Pirinç kepeği yağının metanol ve etanol ile yerinde esterleştirilmesi

Abstract

Pirinç kepeği harmanlanmış pirinç tanelerinin (Oryza Sativâ,) beyaz pirince işlenmesi sırasında elde edilen bir yan üründür. Pirinç tanesinin %8-10'u miktarında elde edilen bu kepek ortalama %20 yağ i çerıği ile Önemli bir bitkisel yağ kaynağı durumundadır. 1984 yılın da Dünya pirinç yağı potansiyeli 4.5 milyon metrik ton olarak saptan mıştır. Pirinç kepeği yağının hem besleyici değerinin hem de potansiyeli nin bu kadar fazla olmasına rağmen yemeklik olarak değerlendirilmesi çok sınırlıdır. Bunun en önemli nedenlerinden birisi değirmenden çı kan kepeğin içerdiği lipaz enzimlerinin yardımı ile, yağın derhal par çalanmaya başlaması ve böyle kepeklerden elde edilen yüksek serbest a sitli yağların rafinasyonunun zorlaşmasıdır. Bu çalışmada özellikle yemeklik olarak değerlendirilemeyen yük sek asitli pirinç yağlarının yağ asidi monoesterleri üretiminde değer lendirilmesi amaçlanmıştır. Sözkonusu bu esterlerden yağ asidi metil esterlerinin önemi son 40-50 yılda giderek artmıştır. Çünkü yağ asi di metil esterleri, pekçok yağ asidi türevlerinin üretiminde giderek yağ asitlerinin yerini alan bir ara hammadde durumuna gelmiştir. An cak bu çalışmada halen yağ asidi metil esterlerinin üretiminde kulla nılan geleneksel yöntemlerden farklı olarak yağın kepekten ekstrakte edilmeden yani yerinde (ins itu) esterleştirilmesi ile metil ve etil esterlerinin üretimi üzerinde durulmuştur. Deneysel çalışmalarda, reaksiyon süresi, katalizör miktarı, ke peğin bekleme süresi gibi değişkenlerin etkisi hem metanol ve hem de etanol ile yürütülen esterleştirme deneylerinde ayrı ayrı incelenmiş tir. Sonuç olarak pirinç kepeğinin metanol ve etil alkol ile yerinde esterleştirilmesi ile, serbest asit içeren yağların tek kademede es terleştirilmesi ile elde edilemeyecek saflıkta metil ve etil esterle rinin elde edilebileceği anlaşılmıştır. Metanol ile esteri eştirmede oldukça saf metil esterlerinin elde edilmesinin en önemli nedeni, me tanol un kepekten yağı oluşturan komponentleri seçimli çözmesi olmuş tur. Etil alkol ile yürütülen esteri eştirmede ise, hem etil alkolün daha iyi bir çözücü olması hem de su içermesi, elde edilen etil es terlerinin safsızlık içeriğini arttırmıştır.

In this study, in situ esterification of rice bran oil methanol and ethanol were investigated. with When harvested from the field, rice is in the form of paddy (or rough) rice, where the Kernel is fully enveloped by the rice hull (Figure 1). After being dried, the first stage in milling is removal of the hull, yielding brown rice. In the second stage of milling the outer brown layer is removed from the brown rice Kernel to yield the familiar white rice. The separeted brown layer is designated rice bran. EMBRYO (OEBM] Figure 1- Diagrammatic representation of a rice Kernel The outer brown layer is actually composed of a number bota nical entities, including several sublayers within the pericarp, and the aleurone layer. Depending upon the severity of milling (degrees of milling), which is abrasive milling, a variable quantity of the subaleurone or endosperm material normally shows up in the bran fraction. In addition, breakage of the white rice Kernel during milling results in small fragments of the endosperm also be coming part of the bran fraction. These broken fragments are n primarily starch and normally comprise 10-20% of the bran. Thus, the starch content and nutritional percentages of bran are a function of Kernel endosperm breakage which occurs during milling. In the case of parboiled bran, the harvested paddy rice is sub jected to soaking and steaming before being dried and then milled. The hull is removed first, followed by removal of the bran to yield parboiled (or converted) white rice and parboiled bran. The soaking and steaming process actually hardens the Kernel such that almost no endosperm breakage occurs during milling. As an consequence, parbo iled rice bran contains substantially less starch than does bran from rice which is not parboiled. Due to this lower level of starch, the parboiled bran shows a concamitant percentage in increase in all other nutrients. The range in composition of brans from typical raw and parboiled rice are listed in Table 1. Rice bran amount to approximately 10 to 11% of the paddy by weight depending upon the variety of paddy, the grade of the milled rice and the degree of milling. Current (1988) world production of rice is estimated to be in order of 460 million metric. tons (MMT). Only about 30% of the world rice crop is milled to separete the bran from the hulls and make the bran suitable for oil extraction and other higher valve uses. Thin gives a current potential of 10 MMT of extractable bran or roughly 2 MMT of crude rice bran oil. Oil can be extracted from bran which may have low or high aci dity depending on the condition and length of storage. The rapid growth of free