The role of the yeast in bread making is the rising of the dough to produce the
characteristic loaf preferred by consumers. Dough rising occurs as a result of the
gases produced by the yeast as it grows within the dough. During growth, the yeast
metabolizes sugars in the dough with the help of a special enzyme system and produces
alcohol and CO2. The leavening power of the yeast depends on its activity and
viability; hence the yeast used must be fully active with a high viable cell count.
Furthermore, the leavening power of any yeast strain depends on its genetic makeup
and on the process of production, and also on the storage conditions before use
(Pyler 1988). The most important function of bakery yeast in bread making is to
leaven the dough during the fermentation process by producing CO2 via the alcoholic
fermentation of sugars. Furthermore, the yeast produces desirable flavor and
aroma compounds as products of secondary metabolism (Evans 1990), thus enhancing
the characteristic flavor and aroma of baked products.
32.2.1 Dates as a Substrate for Bakery Yeast Production
Bakery yeast can be produced from substrates that contain suitable sources of carbon,
energy, nitrogen, minerals and essential vitamins. Dates are said to be a good
substrate for bakery yeast production. Their carbohydrate content is mainly sugars
amounting to 65–87% of their dry matter. Date sugars are mainly glucose and fructose,
which are easily assimilable to most microorganisms (Sawaya 1986). The protein
content of dates is 1–3%. This is a low amount and hence a suitable source of
nitrogen, in the form of inorganic salts, has to be added to the date substrate for
bakery yeast production. Dates also contain vitamins important for yeast growth
such as B1 (0.75 mg/100 g), B2 (0.2 mg/100 g) and nicotinic acid (0.33–2.2 ml/100 g).
Also, the important minerals in 100 g dates are: potassium (650–750 mg), magnesium
(50–58 mg), sulfur (43–51 mg), phosphorus (59–64 mg), iron (1.3–2 mg), calcium
(58–68 mg) and chloride (268–290 mg) (Aleid et al. 2009). Commercial bakery
yeasts produced from strains of Saccharomyces cerevisiae have the following average
chemical composition: 47% C; 32% O2; 6% H2; 7.7% N2; 2% K; 1.2% P; 1% S;
0.2% Mg; 0.1% Na and other trace elements. In addition, the yeast cells contain
small amounts of vitamin B complex, of which D-Pantothenic acid, D-Biotin and
m-Inositol are essential because the yeast cells cannot synthesize them (Bronn
1990). To produce 1 kg of yeast about 3 mg D-Biotin, 150 mg D-Pantothenic acid
and 2 g m-Inositol are needed. These elements and compounds must be provided in
the production medium in sufficient quantities and in metabolizable forms. If dates are used as a substrate for production, their sugars will act as a source of carbon and
energy. According to the date fruit chemical composition given above, 1 mt of dates
used as a carbon and energy source will yield about 325–435 kg of active dry yeast.
The nitrogen content of dates is insufficient to produce such quantities of yeast and
a suitable nitrogen source has to be added. The contents of dates in terms of other
elements and essential vitamins must be determined and any deficiencies remedied
(Aleid et al. 2009).
When comparing date and molasses as substrates for bakery yeast production,
with regard to its nutrient contents, date syrup compares favorably with molasses
which is the conventional substrate for bakery yeast production worldwide
(Table 32.1). Date syrup contains much more sugar, biotin and pantothenic acid than
molasses, about similar content of nitrogen, phosphorus and magnesium, about half
the content of potassium (but still enough for bakery yeast production) and much
less m-inositol. Compounds toxic to bakery yeast detected in date syrup include
formic acid at 3.06%, acetic acid at 2.38% and propionic acid at 0.68% (total acids
6.12%), but no detectable amounts of the toxicants nitrite, sulfite and butyric acid.
Formic acid becomes toxic to the yeast when its concentration in the medium
exceeds 0.25%, whereas the toxicity level of the other two acids is in excess of 3.0%
for the sum of the two (Aleid et al. 2009).
