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Drying 2004 – Proceedings of the 14th International Drying Symposium (IDS 2004)
São Paulo, Brazil, 22-25 August 2004, vol. B, pp. 900-907
900
BIOCHEMICAL AND PHYSICAL CRITERIA OF SPIRULINA
AFTER DIFFERENT DRYING PROCESSES.
Hélène Desmorieux and Fabiola Hernandez
Laboratoire d’Automatique et de Génie des Procédés, LAGEP UPRES-A CNRS Q5007-
Université Claude Bernard Lyon1- CPE - bat. 308G, 43, Bd du 11 novembre 1918, 69622
Villeurbanne Cedex, France.
hernandez@lagep.univ-lyon1.fr; desmorieux@lagep.univ-lyon1.fr
Keywords : spirulina, protein, sugar, convective drying, freeze drying, infrared drying, spray drying.
ABSTRACT
The essential nutritional elements found in spirulina made this cyanobacteruim a
potential food product for the use of spatial crews. After culture and harvesting, the
spirulina must be dried, usually by spray drying in the big farms on the Earth. However,
the obtained powder does not satisfy all criteria for using the spirulina as food for
human consumption. In this study, convective and oven drying, infrared drying, spray
and freeze-drying methods were selected to assess the best technology to dry spirulina.
Protein and sugar content were determined before and after drying. Freeze-drying
showed the highest retention of the analyzable proteins and sugars. Thin layers
spreading out gave better results compared to cylinders. The dried end-product is
characterized by microscopy that allowed to show damage because of the air drying
temperature. The best drying method can be selected by ranking of the drying processes
using the studied parameters.
INTRODUCTION
The spirulina is a cyanobacterium that grows rapidly (Cornet and Dubretet, 1990), has a good digestibility
and contains most of the essential nutritional elements necessary to the human system. Due to its
properties, it can be cultured and used as food, focused in particular for the spatial crews during long-term
space explorations (Vernerey, 2001). To preserve spirulina after harvest and to vary its incorporation in
food it has to be dried. The drying of food usually results in quality reduction compared with the fresh
product. Nevertheless, in the case of spirulina, drying can offer interesting transformations and at the
same time a dried product can be transformed into several different forms to facilitate its regular
consumption. The usually spray dried powder does not satisfy all criteria required for the use of this
901
powder as a food product. Therefore, the high nutritional value of this dried product is not being utilized
as food on the Earth mainly because of the limited consumption of algae and no acceptance of its
characteristic green color. After incorporation in the food, the dried spirulina releases the green color.
Traditional methods used to process fresh into dry spirulina are:
- Spray drying: in most important farms, spirulina is dried by spray drying (Ding-Mei L., 1997),
- Ocean chill: this is a process that combines spray drying with the use of air at low oxygen
concentration usually below 1% (Cysewski,1994) to protect the oxygen-sensitive nutrients,
- Hot air: it is used in semi-industrial farms or artisan production. The paste is extruded into small
cylinders of 2-3mm diameter, placed on trays and then dried by convective hot air (Jourdan J.P,
1999),
- Solar drying: this method presents an interesting solution in hot countries and is used after extrusion
in cylinder forms (Bonnin, 1993),
- Other method proposed by Fox (2001) uses fresh filtrated spirulina which is added to pre-cooked and
dried hot flour, then mix-dry to low moisture content of 5%,
- In some countries such as in Chad, the spirulina is harvested from several lakes by the Kanembu
people. The spirulina is filtrated and dried directly on the sandy shores or tissue support (Gatugel,
2000). Spirulina obtained in this manner is mainly used to prepare a kind of fish or meat and
vegetable broth.
In this work, different drying processes were compared in regard with certain qualities of dry spirulina.
Convective (low air velocity) and infrared drying, spray drying and freeze drying were compared in their
protein and total sugars content before and after drying. The dried end-product was characterized by
microscopy. This study allowed classification of the selected drying methods and also assessing the effect
of air drying temperature in the processing of spirulina.
