Preparation of semi-refined κ-carrageenan: Recycling of alkali solution and recovery of alkali from spent liquor

Transcription

1 Indian Journal of Chemical Technology Vol. 15, January 2008, pp Preparation of semi-refined κ-carrageenan: Recycling of alkali solution and recovery of alkali from spent liquor A S Mehta*, K H Mody, Anita Iyer & P K Ghosh Central Salt & Marine Chemicals Research Institute, Bhavnagar, , India asmehta@csmcri.org Received 27 November 2006; revised received 16 ctober 2007; accepted 30 ctober 2007 The data on the preparation of semi-refined carrageenan (SRC) from Hypnea musciformis and Eucheuma cottonii from the Indian sea coast are presented in the article. Improvements in the process of preparation of SRC such as: reduction in usage of water, reduction in soaking time of seaweed, increase in throughput, reuse of alkali, minimization of waste and regeneration of good quality KH from spent liquor are achieved. Results obtained on lab scale (50 g seaweed) and bench scales (2.5 and 15 kg seaweed) are presented in the article and requirements of alkali, water and steam for a commercial unit are projected. Keywords: Semi-refined carrageenan, Hypnea musciformis, Eucheuma cottonii, Polysaccharide, BD and CD Carrageenan is the name given to a family of linear sulphated food grade polysaccharides obtained from the red seaweeds (Hypnea or Eucheuma spp.). They have the unique ability to form an infinite variety of gels at room temperature, rigid or compliant, tough or tender with high or low melting point. Carrageenan has three main varieties viz., kappa, iota and lambda, which are well differentiated in terms of their gelling properties and protein reactivity. Kappa carrageenan produces strong rigid gels while those made with iota, are flaccid and compliant. Lambda carrageenan do not gel in water, they interact strongly with proteins to stabilize a wide range of dairy products. Carrageenan is a linear polysaccharide, a galactan with the galactose residues linked with alternating β (1-3) linkages and α (1-4) linkages. In addition, the galactose unit link α (1-4) in the general structure often occurs as 3, 6-anhydro-D-galactose and sulphate ester group may be present on some or all galactose units. The use of carrageenan for food has grown in the industrialized countries, by at least 5 to 7 per cent per annum. Carrageenan has unique properties, which cannot be substituted, by other gums and its future is assured in all areas of the world that demand convenience food. It is added in beer, wine and vinegar to accelerate and improve clarity, as stabilizer and viscosity improver in chocolate milk drink, its use in ice cream preparation prevents formation of ice crystals and enhances excellent mouth feel. Carrageenan is used as a stabilizer in tooth paste, its presence in poultry and ham controls dehydration of frozen poultry and enhances juiciness. Carrageenan is being imported in India, but there are no manufacturers of carrageenan in the country. It is mainly used in dairy products, toothpaste and cosmetics in the country. A literature survey pertaining to carrageenan reveals that there are few reports about carrageenan in the form of patents. Larsen 1 has reported the preparation of a carrageenan product which is useful as an emulsifier and for thickening or gelling aqueous systems is made by subjecting a carrageenancontaining material in which 6-sulphated galactose units have been converted into 3, 6-anhydro galactose units to a shear stress treatment, e.g., by means of an extruder 1. An improved method of producing semirefined kappa carrageenan is described having five processing steps 2. Method of recovering clear solution of KH from spent liquor at the end of cooking is also described. Bost et al. 3 have reported a process for producing carrageenans, more particularly kappa and iota carrageenans, containing less than 2% by weight of insoluble material, comprising the steps of preparing an aqueous suspension of a seaweed which contain carrageenans and treating the resultant suspension with one or a mixture of enzyme(s). Tsai et al. 4 have described the preparation of carrageenan products from processed seaweed material using shear stress treatment. The carrageenan products comprise

