Oleoresin represents the total pungency and flavour constituents of pepper obtained by the extraction of ground pepper using solvents like ethanol, acetone, ethylene dichloride, ethyl acetate etc. Oleoresin offers considerable advantages over whole or ground spices in that they are uniform in composition as well as strength. Contaminants like mould and fungus are absent in the oleoresin and hence can be directly added to any food material after adjusting the flavour concentration. The extractives are usually made available in both oil soluble and water dispersible forms and also in dry forms of the extractives.
The process for the recovery of the flavour constituents involves size reduction prior to solvent extraction which is done in stainless steel extractors. At present oleoresins are recovered by a single stage or a two stage process. In the single stage process the oil is recovered along with the resins by solvent extraction whereas in the two stage process the ground pepper is first subjected to steam distillation for the recovery of the essential oil followed by drying and powdering the oil extracted spice before solvent extraction. The extract obtained in the two stage process is blended with the oil recovered by steam distillation to meet the specification laid down by the consumer. The oleoresin obtained by the two processes differ slightly in their quality in terms of the composition of the oil and the yield of the oleoresin. In the two stage process, there is a possibility of the yield of oleoresin to be low and this is attributed to the presence of moisture in the wet cake obtained after steam distillation. To prevent such a loss, drying is resorted to prior to solvent extraction.
For the single stage process, pepper is flaked to a thickness of 1–1.5 mm and packed in stainless steel extractors for extraction with the organic solvent. Normally a solid to solvent ratio of 1:3 is employed and an extraction temperature of 55–60°C is maintained. The solvent is continuously kept recirculated to ensure efficient solid to solvent contact and after three hours of extraction the miscella is filtered and sent for evaporation. The solids are further contacted with lean solvents from other extractors and the entire extraction process is completed after 6 or 7 stages of extraction.
The miscella sent from the extractors are evaporated in a shell and tube evaporator or a tubular evaporator at a temperature not exceeding 80°C and a vacuum of 250 mmHg. When the temperature start rising and no further solvent is recovered from the concentrated miscella, it is pumped into a high vacuum stripper or a wiped film evaporator through a filter. The final desolventization is done at a vacuum of less than 20 mm of Hg and at no stage of operation the temperature is allowed to rise above 100°C. In the initial stages of this vacuum distillation, the condensate recovered will mostly be the organic solvent and towards the end of distillation the essential oil will also start coming out along with the organic solvent. At this stage some entrainer like alcohol or the monoterpene fraction of the essential oil is added to the desolventiser. This is done to remove the final traces of the organic solvent as well as to supplement the monoterpene fraction of the pepper oil which might have been lost during the final desolventization. The product is pumped into storage tanks after confirming that the residual solvent levels are within limits. Suitable blending is resorted to meet the customer requirements in terms of piperine content as well as oil content.
Certain consumers specify the homogeneity of the oleoresin as well as the colour of the resin. This is achieved by passing the oleoresin through a homogeniser which may be a colloid mill or a sand mill. The oleoresin which is dark green in colour is bleached using activated carbon to obtain decolourised oleoresin. The bleaching operation can be done either at the extraction stage itself or prior to the evaporation of the miscella.
In the two stage process, the cake obtained after steam distillation is further dried and ground to 40 mesh size and packed in stainless steel percolators. One volume of solvent is added to the extractor and it is allowed to stand overnight. The concentrated miscella is drained off and equal volume of fresh solvent is added. The resulting miscella is drained off immediately and then fed to the adjacent extractor filled with fresh charge. This operation is repeated so as to effect counter current extraction. The extraction is done in four or five stages. The concentrated miscella obtained from each of the extractors are pooled and distilled for the recovery of oleoresin.
As is evident, the quantity of miscella from the two stage process is less than that from single stage process. This will reduce the energy consumption in the processing plant. Further as the flavour imparting essential oil is recovered prior to extraction, the flavour of the oleoresin, which is very heat sensitive, is superior.