MATERIALS AND METHODS
Spirulina
Spirulina culture was grown in simplified Zarrouck medium (Zarrouck, 1996) using a batch reactor of 30
liters without A4 and A5 solutions under light at 200 W/m2. To recover the biomass the suspension of
medium and algae was filtrated through a 20 µm filter and then rinsed with distilled water which at the
same time separates the salts that come from the culture medium. The consecutive steps were particular
for each of the selected drying process. For each drying experiment, one sample of fresh spirulina was
frozen to perform simultaneous biochemical analysis of the dry biomass and the corresponding thawed
fresh biomass. The dry-based moisture content of the product (X) was expressed in kg water per kg dry
matter and measured before and after drying. The protein and total sugar contents were reported as
protein or sugar mass per mass of dry biomass. The fresh spirulina protein content varied during the
culture in a batch reactor. This deviation is specially due to the variable exopolysaccharide (EPS) content
[Filali Mouhim, 1993]. The more spirulina produced EPS, the more the measured protein content is weak.
Drying conditions
The arrangement and the aspects of the end product were different according to each drying method. By
convective and infrared drying, two arrangements of the fresh paste were investigated: extrusion into
cylinders of 2-3mm diameter and spreading the concentrated and filtrated biomass in a thin layer of 1 to
1,5 mm thickness on a flat support. Freeze-drying and spray-drying were carried out with algae
suspension.
902
-Convective drying was carried out at temperatures of 40, 50, and 60°C both in the layer and cylinders,
the air humidity was between 4 and 7%, the air velocity was 0.15m/s. Oven drying was used to analyze
the effect of temperature. The drying time was between 2 and 3 h.
- Infrared drying was carried out at 40, 50 and 60°C both in the layer and cylinders. Maximum 9 short
infrared lamps (1,16 µm) from Philips was used, with a filament temperature at 2500K. They are placed
at 261 mm away from the sample. The maximal corresponding radiative flux is 2,71kW/m2. The
temperature of the product is measured and controlled through the infrared radiation power.
- Spray drying experiments were carried out at a feed rate of 0.09 l/h of the liquid suspension of 100kg
w/kg dm. Two air drying temperatures were tested, 130°C and 150°C. The drying time was few seconds.
- Freeze-drying was performed using an initial concentration of 20 kgw/kgdm at 45°C during 4.2 h. The
first stage of drying was performed at -20°C and 8 Pa for 18h. The second stage at +20°C and 1.5-5 Pa
for 6.5 h.
Microscope characterization
The aspect of dry product was a function of the process in either powder or coarse material (pieces of 1-3
mm large). To test the effects of drying process on the change of filament, the dried samples were
observed with a LEICA DML microscope with integrated camera. A small sample of dry spirulina was
placed on a slide and moistens with a drop of water. Immediately, the photo was taken with a resolution
of x-20 or x-50.
Protein analysis
Various methods of mechanical and physical extraction were used to extract the proteins from the
samples of diluted spirulina in distilled water. Sonication was the method that allowed measuring the
highest protein contents and a better reproducibility of all dried samples. The protein content was
determined by the use of bicinchoninic acid (BCA). This method avoids interference with others
components in the samples (Nielsen,1994). The proteins content was measured with a spectrophotometer
in the wavelength of 562 nm (Smith,1985). Bovine serum albumine (BSA) was used as the standard
protein for the calibration curve in the range of 0 to 250 µg proteins/l. The precision of the method is
carried out on a commercial spray dried spirulina in powder. For the used spirulina, 8 samples dried in
thin layer by infrared drying at 60°C and also 10 freezed dryed samples are analyzed.
Total sugars analysis
Total sugars were determined with the method of Herbert (Herbert, 1971). Phenol and concentrated
sulphuric acid were used. The absorbance of the obtained colored solutions was measured at 480 nm. A
glucose solution is used as standard in the range of 0 to 100 mg/l.
RESULTS
Precision
The15 analysis on commercial spray dried spirulina give a protein mean value of 75% with a standard
deviation of 9. The mean protein contents of the 8 infrared dried spirulina samples is 63.8% with a
standard deviation of 6.9. For the freezed dryed spirulina, the mean content is 78% with a standard
deviation of 6.7%. For the sugars analysis on commercial spirulina, it contents on average 14,2% sugars
with a standard deviation of 1,6.
