of native algae in

several regions, industrial and experimental efforts

are being made in many countries, with either large

and shallow ponds in areas of high sunlight, or bio-

reactors for the production of unicellular organisms.

0008Macroalgae cultivation is started by providing an-

chorage in sheltered bays for the initial culture; once a

considerable growth of young plants is obtained, they

are transplanted to nitrogen-rich tidal estuaries with

low salinity.

0009Single-cell algae are produced by a variety of

methods, ranging from cultivation in lakes or earthen

ditches or ponds to technically advanced fermenters

or bioreactors. Intensive cultivation was initiated in

the 1940s in Germany, followed by technological

developments in Japan and Taiwan during the 1950s

and 1960s. Later on, commercial production systems

were developed in the USA, Mexico, Thailand, and

Israel. Currently commercial production is carried

out in many countries around the world, though at a

modest scale.

fig0001 Figure 1 Spirulina maxima harvested from Lake Texcoco in Mexico, observed by scanning electron microscopy. Courtesy of Dr Jorge

Sepu

lveda, Universidad Nacional Auto

noma de Me

xico.

tbl0001 Table 1 Main genera of algae used as food and their cellular

organization

Prokaryote organisms

(cyanobacteria

belonging to the

Prokaryotae kingdom)

Eukaryote organism

(true algae

belonging to the

Plantae kingdom)

Unicellular growth Anabaena Chlorella

Nostoc Oedogonium

Spirulina (Arthrospira) Scenedesmus

To l y p o t h r i x S p i r o g y r a

Multicellular growth Caulerpa

Coelastrum

Enteromorpha

Porphyra

Ulva

5270 SINGLE-CELL PROTEIN/Algae

0010 Outdoor cultivation may be in open or closed

systems. Growth occurs not more than 0.5 m from

the water surface, as it is controlled by light penetra-

tion. In clean-culture systems a single species is inocu-

lated and can be maintained over extended time

periods; these are closed photobioreactors operating

either outdoors or indoors, ranging from large closed

areas exposed to sunlight to smaller reactors illumin-

ated with artificial light.

Substrate Requirements

0011 Algae are autotrophic organisms with the distinct

advantage of using carbon dioxide as carbon source.

Carbon dioxide represents the cheapest and probably

most abundant carbon source for microbial growth.

Due to such inexpensive requirements, these organ-

isms are very convenient for SCP production. Some

species can also grow heterotrophically using organic

carbon sources.

0012 The process is limited by the carbon source; the

concentration of dissolved carbon dioxide is rather

low, owing to its low solubility in aqueous solution.

Carbon dioxide can be injected from combustion

gases. Some additional sources of carbon enhance

cell growth: cheap organic materials such as manure,

molasses, or industrial wastes can be used. The added

organic compounds are rapidly degraded by bacteria

to make the carbon dioxide needed for algal repro-

duction. Lake Texcoco in Mexico has high concen-

trations of carbonate and bicarbonate, which are

efficiently consumed by Spirulina maxima.

0013 Nitrogen sources are generally nitrates, nitrites,

ammonia, or urea. Oxidized nitrogen compounds

require energy to be reduced, therefore ammonia is

a more convenient form. Many cyanobacteria are

able to fix atmospheric nitrogen; species of the

genus Anabaena are particularly active in doing this.

0014 Another nutrient of importance is phosphorus that

can be incorporated in inorganic form. Micronu-

trients are needed in minimal amounts.

0015 The use of waters polluted with organic wastes as

input for algal ponds has the advantage of using a

cheap raw material and promoting decontamination

while obtaining a good source of SCP. Even more, the

macronutrients needed (ammonium, phosphate) are

normally present in domestic sewage, animal wastes,

and food industry residual waters. (See Effluents from

Food Processing: On-Site Processing of Waste; Com-

position and Analysis.)