32.2.2 Production Process of Bakery Yeast
Bakery yeast is propagated under optimal conditions of temperature, pH, aeration and
nutrient supply to give maximum yields of time, space and raw materials. The best
fermentation process for bakery yeast production from strains of Saccharomyces
cerevisiae is the fed-batch process, so that the Crabtree effect is avoided. Usually, part
of the mineral medium and a small amount of the substrate is added to the fermentor,
then the inoculum is added and the process started. The rest of the mineral medium
and the substrate are fed to the fermentor at such a rate that the concentration of sugar
in the fermentor does not exceed about 0.1 g/l. Continuous aeration and stirring is necessary to ensure the transfer of sufficient quantities of oxygen and nutrients to the
growing yeast cells. Also, cooling is necessary to remove heat generated by the metabolic
activity of the growing yeast culture and to maintain the fermentation temperature
at about 30°C. When the fermentation process is completed, the final cell
concentration in the fermentor is 4–5% by weight. Yeast cells are harvested by filtration
or centrifugation and processed to the final product (Aleid et al. 2009).
32.2.3 Bakery Yeast Production from Dates
A few investigations into the production of bakery yeast from date extracts have
been conducted (AlObaidi et al. 1985, 1987; Bassat 1971; Mohammed et al. 1986;
Mudhaffer 1978). Comparisons were made between date extract and molasses.
Positive findings were reported and claims were made that there are no technological
constraints to using date extract for bakery yeast production. Nancib et al. (1997)
used waste products from date in the production of bakery yeast from strains of
Saccharomyces cerevisiae. They used a semi-synthetic fermentation medium containing
sugars extracted from the date coat (freshly part), nitrogenous compounds
extracted from seed hydrolysate, 6.0 g/l KH2PO4; 1.0 g/l date seed lipid; 0.6 g/l date
seed ash and 1.0 g/l ammonium nitrate. Although they described this medium as
satisfactory for bakery yeast production, yields obtained were very low with a maximum
of 0.6 g/l biomass concentration in the fermentation medium compared to the
optimum of about 40 g/l expected for an economical production. Khan et al. (1995)
used Saudi Arabian dates in the production of bakery yeast. They propagated six
different strains of S. cerevisiae in fermentation media containing Sefry Beesha cv.
date extract (with 60% sugars) in place of molasses, in addition to 2 g/l ammonium
sulfate and 50 mg/l biotin. Yields were also meager with a maximum of 10.7 g/l
biomass concentration in the fermentation medium from 50 g/l sugar, representing
a yield of only 42.8% of the theoretical. Date extract as a carbon and energy source
for the propagation of bakery yeast on a pilot-plant scale, in comparison with molasses,
was investigated by AlObaidi et al. (1986). Results showed that higher productivity
of bakery yeast was observed when date extract was used. It was concluded
from their study that date extract holds promise as a source of carbon and energy for
the production of bakery yeast, although the average yields were only 47%. None of
the authors discussed the Crabtree effect as a major technological problem encountered
with bakery yeast propagation.
Aleid et al. (2009) used substrates from pure date syrup and pure molasses for the
propagation of the bakery yeast strain Saccharomyces cerevisiae. All runs were fedbatch
processes, at pH 4.5, 30°C, 8 g/l inoculum size and sugar concentration in all
substrates of 200 g/l. The overall biomass yield from pure date syrup substrate was
significantly lower than the yields from pure molasses substrates. Reduced yields
could be attributed to the effect of yeast toxic organic acids contained in date syrup
at high concentrations. The performance of the bakery yeast, propagated on date
syrup as a fermentation substrate, in an Arabic bread test was excellent (Fig. 32.2).
32.3 Single Cell-Protein Production from Dates
The technology of single-cell protein (SCP) production was established in the
1970s (Martin 1997). SCP is produced from bacteria, yeasts, molds and algae
using different substrates as sources of carbon and energy such as food crops,
by-products of agriculture and industry, wastes and also sunlight and atmospheric
CO2 (Israelidis 1987). Abduljabbar et al. (2008) reported about a yeast
and a bacterium used for the production of SCP from ethanol, kerosene and gas
oil. They found that optimum substrate concentrations during propagation were
0.5–4%. These are very low concentrations and will not yield more than about
2% biomass concentration in the bioreactor, hence the economic feasibility of
the process is doubtful. No citations on SCP production from dates were found
in the literature.