2 46 INDIAN J. CHEM. TECHNL., JANUARY 2008 at least about 65% by weight of carrageenan and at least about 2% by weight of acid insoluble material. A detailed critical analysis of the available literature reveals the fact that higher temperature (>100 o C) and pressure (>10 psig) causes the carrageenan present in the cell wall of the seaweed to leach out. In some cases formation of 3, 6-anhydro linkage takes place in presence of mild alkali 5. When extraction is carried out at ~ 80 o C and atmospheric pressure using concentrated alkali solution (~ 1.5 M KH), carrageenan gets loosened from the cell wall, but it doesn t come out in the solution and gets fixed on the surface of weed itself. This type of product is called semi-refined carrageenan (SRC) 5. Earlier this product was used for non-food gelling applications. However, recently it has been accepted in the pet food market. A process flow sheet used for the manufacture of SRC is shown in Fig. 1. From the process flow sheet it can be seen that soak liquor, spent liquor and washings are liquid effluents in the process. Since the alkaline solution used is about 1.5 molar KH, a fairly good amount of KH goes un-reacted in the spent liquor. It is, therefore, essential to either reuse the spent solution in the process or recover alkali from the spent liquor for an economically viable process. Both these issues are addressed in the present paper. Experimental Procedure Bench scale preparation of semi-refined carrageenan (SRC) Experiments were carried out using 8.0 to 9.0% KH solution. Cooking temperature was maintained at 80 C (78-82 C) and cooking time was 2-3 h after achieving 80 C temperature. Soaking of seaweed was done in tap water and soaking time varied from overnight (16 h) to just before the extraction (2 h). When seaweed was soaked for a long time its water uptake increased (up to 9 times its weight) and hence the volume of wet seaweed increased. For small duration of soaking time, water uptake (6 times) and volume of wet weed remained low. Alkali solution used is litres depending upon the volume of wet seaweed. Experimental set-up used for 2.5 kg weed scale is shown in Fig. 2 and experimental observations are given in Table 1. Circulation (stirring) of alkali solution in the reactor was done with the help of a pump. A lot of frothing was observed on the surface of the solution in the vessel. Periodically froths were removed or disturbed to avoid spillage of the alkali solution. A layer of froth acted as an insulation on the surface of the hot Fig. 1 Process flow sheet for preparation of semi-refined carrageenan Table 1 Data sheet for preparation of semi-refined κ-carrageenan from Hypnea musciformis and Eucheuma cottonii Experiment No Input Type of seaweed (algae) Hypnea musciformis Eucheuma cottonii Dry wt. (kg) Strength of KH soln.(%) Vol. of KH soln. (L) Temp of cooking ( C) Time of cooking (h) utput Vol. of spent liquor (L) Vol. of 1 st Wash (L) Vol. of 2 nd Wash (L) Vol. of 3 rd Wash (L) Time of drying (Days) Wt of dry product (Kg)

3 MEHTA et al.: PREPARATIN F SEMI-REFINED κ-carrageenan 47 taking a known organic compound having organic sodium sulphate (sodium dodecyl sulphate). Experimental determination gave 32.83% sulphate in the compound against 33.33% theoretical value, which showed about 1.4% experimental error in the determination. Endeavors were made to estimate free sulphate in the product by first dissolving SRC in hot water and adding solution of barium chloride. White precipitates of barium sulphate were not obtained. Hence it was confirmed that the product SRC contains only sulphate bound with carbon. Fig. 2 Sketch of experimental set-up for extraction of semi refined carageenan solution. As a result, a very small amount of steam was required to maintain 80 C temperature in the reactor. Volume of steam condensate coming out of the jacket of the reactor was collected and measured and was found to be less than ten litres over the 3 h duration of extraction process. At the end of extraction, hot alkali solution (spent liquor) was removed from the reactor and washing of the product (SRC) was carried out in the same vessel using tap water. Three washes of almost equal volume of water were given for 20 min each. During washing also, water was circulated with the help of pump in order to have better removal of adhering alkali from the product. After the third wash, the product was sun dried till moisture content became less than 10%. Spent liquor and washings were analyzed for alkali content to estimate consumption of alkali in the extraction. Dried product was pulverized and sieved before packing in a bottle. Measurement of gel strength of SRCs For measurement of gel strength of SRC, 1% SRC solution was