Mass-Culture Open Systems

0016 For open systems, lakes, ponds, and ditches are used.

They can have an earthen floor or be lined with con-

crete or plastic. Open systems have low cell densities

with large variation in productivity, depending very

much on water properties and environmental condi-

tions. In these kinds of systems the weather condi-

tions, especially temperature and sunlight radiation,

are critical. On the other hand, the needed facilities

are constructed at low cost, and large areas are

available.

0017It is a challenge to keep a monoalgal culture in an

open-air system. The propagation of mixed popula-

tions and frequent problems of contamination by

bacteria, fungi, protozoa, and invertebrates usually

disturbs the productivity and lowers the quality of

the product. However, under such nonsterile, mixed-

culture conditions some algal species tend to predom-

inate. The surface of the water in some ponds or

ditches used for mass production is covered with

polyethylene or other plastic material to reduce the

risk of infection. Another possibility of avoiding con-

tamination is the common practice of seeding a large

inoculum to dominate the culture, at least during

the first growth phase. Other approaches to promote

single-species culture have been evaluated; for in-

stance, the use of nitrogen-fixing species of the genus

Anabaena in media without other nitrogen source has

proved useful. Similarly, the selection of thermophilic

species, such as Scenedesmus obliquus, has shown

some preliminary potential.

0018Gentle agitation is very important to achieve high

productivity. It prevents sedimentation, allows a

more homogeneous exposure of algal cells to light,

and reduces nutrient and temperature gradients along

the depth of the culture. To this end, several designs

have been implemented in ponds and ditches, includ-

ing paddlewheels, gravity flow, and pump recycle,

combined with special designs of slopes in oval

ponds and horizontal raceways.

0019Productivities rarely exceed 30 g m

2

day

1

and

cell densities of 2 g l

1

, which are much lower than

the values for other industrial fermentation processes.

Some experimental improvements have been achieved

by optimizing the use of wastes through the addition

of nitrogen sources, adding aeration ports, and inocu-

lating selected bacteria that efficiently degrade the

diluted organic materials. By such means, the dry-

weight productivities reach about four and three

times the average figures obtained for maize and soy-

bean respectively on a yearly basis. With intensive

modes of cultivation, algal cultures can produce up

to 20–35 times more protein than soybean for the

same area of land.

0020An example of a typical mass-culture open system

is that conducted by the Sosa Texcoco Company in

Mexico, mainly during the 1970s and 1980s, in

which an alkaline (pH 9–11) lake with 900 hectares

of surface produced up to 1000 metric tons per year

SINGLE-CELL PROTEIN/Algae 5271

of Spirulina maxima (equivalent to 0.111 kg m

2

Weather conditions in Central Mexico, with high

and practically constant temperature and solar radi-

ation through the year, and the high alkalinity of the

water favor effortless predomination of S. maxima on

Lake Texcoco. The water of the lake flows by gravity

into a sloped spiral raceway, maintaining gentle

agitation. This factory stopped its production in the

mid-1990s due to a long strike that led to the bank-

ruptcy of the company.

0021 The most advanced system developed so far is the

high-rate algal ponds (HRAP) which combines the

treatment of sewage with the simultaneous massive

production of algae. The project was developed by

the Technion Research Center at Haifa, Israel. Basic

infrastructures are shallow canals that add up to a

maximum of 1000 m

, equipped with systems for

gentle agitation and aeration. The process is operated

continuously with retention times varying between

2 and 6 days, depending on the season. A steady multi-

culture is established in the system within a few days

of operation. This includes bacteria that degrade or-

ganic compounds, and well-defined algal species,

with Euglena, Chlorella, and Scenedesmus predomin-

ating. The maximum daily productivity reported at

times of maximum solar radiation is 30 g m

2

, with

an average annual production of 7 kg of algae per m

To recover the cells, aluminum sulfate is added as a

flocculant; the float is then dewatered by centrifuga-

tion and dried in a drum drier to reach a final mois-

ture content of 10%. The final product has been

shown to have an excellent nutritional quality, con-

taining 57.4 g of crude protein per 100 g, and an

amino acid profile superior to the average for soybean

protein. It has been used to complement at least 25%

of fish diet and 10% of poultry diet with no toxic

effects. The resulting effluent can be used directly for

crop irrigation.