SCP is mainly considered a protein source and hence it is used to replace protein
concentrates in animal feeds. SCP contains 50–70% protein, about 30% carbohydrates,
6% lipids and 8% minerals. In addition SCP is rich in vitamins, especially
the B-complex group (Hamad 1986; Herbert 1976; Robinson 1986). Other advantages
that support the use of SCP as animal feed include short production time
(a few days compared to months for crops; years for animals), small land areas, no
seasonality and use of cheap raw materials which are usually wastes that contaminate the environment.
32.3.1 Organisms Used for SCP Production
Desirable characteristics of the production organism include freedom from toxicity
and pathogenicity, high content of protein with a well-balanced amino acid composition,
thermo-tolerance, no growth factors needed, high yields and high growth rate
(Hamad 1986).
32.3.1.1 Bacteria
Bacteria can be used for SCP production. The advantages are many e.g. they have
high growth rates; are more stable in adverse growth conditions such as high temperatures;
can utilize a wide range of substrates as sources of carbon, energy and
nitrogen; and have high protein content. However, the main disadvantages of bacteria
are: the cell wall makes digestion difficult, the high content of nucleic acids and
the small size makes separation difficult (Holts 1994; Madigan et al. 1997).
32.3.1.2 Yeasts
Yeasts like Candida utilis, Kluyveromyces marxianus and Saccharomyces cerevisiae
are used for SCP production. Yeasts used in SCP production have the advantages
of better digestibility, lower nucleic acid content and easier handling during
harvest (Hamad 1986; Ray 1996). On the other hand they have lower protein content
than bacteria, lower growth rates and are less thermo-tolerant.
32.3.1.3 Algae
Algae are also used for SCP production, but less frequently. Algae are produced in
bonds exposed to sun where they perform photosynthesis. Production costs are low,
but much water and sunshine are needed.
32.3.2 Substrates for SCP Production
SCP can be produced from substrates that contain suitable sources of carbon, energy,
nitrogen, minerals and vitamins Substrates range from food materials like grains
and dates; by-products like molasses; alkanes, whey and wastes like sulfite liquor;
residues of foods, plants and animals; and waste water. Some substrates contain
easily-metabolized carbon and energy sources such as mono and disaccharides in
molasses, dates and whey. Others contain complex carbohydrates such as starches
in grains and cellulose in plant residues. Only a limited number of microorganisms
are able to metabolize complex carbohydrates. In most cases such substrates need some treatments before use for SCP production. The use of wastes for SCP production
serves two goals. First, it is possible to get rid of these environment-polluting wastes
and, second, the process results in obtaining valuable products of commercial use
(Allison 1975; Einsele 1975; Oura 1983). The yields on substrates are about 0.5 mt
dry biomass per 1 mt carbohydrate (Allison 1975; Einsele 1975; Oura 1983). In
most cases the substrates used for SCP production are low in nitrogen content and a
suitable nitrogen source must be added. Usually inorganic nitrogen salts are added
e.g. ammonia, ammonium salts, nitrates, etc.; sometimes urea is used. The process
is therefore an upgrading of such inorganic nitrogenous compounds to the highly
valuable organic nitrogen, the proteins.
32.3.2.1 Dates as a Substrate for SCP
Dates are a good potential substrate for SCP production. Their carbohydrate content
is mainly sugars, amounting to 65–87% dry matter. The sugars are sucrose, glucose
and fructose, which are easily assailable to most microorganisms (Aleid 2006; Sawaya
1986). This means that 1 mt of dates dry matter can produce up to 435 kg dry SCP.
The protein content of dates is 1–3%; a low amount and hence inorganic nitrogen has
to be added to the date substrate for SCP production (70 kg ammonium phosphate per
mt of dates). The vitamin content of dates includes: thiamine (B1), 0.75 mg/100 g;
riboflavin (B2), 0.2 mg/100 g and nicotinic acid (niacin, B5), 0.33–2.2. The content of
some important minerals (in 100 g dates) is: K, 650–750 mg; Mg, 50–58 mg; S,
43–51 mg; P, 59–64 mg; Fe, 1.3–2 mg; Ca, 58–68 mg and CI 268–290 mg).