fatty acid in the bran after milling has been recog nized as a serious problem for the rice bran oil industries in the rice producing areas. The principal causes of deterioration of oil in the bran during storage are due to activity of enzyme lipase in the presence of moisture. Temperature and the relative humidity of the storage atmosphere favour the hydrolysis of oil in the bran. Active lipase is located in the testa and oil is located in the aleurone and sub-aleuron layers. Milling abrades the external cell layers down to the endosperm, and the bran material is thoroughly mixed, resulting in rapid hydrolysis of the neutral fatty acids (FFA) and glycerol. FFA release of 5-7 or up to 70 % in a month has been widely recorded, the refining loss for potential edible oil production increases more rapidly serice losses during refining are two to three times, the FFA percentage. Therefore, it is important to extract the oil within a few hours after milling or to stabilize the bran by inactivating the lipases through heat-treatment. oil to free % in single day As FFAs increase. m Another major problem is the fine physical nature of the bran which causes difficulties in many phases of solvent extraction. These include:.In percolation extractors, fines tend to cause channeling and binding.In the total submergence- type extractors, fines inherent in the bran severely limit capacity..The rich miscella produced from all types of extractors is difficult to clarify..The large quantity of fines in the vapors from the desolventizers presents both operationel and capacity problems. Edible rice bran oil at present is obtained in Japan and India only in those cases where the bran can be extracted within a short time after milling prior to FFA build up» In those cases the bran is steam- agglomerated (Japan) or pelleted (India) prior to hexane extrac tion. Rice bran oil is seen as a superior oil, rich in vitamins and low in ingredients responsible for cholesterol. In Japon, rice bran oil is called the "heart oil" because the food cooked in it is found to be very delicious and the oil has a longer shelf life. Rice bran oil is more stable under frying conditions than any of the other com mon vegetable oils due to a more even balance between linoleic and oleic acid, a very low level of linolenic acid, and a high level of powerful antioxidants# (both tocopherols and ferulic acid esters). Several other commerci a 1 products also can be isolated from the crude rice bran oil (Table 2). Table 2 Products from "Edible Rice bran In ı Free Fatty Acids Glycerol Oryzanol Phosphalipids Wax Sterols, Triterpenes, crude rice bran oil Tocopherols, Over the past 40 years, fatty acid esters have constantly grown in commercial importance. Considering the products actually used in technology fatty acid esters can be classified into three groups: ester of polyfunctional alcohols; ethoxylated of fatty acids or esters; and esters of monoalcohols. Their most important applications are in the cosmetics industry, textile and fiber technology, the manu facture and processing of plastics, metal treatment and lubricants. Methyl esters are also intermediate raw materials for further chemi cal conversions into fatty alcohols, esters, amides and ester sulona- tes. Methyl esters, so to speak "masked fatty acids" should have a future as basic oleochemicals. From the viewpoints of physical and chemical properties, fatty acid methyl esters have the following ad vantages as compared with fatty acids: easy to handle because of lower melting points, more stable in storage, noncorrosive in the :.. equipment used, and easy to distill because of lower boiling points. Methyl and ethyl esters are also excellent substitutes for Diesel fuel. Two routes are available for manufacturing fatty acid methyl ester: esterification of fatty acid obtained by fat-splitting, and direct interesterification of fats and oils with methanol (metha- nolysis process.) Industrial methyl ester production is carried out from fats and oils by alkaline-catalyzed interesterification (methanolysis) or by the direct esterification of inexpensive fatty acids. In the latter case the esterification can be carried out continuously with excess methanol at pressures of about 10 atmospheres with sulfuric acid as catalyst; about 88-89% conversion to methyl esters is achieved with out the necessity for removal of the water. Excellent yields of methyl esters can also be obtained by conventional batch esterifica tion procedures in which many acidic catalysts are employed with molar excesses of 5-7:1 of methanol and fractional distillation to separate the water from the methanol. The products may be conveniently dis tilled if required. The predominant process for the manufacture of methyl esters is the methanolysis of fats and oils. The ester interchange, i.e., the replacement of the alcohol component glycerol by methanol takes place quite easily at