Because of variations in the initial protein content, the results were expressed in a ratio of protein (or-
sugars) concentration in dry spirulina / initial protein (or-sugars) concentration of fresh spirulina before
drying.
903
Influence of drying air temperature on the proteins content.
Figure 1 shows the effect of the air drying temperature on the protein content after drying. As can be
observed, minimal loss was obtained at moderate temperature of 40°C. The protein loss was proportional
to the increase in temperature with losses of 10 at 40°C and 20% at 70°C. All the results are refereed to
the fresh spirulina protein content. Then, the figure shows only the relative loss, but not the deviation
between the fresh and the dry spirulina. For example when the fresh spirulina contains 70%, a protein
ratio after oven drying at 40°C of 90% (or a relative protein loss of 10%) means a absolute protein loss of
7% (90-0.7x90).
Figure 1. Ratio between protein content after drying and initial protein content (Cprot/Cproti) for 5 samples in function of the
drying air temperature in the oven dryer.
Influence of the drying process
Figure 2 shows the proteins loss in function of the drying processes.
Figure 2. Ratio between protein content after drying and initial protein content, C/Ci, in samples dried by oven-drying and
convective-drying (C), short infrared drying (IR), spray drying and freeze drying (lyo).
50
60
70
80
90
100
Fresh biomass 40°C 60°C 70°C
Drying air temperature
C
prot
/ (C
prot
)
i
, %
S1
S2
S3
S4
S5
mean
0,50
0,60
0,70
0,80
0,90
1,00
oven +
C40°C layer
C40°C cyl
oven +
C60°C layer
C60°C cyl
IR 40°C layer
IR 40°C cyl
IR 60°C layer
IR 60°C cyl
spray-dr
(130- 70°C)
spray-dr(150-
95°C)
Lyo N°1 and
N°2
Lyo N° 3 and
N° 4
Drying proce sse s
Ratio C/Ci in proteins
1st analysis
2d analysis
3rd analysis
4th analysis
5th analysis
6th analysis
mean
Oven + conv
IR
Spray dr.
Freeze dr.
Cylinders and layer Powder
904
The results obtained with oven drying and those obtained after convective drying were regrouped due to
the low value of the air velocity during convective drying that brings similar drying conditions for the
both processes.
The dispersion seems to depend much more on the type of drying. The dispersion is low for the infrared
drying in a thin layer and for freeze-drying. The highest dispersion was obtained by convective and oven
drying. The freeze-drying method had the best reproducibility of the results probably due to homogeneity
of the dry powder. This powder has a high solubility.
Freeze-drying showed the lowest protein loss below 10%. As seen in figure 2, 90% of the initial proteins
remained in the dry product. All others methods resulted in 10 to 25% protein losses.
In spray drying, where two experiments were carried out, the losses were about 10-15%. The end product
obtained by this method did not have the same aspect and color at 130 and 150°C. Therefore, more work
needs to be carried out involving other parameters to study the effect of air temperature.
Infrared drying on thin layer underwent also losses of about 10-15% compared to 25% loss for
experiments with cylinder form. The infrared dried thin layer material had a specially polished and shined
aspect on the surface.
Although the convective and oven drying methods resulted in a great dispersion, the figure allows to
remark that for the drying of a thin layer at 40°C the loss was minimal.
Figure 3 shows the mean protein content after drying in function of the drying processes. The different
methods of processing in this study were arranged from the one with the highest loss to the one that
presented the smallest protein loss (obtained for freeze-drying). The type of arrangement for the dry final
product was represented in the rectangle. The numbers of the corresponding analysis were noted for each
process.
Figure 3. Effect of different drying methods on the ratio protein content after drying/ initial protein content of the spirulina
(Cprot/CprotI) for oven-drying, convective-drying (C), infrared drying (IR), spray drying and freeze-drying (Lyo). Oblique
lines correspond to the spreading out in cylinders, the full grey for thin layer, the horizontal line for spray drying and the points
for freeze drying.
It appears clearly that drying in cylinders caused more protein losses (20-25%) than the form in thin
layers (15%).