32.3.2.2 Chemical Composition of Date Substrate
Date syrup for the production substrate should contain nutrients needed by yeasts
such as sugars, protein, minerals, D-Biotin, D-Pantothenic acid and m-Inositol. As
shown in Table 32.2, date syrup is deficient in P, K, Mg and proteins, it serves
mainly as carbon and energy source. As a result, the deficient minerals need to be
supplied to the yeast in the mineral medium in form of (NH4)2SO4, KH2PO4, and
MgSO4 salts, and in case of the Saccharomyces cerevisiae strain, inositol needs to
be added to the medium (Aleid et al. 2010).
32.3.3 Process of SCP Production SCP is produced in fermentors using the fed-batch or continuous fermentation processes
(Brauer 1985; Bronn 1990; Roels 1983). At first the substrate and the media
are prepared by dilution, mixing, sterilization and purification if needed. Also the
fermentor is cleaned and sterilized as needed. The process begins with the addition
of a starter culture of the production organism to the fermentor containing some
medium and substrate. The rest of the medium and substrate are then fed to the
fermentor. In the continuous fermentation process, feeding and harvest continue
simultaneously and the process goes on as long as it is in a steady-state and no contamination
of foreign microorganisms occurs. In the fed batch-process, feeding continues
until a certain broth volume in the fermentor is reached, after which the
process is stopped and the biomass harvested. Continuous aeration is needed because
the process is mostly aerobic. Also cooling is necessary because large amounts of
heat are produced during microbial growth. Stirring is needed to intensify cooling
and air transport to the microorganisms. The final biomass concentration in the
fermentor is about 4% on a dry weight basis. The biomass is harvested by filtration,
centrifugation or sedimentation. The biomass is then dried to about 95% dry matter
in dryers or under the sun. Drying increases the shelf-life by killing the cells of the
production organism and preventing the growth of contaminants. According to use,
the product is dried into powder, granules or flakes. Finally the product is packed in
suitable containers and sent to the market.
For the assessment of the safety and nutritional value of SCP, factors such as
nutrient composition, amino acid profile, vitamin and nucleic acid content as well
as palatability, allergies and gastrointestinal effects should be taken into consideration
(Litchfield 1968). Also, long-term feeding trials should be undertaken for
toxicological effects and carcinogenesis.
32.3.4 SCP Production from Dates
Aleid et al. (2010) described using date syrup as the production substrate for singlecell
protein and steps taken to formulate a suitable substrate from it. Fermentation
process optimization experiments for both Candida utilis and Saccharomyces 32.3.5 The Problem of Nucleic Acids
About 70–80% of the total cell nitrogen is represented by amino acids while the
remainder occurs as nucleic acids. This concentration of nucleic acids is higher
than other conventional proteins and is characteristic of all fast-growing organisms.
This has two implications for the nutritional value of SCP. For use of SCP
in animal feeds the major implication is simply that nucleic acid is not protein and
essentially dilutes the protein, although there are at least some possibilities of
physiological effects (Bull et al. 1997). As far as the potential use of SCP in
human food is concerned, nucleic acids are undesirable because their digestion
leads to unacceptably high levels of uric acid in the blood, sometimes resulting in
gout disease (Edozien et al. 1970; White et al. 1964). Uric acid is a product of
purine metabolism.
The bulk of the nucleic acid in microorganisms is RNA, which has a critical role
in protein synthesis. Thus, it may be anticipated that the faster the rate of protein
synthesis in a particular cell, the higher the nucleic acid content (Bull et al. 1997).
The removal or reduction of nucleic acid content of various SCPs is achieved with
one of the following treatments (Zee and Simard 1974): (a) chemical treatment with
NaOH; (b) treatment of cells with 10% NaCl; (c) thermal shock. These methods aim
to reduce the RNA content from about 7% to 1%, which is considered within acceptable
levels. Thus SCP is treated with various methods in order to kill the cells,
improve digestibility and reduce the nucleic acid content
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