0022 Chlorella ellipsoidea is produced in Taiwan for

food use in open ponds with agitation or circulation.

The product is recovered by filtration. In China, pilot-

scale open systems have been used for the production

of Spirulina, mainly S. platensis. Ponds about 100 m

covered with polyvinylchloride sheets, lead to a daily

production of 10 g m

2

(equivalent to more than 3 kg

per year per m

if the production could be maintained

constant through the year). Other microalgae pro-

duced in China in open-air systems are Scenedesmus,

Chlorella, and Anabaena.

Photobioreactors

0023 Photobioreactors are closed systems working either

outdoors or indoors, in which a single species is inocu-

lated to keep a clean-culture operation. Closed culti-

vation systems offer better control of contamination

and cell physiology than open systems, leading to

higher growth and quality of the harvested product,

but manufacturing costs increase.

0024Large systems operating outdoors consist of tubes

covering large areas exposed to sunlight and can be

operated either in batch or continuously. Many

designs have been constructed or proposed at pilot

scale. Tubes are made of either glass or plastic, such as

polyethylene. Since the tubes behave as solar collect-

ors, overheating is a problem. Hence, the tubular

solar receptors must have a temperature control

system, which is usually a water pool. Alternatively,

the use of thermotolerant strains has been proposed

to avoid cooling facilities. Generally, tubes are

grouped in several modules to facilitate control, oper-

ation, and to offer flexibility to the system in terms of

culture volume. CO

supply systems such as carbon-

ation towers, pumps for circulating the medium, and

tanks to mix nutrients are attached to the solar recep-

tors. These photobioreactors have been used for the

production of Chlorella, Spirulina, and Scenedesmus.

0025Other designs constructed in Chile up to a scale of

110 m

of solar irradiation area consist of a pond

made of cement lined with epoxy resin and covered

with a polyethylene dome. The agitation system is a

paddlewheel. It has been used to produce Spirulina

biomass, reachinga growthdensity of 450–750 mg l

1

0026An innovative design, operated in the USA, is based

on the use of oval plastic bags floating on thermal

waters.

0027Photobioreactors operating indoors necessarily

have smaller sizes because artificial light is needed.

Designs can be either plastic tubular systems, or stain-

less-steel fermenter-like reactors with internal illu-

mination to allow maximum light incidence. Their

use is rather limited for SCP production because of

low throughputs; however, they are quite adequate

for the production of algal metabolites with high

added value, such as polysaccharides, carotenes, and

other pigments, polyunsaturated fatty acids, etc.

Harvesting

0028The recovery of microalgal biomass after production

is rather difficult, particularly in large-area lakes, or

when low concentrations occur. Some species, such as

Spirulina platensis, S. maxima,andCoelastrum pro-

biscideum, are easily skimmed off or harvested by

filtration through cloths or screens. Filter presses

can be used as well. Owing to their small cell size

(10 mm), other species need to be harvested by centri-

fugation or flocculation, adding flocculants such as

lime, alum, or polyelectrolytes.

0029After harvesting, the algal biomass must be

dewatered by centrifugation and/or dried. Oper-

ations to dehydrate biomass are normally done by

5272 SINGLE-CELL PROTEIN/Algae

drum-drying, sun-drying, or spray-drying; drum-

drying is the most widely preferred. In some cases,

the product is not dried; for example, in China, Sce-

nedesmus and Chlorella are fed to swine as fresh algal

slurry.