low temperatures of Ca. 50-70 C and under at mospheric pressure with an excess of methanol and in the presence of alkaline catalyst, usually sodium methoxide. These mild reaction conditions, however, require a pre-neutralization of the fat by -> means of e.g., alkali refining, steam refining. The removal of free fatty acids is not required if the reaction is carried out under pressure, e.g., at 90 atmospheres and at a higher temperature, e.g., at 240°C. Under these conditions, even inferior grades of fats and oils with a high FFA content can be converted into methyl esters without preneutralization, since a simultaneous esterification takes place. The methyl esterification of acidulated vegetable soapstacks, such as that of cottonseed, soybean presents a unique problem in that these materials usually contain, in addition to fatty acids, 15-25% of neutral oils, and mono-and diglycerides. Attempts to prepare methyl esters with acidic catalysts and excess methanol af fords rapid and essentially complete conversion of the free fatty acids but, unfortunately, slow alcoholysis of the oils, and mono- and diglycerides, even at temperatures of 100-20U°C and pressures up to 20 atmospheres occurs. On the other hand, while alkali-cataly zed alcoholysis of oils is reasonably rdpid, alkali-catalyzed direct esterification of fatty acids is quite slow. For this reason, a rapid and efficient conversion of acidulated soapstocks to methyl esters by a one-step acid catalyzed process is not yet practical. For acidulated soapstocks it is usually best to split the soapstocks by the Twitchell process, remove the glycerol, and directly esterify the resulting acids with Twitchell reagent catalysts; thus about 94% conversion to methyl esters can be readily achieved. Otherwise, the acid-catalyzed methyl esterification of unsplit soapstocks atfforts only 70-76% esterification of the fatty acid radicals.:.; An alternate method consists in first esterifying the free fatty acid with the proper amount of glycerol (uncatalyzed) at temperatures in the range 210-230°C at 5-10 mm pressure, followed by interesterifi- cation the triglycerides so produces with methanol using NaOH, KOH or ZnO. The concept of methanolysis of sunflowerseed oil in situ was described by Harrington and D'arcy-Evans (1985) and it was demons trated that significant increases in ester yields could be achieved by such a method. The processes of conventional and in situ metha nolysis are summarized schematically in Figure 2. Figure2- Shematic summary of conventional and in situ methanolysis processes. The aim of this study was the investigation of in situ este- rification of rice bran oil which is not suitable for edible purposes, The rice bran sample used for this investigation was obtained from a rice factory in Istanbul. The changes of the oil and moisture contents of the bran and FFA content of the oil were determined du ring all the study. In situ esterification of rica bran oil was carried out according to the following example: 50 g of the bran was mixed with 200 ml of alcohol <%99.7 methanol or £96 ethanol) and the known amount of concentrated sulfuric acid was added to this mixture. After heating at reflux for several hours, the reaction mixture was filtered and washed with fresh alcohol. During the reaction, 1-2 ml of the alcoholic phases were taken from the mixture in each 15 minutes and were examined by thin layer chromotographic analysis. The filtrate was extracted with'hexane three times and the hexane solution thus obtained was washed with water until the washings vi were neutral. The organic layer was dried over sodium sulfate, fiV tered and evaporated to give the esteri fied product. The residue from the above filtration was re-extracted in a Soxhlet apparatus for 2 hours with hexane and the amount of oil left in the bran was determined. Based on these experiments, it was concluded that in order to achieve the esterification, it was necessary to use an acidic catalyst. Using 5 ml H2SO4, it was possible to esterify all the free fatty acids dissolved in methanol in 15 minutes. It was also possible to obtaine a very pure methyl esters under these circumtances because of methanol dissolved free fatty acids selectively, It was also noted that the amount of methyl ester fraction and the amount of the oil left in the bran after in situ esterification were proportional with the FFA content of the rice bran oil. On the other hand, it was not possible to obtaine very pure ethyl esters even using much more catalyst and longer reaction time, because rice bran oil components were more soluble in ethanol than in methanol. Another reason of this result was the water content of ethanol. As a result of this study, by in situ esterification of high acidity rice bran oil, it was possible to obtained fatty acids methyl and ethyl esters which could not be obtained by conventional one-step esterification processes.