prepared in 1% hot aqueous KCl solution. Gel was allowed to form in the beaker at room temperature by natural cooling of solution and then kept in the refrigerator in the inverted position overnight at C temperature. Next day the gel was brought to room temperature and gel strength was measured by using Nikkansui type gel tester. Gel strength of the product ranges from and g.cm -2 for the product obtained from Hypnea and Eucheuma respectively. Estimation of sulphate and carrageenan in SRC Before estimating sulphate in SRC, accuracy of the method 6 of estimation of sulphate was checked by FT-IR The IR spectra of SRC product obtained from Hypnea and Eucheuma were recorded in KBr pellets using FT-IR spectrophotometer (Perkin-Elmer Spectrum GX). Results and Discussion Characterization of cations and anions present in Hypnea, Eucheuma and product SRC was carried out using standard methods 6 and results are given in Table 2. Moisture content of SRC, as estimated through oven drying at 110 C, varies from %. This shows that sun drying is quite effective. But low rate of drying is its drawback. Ash content in the Eucheuma derived SRC as compared to Hypnea derived SRC is higher. This might be because of an inherent property of the weed. Concentration of K + in SRC is in the range of and % for Hypnea and Eucheuma based SRC respectively. Potassium content in Eucheuma is 12-14% compared to about 1.0% in Hypnea (Table 2) and this is the reason for higher K + in SRC obtained from Eucheuma. However, as such there is no apparent relation between K + in SRC and gel strength of SRC (Table 2). Pure κ-carrageenan of idealized structure, 1, where every repeat unit has one sulphate group, has a sulphate content of 22.64%. It is believed that Hypnea and Eucheuma seaweeds are rich in 1. Additionally, 1 is derived from structure, 2, (μ-carrageenan), during the alkali treatment process 5 : - CH 2 H CH 2 S 3-3 S H H H μ - Carrageenan CH 2 H - 3 S (+) KH (-) (-) H 2 20 & (-) (-) K 2 S 2 S0 4 4 CH 2 H H 4--sulphato-β-D-galactosa β-d-galactoseα-d-3, α-d-3, 6-anhydrogala 6 anhydrogalactose κ-carrageenan κ - Carrageenan (repeating (repeating units) units ) (1)

4 48 INDIAN J. CHEM. TECHNL., JANUARY 2008 Besides 1, the carrageenan obtained from Hypnea and Eucheuma also presumably contain other types of polysaccharides. For example, some of the molecules may have even less than one sulphate ion per repeat unit, i.e., some repeat units may have no sulphate at all, while other molecules may have two or even three sulphate per repeat unit. All of these could lead to variations from the idealized sulphate content of 22.64% for κ-carrageenan. For example, sulphate content in refined carrageenan obtained from Hypnea 7 is around 14-16% while it is around 18-20% for refined carrageenan obtained from Eucheuma. In the case of semi-refined carrageenan, interpretation of sulphate analysis could be further complicated by the presence of non-carrageenan impurities. Thus Eq. (2) below may be considered as a rather simplified formula for evaluation of carrageenan content and product quality. Percentage of carrageenan = (% of sulphate in the product 100)/22.64 (2) During cooking in alkaline medium carrageenan present in the cell wall presumably gets detached from the cellulosic species. At low alkalinity, high temperature (>100 o C) and pressure (2 atm), the carrageenan is released into solution whereas at high alkalinity, lower temperature and atmospheric pressure it gets loosened from cell wall and gets fixed in the seaweed. Relative contributions of the above parameters in determining whether carrageenan is extracted in water or remains fixed in the seaweed need to be established, together with the effect of different types of alkali. Based on Eq. (2), carrageenan content varies from % and % for the product obtained from Hypnea and Eucheuma respectively. When sulphate content is used as the basis of estimation of carrageenan in SRC, it is found that carrageenan content appears on lower side for products derived from Hypnea compared to SRC prepared from Eucheuma. Based on the sulphate content in the weed and the final product, it appears that 48-57% sulphate is removed during extraction in the bench scale studies. This indicates that the extent of desulphation is same in both the weeds. As per the stoichiometry of Eq. (1) and assuming 50% reduction of sulphate based on the overall initial sulphate content of ca. 12% in 2.5 kg of dry seaweed, the requirement for KH is ca. 1.5 moles or ~ 