Nutritional Value and Human

Consumption

0030 Algal SCPs have nutritional values similar to other

SCP sources. The crude protein content (N 6.25)

varies between 45 and 73%, while lipid and mineral

contents are 2–20% and 5–10% respectively. The

chemical composition of some algal species is shown

in Table 2. The protein content of algae is higher than

the value for soybean (40 g 100 g

1

0031 The amino acid content of algal SCP shows an

adequate balance except, as any other microbial

biomass, for the sulfur-containing amino acids me-

thionine and cystine. They are rich in vitamins, espe-

cially some of the water-soluble ones, and essential

fatty acids. The content of some vitamins, such as

thiamin, riboflavin, folic acid, and carotene, is higher

in algae than in many vegetable foodstuffs. However,

the content of nutrients is highly dependent on culti-

vation and processing conditions. Table 3 shows

some indices of protein quality of some algae. The

protein efficiency ratio (PER), net protein utilization

(NPU), and biological value (BV) of algal proteins are

somewhat lower than casein. Nutritional tests have

shown promise when algae supplemented with

methionine and cystine are fed to broilers. However,

monogastrics have problems digesting the whole cells

and some processing is therefore needed.

0032Algal cell wall is not readily digestible, there-

fore any treatment to disrupt the walls will increase

tbl0002 Table 2 Composition of algal species. Main components in g 100 g

1

dry wt. Amino acids and vitamins as specified

Spirulina maxima Chlorella Scenedesmus obliquus Scenedesmus acutus

Crude protein (N  6.25) 55–71 40–58 50–56 46–64

Tr u e p r o t e i n

Amino acids (g 16 g

1

48–61 44–48

Alanine 5.0–6.1 4.2–7.4 5.3–10.4

Arginine 4.5–9.3 5.8–10.2 4.6–7.1

Aspartic acid 6.0–15.2 6.9–8.8 6.5–11.1

Cystine 0.6–2.2 0.3–0.9 0.6–1.6

Glutamic acid 8.2–21.8 8.0 5.3–10.7

Glycine 3.2–4.0 4.9–5.5 3.4–7.0

Histidine 0.9–1.6 1.4–3.0 1.5–2.3

Isoleucine 3.7–4.5 3.1–6.4 2.2–4.9

Leucine 5.6–7.7 6.8–9.7 5.0–10.6

Lysine 3.0–4.5 4.9–9.4 5.0–6.4

Methionine 1.6–2.2 1.0–2.0 1.4–2.7

Phenylalanine 2.8–4.0 3.2–5.1 3.6–6.4

Proline 2.7–3.2 2.2–6.4 3.1–6.1

Serine 3.2–4.3 3.0–4.1 3.2–5.4

Threonine 3.2–4.5 3.6–4.7 3.0–5.8

Tryptophan 0.8–1.2 1.0–1.5 0.3–1.8

Tyrosine 3.9 2.6–4.1 2.0–4.6

Valine 4.2–6.0 4.8–6.0 4.7–7.4

Lipids 4–7 6–16 12–14 8–14

Carbohydrates 13–16 10–17

Minerals 4–9 6–9 4–9 6–17

Vitamins (mg 100 g

1

)

Thiamin 5.5 0.6–2.3 1.2–8.2

Riboflavin 4.0 2.0–6.0 3.4–36.6

Pyridoxine 0.3 0.1–3.2 1.1–2.5

Nicotinic acid 11.8 10–22 12–16.7

Pantothenic acid 1.1 1–10 1.5

Folic acid 0.05 0.1–4.0 0.7

Biotin 0.04 0.015–0.064 0.02–0.2

Cyanocobalamin 0.02 traces 0.04–0.44

Ascorbic acid 18–370 165–181

b-Carotene 0.17

g-Tocopherol 19.0 26–33 14–18.5

SINGLE-CELL PROTEIN/Algae 5273

digestibility and hence nutritional value. Many treat-

ments have been suggested, such as mechanical dis-

ruptions, extractions with organic solvents, and

treatments with alkalis and/or acids. The method of

drying affects product bioavailability. In drum-dried

algae compared to the air-dried product, NPU is

increased to around 100%, while digestibility is in-

creased to about 60%. This phenomenon may be due

to the rupture of the algal cell walls when water is

removed at controlled conditions. Figure 2 shows

spray-dried cells of Spirulina maxima; it is clear that

the drying process disrupts cells if the cellular struc-

tures shown in Figure 1 and Figure 2 are compared.

The micrograph shown in Figure 2 was taken from

a sample of the spray-dried commercial product

(powder) shown in Figure 3.