Although the spreading out in cylinders or thin layer involves a great dispersion, the hardening on the
surface may be required for special organoleptic qualities and the possibility of retaining the pigments.
50
60
70
80
90
100
conv40°C
cyl
IR 40°C cyl IR 60°C cyl conv60°C
cyl
oven and
Conv 60°C
layer
IR 40°C
layer
spray-dry
(150-95°C)
oven and
Conv 40°C
layer
IR 60°C
layer
Lyo N°1
and N°2
Lyo N° 3
and N° 4
C
prot
/ (C
prot
), %
3p
3p
2p
7p 2p 3p
6p 2p
4p 3p
3p
905
Although there is no hierarchy of the different drying methods concerning organoleptic quality criteria,
there is lot of information in regard with the use of cylinders as the best way to incorporate spirulina in
foods. The use of this technique avoids a great dispersion of the blue green color in the food. Although no
scientific papers report these organoleptic qualities, the classification of the processes according to the
protein loss criteria is exactly in the opposition of the classification of these methods according to the
widespread qualities of taste.
This observation shows that other criteria have to be taken into account before choosing the best method
to process spirulina to use it as food.
Effect of drying air temperature on the total sugars content
Figure 4 shows the total sugars loss in function of the drying air temperature.
The total sugar loss was more significant (30%) compared to the lost of proteins (10-15%). The mean loss
was 30% at 40°C and remained constant at higher temperatures.
The loss was significant even at the lowest temperature used in this study.
Figure 4. Ratio between total sugars content after drying and the initial total sugars content (C/Ci) for 7 samples dried as the
function of the drying air temperature in the oven.
Photomicroscopy analysis of the dry spirulina.
Figure 5 shows the spirulina filament dried at different temperatures. This picture was taken
immediately after rehydration of the sample.
At 40°C (figure 5 at 40°C) no damage was observed. This result
confirms the good results for the proteins loss at this temperature,
by convective or infrared drying. In the figure, damage on the
filament was observed for temperatures higher or equal to 60°C.
The border of the filament became neither smooth nor clear. After
drying of the spirulina, the brightness of the filament decreased as
a function of the air drying temperature. Rehydration of the
samples did not allow obtaining the initial width of the filament.
50
60
70
80
90
100
Fresh biomass 40°C 60°C 70°C
Drying air temperature
C Tsugars / C Tsugars
i
, %
S1
S2
S3
S4
S5
S6
S7
mean
a) Fresh
906
Figure 5. spirulina a) fresh, and oven dried at b)40oC, c)60oC, and d)120oC air temperatures.
CONCLUSION
The decrease of protein and total sugar content was analyzed after convective drying, oven-drying,
infrared drying, spray drying and freeze-drying. The dispersion in the percentage of losses was a function
of the method used, the drying air temperature, and the spread-out method.
In drying oven, the total protein and sugar losses varied according to the used air temperature. These
losses were more significant in total sugar percentage (about 30%) compared to proteins (10-20%).
Protein losses using hot air drying between 40 and 70°C is proportional to the drying air temperature.
However the loss of total sugar of 30% remained constant from 40°C to 70°C.
The damage on the border of the filaments observed by microscopic analysis depended on the air
temperature by oven drying up to 40°C.. This result confirms the good results for the proteins loss at
40°C, by convective or infrared drying and the higher loss for a temperature equal or up to 60°C.
The best method to recover proteins and total sugars was freeze-drying, which resulted in minimal loss
(<10%) for each component. The highest loss of proteins and total sugars was obtained by convective and
infrared drying in spreading in cylinders. Thin layer drying rendered minimal loss compared to cylinders.
Relevant biochemical properties are obtained using drying methods that provide a powdered end product.
However, for the purpose of this study, the end product did not have acceptable organoleptic properties.
Thus, to select a drying method it will be necessary to take into account the importance of all the
intervening factors. These results showed that a systematic analysis has to be used in order to select the
appropriate drying method.
ACKNOWLEDGEMENTS
The authors would like to thank the European Spatial Agency (ESA) and the Universidad Autonoma de
Barcelona (UAB) for their financial support.
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907
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