85 g. The actual amount of KH consumed was found to be kg, which indicated that there are perhaps, unaccounted losses. Cations and anions in the product (SRC) Ash content indicates inorganic component in the product as well as in the raw material. This varies from 17-20% in SRC, which is well within the requirement of 35% maximum 8. The ash content of SRC agrees with the sum of the individual concentrations of the various cations (sodium, potassium, calcium and magnesium) and sulphate anion. An attempt was made to balance cation and anion in the final product. Cation and anion balance of each product was worked out and typical results of calculation of SRC are given in Table 3. Semi-refined κ-carrageenan contains only sulphate as anion. It has -1 charge instead of -2 when it is bonded with organic carbon. Calcium and magnesium are divalent whereas sodium and potassium are univalent cations. This is accounted for by expressing their concentrations in terms of moles of charge. It was observed that concentration of cation was more than anion in all the products. This is mainly because of adhering KH in the product (this might also help to explain the higher apparent consumption of KH as compared to the extent of Table 2 Physicochemical analysis of Hypnea, Eucheuma and semi refined carrageenan SRC-Hypnea based SRC-Eucheuma based Content in (%) Type of sea weed Experiment No Hypnea Eucheuma Moisture Ash Sulphate Sodium Potassium Calcium Magnesium Carrageenan Gel strength g.cm

5 MEHTA et al.: PREPARATIN F SEMI-REFINED κ-carrageenan 49 sulphate removal from seaweed discussed above). In other words part of potassium ion is in the form of KH in the product. Excess cation in SRC varies from to moles as is evident from the calculation. Considering this as K +, the product contains corresponding moles of KH. This is equivalent to less than 1% KH in the final product. Preparation of semi refined carrageenan (SRC) from Hypnea musciformis by recycling KH solution (spent liquor) Experiments were carried out to prepare SRC from Hypnea musciformis by recycling KH spent liquor on lab scale (starting with 50 g seaweed), with a view to find out maximum number of batches of SRC that can be obtained in 24 h and to study the effect of recycling of alkali solution on the quality of the product, by following the process described earlier. Fifty grams of dry and cleaned Hypnea were soaked for 1 h in water, drained and its wet weight was taken. This was transferred to a beaker containing 1 L of approximately 8% KH solution maintained at C in a constant temperature bath. Cooking was carried out for 3 h. The product obtained was then drained and the alkali solution (spent liquor) was recycled. The strength and volume of the spent liquor was made up to approximately 8% and 1 L respectively by adding required amount of solid KH and water respectively. The next cycle was performed using this solution. It was observed that the temperature of spent alkali had decreased to 60 C when it was replaced in the bath for a new cycle. The product was subjected to three washes with tap water for 20 min each and then air dried at room temperature for 24 h and later on at 60 C in the oven. The alkali content of washes was estimated and the product was characterized. In 24 h, 6 cycles were repeated and a total of ten batches were carried out. Table 3 Cation and anion balance in the semi-refined carrageenan Constituents Percentage Percentage on dry wt basis Moles Moles of charge Total cations & anions Moisture Ash Sulphate (-ve) Sodium Potassium Calcium Magnesium (+ve) Important process parameters and data of overall 10 cycles are given in Tables 4 and 5. Based on the data given in Table 5, the amount of KH consumed in each batch is calculated and results are given in Table 6. ne can observe that except one or two batches, the average requirement of KH works out to be12-13 g per 50 g of dry Hypnea. Yield of SRC is calculated on dry basis, i.e. considering no moisture in the raw material. Average yield works out to be 33% as shown in Table 7. It is possible to recover alkali from spent liquor and washes and hence considering average yield of SRC as 33%, alkali requirement works out to be about ton per ton of SRC, which is on a higher side than stoichiometric requirement as discussed earlier. Product characterization of SRC obtained in the above-mentioned experiments is given in Table 7. From the results it can be seen that gel strength of the product ranges from g.cm -2 except 430 g.cm -2 in one set. This is comparatively on a higher side than the results obtained on bench scale study shown in Table 2. Carrageenan content of SRC, estimated based on sulphate content of the product and formula given in Eq. (2) varied from %. This indicates that recycle of alkali solution has no adverse effect on the quality of the final product. IR Spectra of SRC The IR spectra of SRC products obtained from Hypnea and Eucheuma are shown in Fig. 3. The spectra show bands at approximately 845 and 930 cm -1 assigned to D-galactose-4-sulphate and 3,6- anhydro-d-galactose respectively, which are typical of κ-carrageenan. In addition to this, SRC obtained from Hypnea sp. contained an additional band at 1714 cm -1, which was indicative of C - group. Management of effluent disposal Spent liquor contains a major amount of unused KH and organic matter extractable in alkali. It is Table 4 Important process parameters for experiments carried out for recycling of spent liquor Parameter Value Dry wt. of Hypnea in each batch 50 g Volume of water used for soaking 1.75 L Time of soaking 1 h Temperature of cooking C Volume of alkali used for cooking 1 L Time of cooking 3 h Volume of each wash 2 L Time of each wash 20 min Number of washes 3

6 50 INDIAN J. CHEM. TECHNL., JANUARY 2008 advisable to reuse the spent liquor in the subsequent batches by adjusting KH concentration. It is reported in the literature that after using appropriate filtration technique, clean filtrate can be recycled in the process 2. But during filtration of the spent liquor temperature drops down to almost room temperature and hence there is loss of thermal energy present in the spent liquor. Instead of filtration, if spent liquor can be directly utilized in the process, thermal energy can be conserved and utilized in a meaningful way. However, at the end of minimum 25 such cycles 9, spent liquor is thrown out as effluent because of increased organic matter and dirty appearance. In the present study a method has been developed to recover good quality KH from such waste and the same is described hereunder. Three tap water washes to remove adhering alkali from SRC are given in a cascading manner, i.e. third wash of first batch is used as second wash in the next batch and so on. As a result only one liquid wash rich in KH, comes out as effluent. Actually, KH is recovered from this also as described below along with spent liquor. Soak liquor obtained after soaking Cycles Fig. 3 IR spectra of SRC prepared from: (a) Hypnea and (b) Eucheuma Table 5 Data sheet of experimental observations on recycling of spent liquor Input utput Alkali solution Water KH Spent liquor KH in washes, g Vol. Strength of added (L) added (g) Vol. (L) Strength (L) soln.(%) of soln.(%) Table 6 KH consumption during SRC preparation by recycling of spent liquor Cycle no. KH KH in various streams (g) -output Total KH KH taken (g) Spent Washes output (g) consumed (g) liquor

7 MEHTA et al.: PREPARATIN F SEMI-REFINED κ-carrageenan 51 of dry seaweed in tap water is the only one effluent stream of the process which is to be thrown out. Data collected on dissolved oxygen and TDS of washes and soak liquor of a representative experiment are given in Table 8. The difference in the values of dissolved oxygen of fresh effluent (first day) and effluent after fifth day of storage in closed container is the value of biological oxygen demand (BD). These range from 0.71 to 6.27 mg.l -1 for soak liquor as well as washes. The upper permissible limit according to the standards of Central Pollution Control Board of India 10 is 20 mg.l -1. Thus the effluent does not require any treatment prior to discharge. CD measurement was carried out for soak liquor and washes. It was observed that CD values are 275 mg.l -1 for soak liquor and 270 mg.l -1 for washes which are very close to the permissible upper limit which is 250 mg.l -1. The spent liquor is to be recycled in the process hence it is not considered as effluent and therefore its CD was not measured. These results indicate that the effluent generated (soak liquor and washes) during preparation of SRC is not hazardous and does not require treatment before its disposal except perhaps some ph adjustment. Soak liquor is a good source of nutrients and growth promoting agents for plants. This may be discharged in the coconut plantation 11. KH can be recovered from final wash as described below. Recovery of KH from spent liquor and final wash Efforts were made to develop a simple method of