0033Many uses have been made of algal SCP, including

the cultivation of daphnid and similar species that

thrive on plankton as a food source in aquaculture.

Particularly, some algal species rich in carotenoids

have been used to feed salmon and trout to enhance

the color of their flesh, and the US Food and Drugs

Administration (FDA) has approved the ‘all natural’

statement for farm-raised fish. In addition to its use as

feed for chicken and swine, algal biomass has been

used as food worldwide. Chlorella is produced in

Taiwan for food use; Spirulina has been produced

commercially in the USA, Mexico, Taiwan, Japan,

Thailand, and Israel with the same purpose.

0034Algal SCP has mainly been used for the preparation

of nutraceuticals or dietary supplements, alone or

mixed with other sources of protein and other food

ingredients, sold as tablets, caps, powders, and other

products available mainly in health food stores or

nutrition centers, but also sold in groceries, drug

and discount chain stores; they are promoted as pro-

tein and vitamin supplements, or to help people lose

fig0002 Figure 2 Cells of Spirulina maxima disrupted by the spray-drying process, observed by scanning electron microscopy. The sample

was taken from a commercial powder.

tbl0003 Table 3 Parameters of protein quality of some microalgae

Product NPU PER BV

Spirulina platensis 52.7 68

S. platensis þ methionine 62.4 82.4

Spirulina spp. 65 1.80 75

Spirulina spp. þ methionine 73 82

Chlorella spp. 66 72

Chlorella spp. þ methionine 78 91

Scenedesmus spp. 67 1.93 81

Casein 83 2.50 88

NPU, net protein utilization; PER, protein efficiency ratio; BV, biological

value.

5274 SINGLE-CELL PROTEIN/Algae

weight. However, very few indepth studies, if any,

have been conducted to evaluate the nutritional or

health-associated benefits of algal SCP. In countries

such as the USA, Mexico, and Chile algal biomass is

sold as tablets or powders, while in Japan and Taiwan

it is sold either as dry powder or as pellets. Figure 3

shows two commercial products elaborated with

100% biomass of Spirulina maxima.

0035 The FDA considers SCP from algae to be a dietary

supplement; like any other dietary supplement,

manufacturers do not need FDA approval to sell

their product; this means that the FDA does not

keep a list of manufacturers or products. However,

claims on the product label other than nutrition con-

tent or approved health claims must be notified to the

FDA. Examples of such claims are ‘free radical de-

fense’ or ‘helps combat free radicals,’ used in some

products containing algal SCP rich in astaxanthin, a

carotenoid with antioxidant capability, or ‘structure/

function’ claims such as ‘promotes cellular health’ or

‘support immune system health’ for a mixture of

Spirulina and Chlorella, statements which had not

been evaluated previously as health claims by the

FDA. Another example is ‘helps maintain healthy

eyes’ and ‘helps support immune function,’ claims

used in a product sold as powder or tablets made by

a strain of S. platensis rich in carotenoids.

0036 A wide range of food formulations has been pre-

pared with algal biomass. For instance, powder meals

containing Spirulina plus soy proteins, oat bran,

apple pectin and vitamins, either chocolate- or van-

illa-flavored, are sold as energy supplement powders

to mix with milk or juices. Algal SCP has been widely

used as an additive or supplement to cereal food-

stuffs, or as a garnish to salads. Experiences related

to supplementation of cereal foods with algal SCP

include mixtures with doughs for baked goods and

pasta, such as bread, rolls, cookies, noodles, etc. In

Mexico, S. maxima has been used as a supplement for

cookies produced by a state company as part of a

national breakfast program for schoolchildren. Also,

the use of S. maxima has been suggested to enrich

tortillas, increasing their protein content; however,

these products have not found success in the market

place.

0037Concerning functional properties, it has been

reported that S. platensis flour had similar emulsion

and foam capacities, slightly lower water and fat

absorption capacities, and lower foam stability than

soybean meal. Using the protein concentrate obtained

from the flour, improved functional characteristics

with the exception of water absorption ability are

obtained. Some values obtained for functional cap-

acities of a protein concentrate obtained from S. pla-

tensis are given in Table 4.