In both types of extractions the final adhering solvent is recovered from the extractors by sparging steam. The solvent rich steam is condensed and the solvent is recovered. The entire operation is done in such away that the solvent losses are kept as low as possible. The total loss of solvent by the industry is estimated to be about 50 kg per ton of spice processed.
The chlorinated solvents which are presently employed for oleoresin recovery are slowly being replaced by other solvents or solvent mixtures. Ethyl acetate and acetone are good competitors for the chlorinated solvents but the miscibility with water, flammability and low boiling point are major drawbacks. Hexane-acetone mixture in the ratio 2:3 is a good alternative for the presently employed solvents. Research work is underway at the Regional Research Laboratory, Thiruvananthapuram, India to find an alternate solvent system by a suitable blending of certain fractions of a naturally occurring essential oil with permitted organic solvents so as to match the polarity and other favourable characteristics of the organic solvent presently employed.
Oleoresin extraction, all over the world, is done mostly in batch extractors. A recent investigation conducted at Regional Research Laboratory, Trivandrum () on the kinetics of extraction has revealed the possibility of extraction being done at ambient conditions and with very low residence time. The present day extractions, which take more than 18 hours for processing one batch, can be dispensed with by employing continuous countercurrent extraction. Also the quantity of solvent required for extraction as well as its loss can be reduced.
Extraction of pepper has been done at laboratory level by enzymatic breakdown of spice cell walls. Spice is mixed with water, pH adjusted by the addition of citric acid, treated with enzymes individually as well as in combination followed by centrifugation. The enzymes used were commercially available cellulase, pectinase, hemicellulase and liquefaction enzyme preparations. Pepper extracts with good sensory and compositional properties were obtained with some of the enzyme combinations. Optimum results were obtained using a combination of cellulase and pectinase preparations. The addition of hemicellulase did not show any improve ment in flavour. This solvent-free extraction route offers an alternate process for the recovery of flavour compounds which however is not commercially viable at present.
Another solvent-free extraction which has shown promising results is the supercritical carbondioxide extraction. The major advantages of the process include low energy consumption for solvent recovery, high purity of the extracts, ecofriendly and the possibility of fractional separation of the components present in the spice. The prohibitive cost of machinery is to be compared to the benefits derived like selectivity of extraction, no residual solvent, no bacterial contamination and standardisation of flavour strength.
Now a days, the demand for piperine is on the increase. Piperine in the concentrated form can be produced from the oleoresin by centrifuging the oleoresin in a basket centrifuge. Part of oil along with some resin will come out of the centrifuge and the centrifuged cake containing as high as 60 per cent piperine is obtained. The piperine concentration can further be increased by washing the cake obtained with fresh pepper oil and further centrifuging it.
From pepper oleoresin, numerous secondary products have been developed to improve the solubility in food substances and are marketed under various trade names by the manufacturers with their flavour strength indicated on the respective label. These include standardised seasonings compatible with and are able to withstand almost all processing conditions. Products such as emulsions, solubilised spices, dry soluble spices, encapsulated spices, heat resistant spices, fat based spices etc. are a few of them.
Emulsions are liquid seasonings prepared by emulsifying blended pepper oil or oleoresins with gum acacia or other permitted emulsifying agents. A stabiliser is added to prevent creaming and the products have only a very limited shelf life. Solubilised pepper is blended pepper oil and/or oleoresin mixed with one of the polysorbate esters in such a concentration as to give a clear solution when mixed with water. Dry soluble pepper is prepared by dispersing the standardised oleoresin onto an edible carrier like salt, dextrose, rusk etc. to give a product having a flavour strength equal to that of a good average quality ground pepper.
Selections from the book: “Black Pepper. Piper nigrum”. Edited by P.N.Ravindran. Series: “Medicinal and Aromatic Plants — Industrial Profiles”. 2000.