recovering KH from spent liquor and the washing as mentioned above. Adsorbents like clay, activated charcoal and synthetic resins were used to remove pigments extracted from the weed into solution. But these adsorbents could not improve the appearance of the spent liquor. As a result a novel method was developed as described below. Recovery of KH from spent liquor was done by: 1. Initial concentration of spent liquor by steam heating followed by heating on oil bath. 2. Calcination of concentrated spent liquor (sludge) in a muffle furnace at 550 C. Calcination at high temperature converts all the organic matter present in the sludge (mainly pigments and dissolved polysaccharides) into C 2 and inorganic matter, mainly KH, remains in the container as ash, which is white crystalline solid. This can be reused in the cooking process. Projection of raw material and utility requirements In addition to seaweed, potassium hydroxide and tap water are the main raw material for the manufacture of SRC. Based on the data generated in the bench-scale study, the amount of KH required for manufacturing 1 ton SRC works out to be ton. Volume of water required for soaking the seaweed and preparation of KH solution is in the range of kilo-liter for one ton of SRC. Steam and electricity are major utilities required in the process. Steam required for cooking of seaweed is about 10 ton for one ton of product. Table 7 Physicochemical properties of semi-refined carrageenan obtained in KH recycling experiments Product code Yield (%) Gel strength (g.cm -2 ) Ash (%) Sulphate (%) Na (%) K (%) Ca (%) Mg (%) Carrageenan content (%) I I I I I I I I I I Table 8 Dissolved oxygen and TDS of soak liquor and washes Parameters First day analysis Fifth day analysis Soak liquor First wash Second wash Third wash Soak liquor First wash Second wash Third wash TDS (mg.l -1 ) D (mg.l -1 )

8 52 INDIAN J. CHEM. TECHNL., JANUARY 2008 Conclusions Based on the data obtained on preparation of SRC from Hypnea musciformis and Eucheuma cottonii from the Indian sea coast and product characterization it can be concluded that it is possible to manufacture semi refined carrageenan of international requirement on a commercial scale in India. Improvements in the process of preparation of SRC such as: reduction in usage of water, reduction in soaking time of seaweed, increase in throughput, reuse of alkali, minimization of waste and regeneration of good quality KH from spent liquor, etc. provide insight in the chemistry and engineering aspects of the process. Results obtained on lab scale (50 g seaweed) and bench scale (2.5 and 15 kg seaweed) indicate good consistency and reproducibility in the experimental findings. It is possible to recover good quality KH from the spent liquor and washes. Moreover, soak liquor as such doesn t require further processing before its final discharge. Requirements of alkali, water and steam for a commercial unit are projected to give an idea about their requirement on a commercial scale. Acknowledgement Authors thank all the colleagues who have provided useful suggestions and scientific inputs in the research problem outlined here and M/s Pepsico India Holdings Private Limited, Gurgaon for providing financial assistance. References 1 Larsen P F, US Pat 5, 502, Rideout C S & Bernabe M G, US Pat 5, 801, Bost P E, Recabarren A H & Palma J Z, US Pat Appl No Tsai A G, Ledwith L K S, Kopesky R, Lynch M G, Blakemore W R & Riley P J, US Pat Appl No Towle G A, Industrial Gums, Polysaccharides and Their Derivatives, 3 rd edn, edited by R L Whistler & J N BeMiller (Academic Press, New York), 1973, Vogel A J, A text book of Quantitative Inorganic Analysis, 3 rd edn (The English Language Book Society and Longman, Green & Co Ltd.), Report on preparation of κ-carrageenan using Indian seaweed and cultivation of carageenophytes, Submitted to PepsiCo India Holdings Private Limited by Central Salt and Marine Chemicals Research Institute, Bhavnagar, India, Food Chemical Codex, 3 rd edn (National Academy Press, Washington D.C.), Private communication with a manufacturer in Indonesia, Gujarat Govt Gaz Part IV-C, Notification issued by Gujarat Pollution Control Board, Gandhinagar, Gujarat, India, Private communication with agar manufacturer in Tamil Nadu, India, Bhalal B J, Bhatt J & Pandya V P, Indian J Environ Prot, 16 (1996) Gordon M & Duri B A, Ind Eng Chem Res, 30 (1991) Anand P S, Popat K M & Dasare B D, Third National Conference on Desalination held at CSMCRI, Bhavnagar, 1984, 213.