0038The major problems in acceptability of algal bio-

mass are its unfamiliar and sometimes bitter flavor, as

well as the presence of dark green pigments which are

difficult to mask. In addition to chlorophyls, other

pigments such as carotenes, xanthins, and phycocya-

nin are present in varying amounts. Flavor and color

may be improved if algal biomass is treated during

downstream processing to remove undesirable com-

ponents.

Toxicological Problems

0039Algae have been consumed as food for generations,

without ill effects. No toxic effects have been reported

in animal evaluations. However, the following con-

siderations must be taken into account.

0040A common problem for SCP from any microorgan-

ism is the high content of nucleic acids present in

microbial cells. Consumption by humans of nucleic

acids in amounts higher than 2 g day

1

can lead to

accumulation of uric acid, which increases the risk of

gout and/or kidney stones. The concentration of

nucleic acids in algal biomass depends on several

factors, such as species and growth conditions. Cy-

anobacteria have a nucleic acid content of 2.9–5g

tbl0004Table 4 Functional capacities of a protein concentrate

obtained from Spirulina platensis

Water absorption (g 100 g

1

protein) 145

Fat absorption (g 100 g

1

protein) 373

Emulsifying capacity (ml oil 100 g

1

protein) 113

Foam capacity (%) 205

Foam stability (1 h) (%) 27

fig0003 Figure 3 (see color plate 126) Two commercial products elab-

orated with 100% dried Spirulina maxima biomass, sold as nutra-

ceuticals in health food stores in Mexico City.

SINGLE-CELL PROTEIN/Algae 5275

100 g

1

while microscopic plant algae have 1–17 g

100 g

1

. These amounts are higher than in most

other foodstuffs.

0041 In order to reduce nucleic acid content, protein

concentrates or isolate can be prepared by cell disrup-

tion and protein separation. Figure 4 shows a flow

diagram of a process for the preparation of protein

isolate from Spirulina. However, this increases the

cost of the product. In general, algal biomass is con-

sumed by humans in small amounts and so the con-

sumption of nucleic acids is below risk.

0042 The cell wall of microalgal biomass represents about

10% of its weight. It is mainly composed of indi-

gestible polysaccharides and some other compounds,

e.g., murein in cyanobacteria. The bioavailability of

protein from whole cells is therefore very low. The

preparation of protein concentrates or isolates can be

used to obtain products with a high nutritional value

and free of undesirable pigments, although it repre-

sents a costly alternative.

0043 Algae have the ability to remove heavy metals from

polluted waters. Similar physiological phenomena ac-

count for the accumulation of pesticides and organo-

chlorine compounds. This represents an objection

when algae are intended for use as SCP. However,

the causes are well identified and some steps can be

implemented to maintain the final product compos-

ition within safety levels.

0044The origin of the water used in cultivation ponds

determines the need for pretreatment. In general,

wastes generated in food industries carry low amounts

of the contaminants mentioned. However, urban

sewage and run-offs show high variability in the con-

tent of heavy metals and other toxicants. When these

waste streams are subjected to standard secondary

treatments, most organics are degraded, whereas

metals remain associated with the activated sludge,

rendering them safe as an input for algal growth.

Furthermore, some studies have demonstrated that

biological absorption of metals is a rather slow pro-

cess, requiring more time than the usual retention

time of the water within the bioreactor. It appears

that fears concerning the presence of recalcitrant toxi-

cants in algae might be excessive, although more

research is needed to get a clear picture.

0045Another problem to consider is the possibility

of contamination by pathogenic microorganisms.

Certainly, the culture practices in open systems in-

crease the risk of pathogenic infections. The down-

stream processes of SCP products are designed to

destroy most of the viable forms present, although

some could survive in the product. Recommendations

have been established for microbiological standards

of SCP products for use in animal feeds.

See also: Single-cell Protein: Yeasts and Bacteria;

Yeasts