Lipids-chemical properties

1、 Chemical properties of lipids油脂的化學(xué)性質(zhì) Rancidity of Lipids 酸敗 - 水解酸敗短鏈脂肪酸, - 氧化酸敗不飽和脂肪酸, 被氧化脂肪酸的氣味及閾值Fatty acidQuality of tasteAroma threshold (mg/kg) in奶油椰子油氣味Taste口感odortaste4:0Rancid 酸敗5060351606:0Rancid, like goat8510525508:0Musty,(霉味) rancid, soapy20012010002510:0Soapy肥皂味4009010001512:0Soapy4001

2、3010003514:0Soapy 400400100075- Food Chemistry, H-D Belitz et al.1 Hydrolytic rancidity (水解酸敗) heat222-monoacylglycerolH2O 反應(yīng)機(jī)制- 化學(xué)水解: high temperature + water- 酶水解: lipase 脂肪酶化學(xué)水解high temperature (225-280)-油炸- 發(fā)發(fā)煙煙點(diǎn)下降點(diǎn)下降- 油變暗,吸油率上升FFA (free fatty acid) (%)Smoke point (F)0.011.010.0100.0450320260200

3、棉籽油：酶水解- 大多數(shù)脂肪包含脂肪酶 e.g. milk-goaty odor and flavor- 微生物產(chǎn)生脂肪酶 e.g. cheese-desirable and characteristic odors and flavor 脂肪酶水解酸敗的抑制- 熱處理: 滅脂肪酶 e.g. lard refining, blenching (漂燙)- 冷藏: 降低反應(yīng)速率防止或降低措施減少油脂中的游離脂肪酸數(shù)量-用堿處理中和- 脫臭處理, remove volatile compounds 2 氧化酸敗氧化酸敗 是由空氣中的氧氣和油脂（磷脂、飽和脂肪酸和不飽和脂肪酸） 之間的一些化學(xué)反應(yīng) 其

4、產(chǎn)物包括酸、醛、醇類、羰基化合物以及酮類化合物； 產(chǎn)生強(qiáng)烈的氣味氧化酸敗 Autoxidation 自動氧化 Photosensitization oxidation 光敏氧化 形成脂類的氫過氧化物 脂類的氫過氧化物降解自動氧化的機(jī)理 free-radical chain reaction (自由基鏈?zhǔn)椒磻?yīng)自由基鏈?zhǔn)椒磻?yīng)) Initiation(引發(fā)引發(fā)): 產(chǎn)生自由基 Propagation(傳遞傳遞): 自由基向其它化合物傳遞未成對電子 反應(yīng)終止: 兩個自由基結(jié)合 自由基: R 分子中包含一個或多個未成對電子 傳遞 R + O2 ROO (peroxyl radical 過氧化自由基過氧化

5、自由基)ROO + R1H ROOH (hydroperoxides 氫過氧化物氫過氧化物) + R1 終止 R + R RRROO + ROO ROOR + O2 RO + R RORROO + R ROOR2RO + 2ROO ROOR + O2 R-H R + H (alkyl radical 烷基自由基）O21引發(fā)引發(fā): R-H R + H CH3(CH2)4 CH = CH CH CH = CH(CH2)7COOHCH3(CH2)4 CH = CH CH CH = CH(CH2)7COOHH -H: (CH2-CH = CH-CH2) O21 單線態(tài)氧, electrons in 2

6、p orbitalO21O23 Triplet oxygen (三重態(tài)三重態(tài)), electrons in 2p orbital O2: 更強(qiáng)的電斥力, 更高的能量狀態(tài), 親電性強(qiáng), 極易和具有雙鍵等高密度電子云的分子反應(yīng)1or 單線態(tài)氧的產(chǎn)生O23 + sensitizer O21光敏劑: 天然色素如核黃素、葉綠素sensitizer(光敏劑光敏劑) sensitizerhv單線態(tài)氧的猝滅劑: carotenoids (類胡蘿卜素) O21 + quencherO23 傳遞R + O2 ROO (peroxyl radical 過氧化自由基過氧化自由基)ROO + R1H ROOH (hyd

7、roperoxides 氫過氧化物氫過氧化物) + R1 單單烯烯酸的氧化酸的氧化R1R2+R1R2R1R2R1R2R1R2光、熱或金屬O2（增殖期）H+R1R2R1R2R1R2R1R2OOHOOHOOHOOH89108101191110-ROOH8-ROOH11-ROOH9-ROOHlight, heat, metalpropagation, free radicalRH多多烯烯酸的氧化酸的氧化共軛雙鍵共軛雙鍵: have absorbance at 234 nmR1R2R1R2R1R2R1R2O2HR1R2R1R2OOHOOH11狻 蚪 鶚?鮒 913119139-ROOH13-ROOHl

8、ight, heat, metalpropagation., free radicalRH DR-H (kJ/mole)HCH2-422 HCH3 CH-410 H-CH-CH=CH-322 H-CH=CH-CH-CH=CH-272Energy requirement for a H-atom abstraction 反應(yīng)終止R + R RRROO + ROO ROOR + O2 RO + R RORROO + R ROOR2RO + 2ROO ROOR + O2 脂肪的氫過氧化物的降解 脂肪的氫過氧化物不穩(wěn)定，會快速降解成 - 短鏈脂肪酸 - 短碳鏈的醛 - 酮 - 羰基化合物 Small

9、amounts (0.1%)of these compounds are flavored and have strong odors, and can render a fat or oil inedible氫過氧化合物的降解過程是- 形成烷氧基自由基 - 烷氧基自由基的進(jìn)一步降解CH3-(CH2)4-CH=CH-CH=CH-CH-(CH2)7COOHOOHO2e.g. 亞油酸CH3-(CH2)4-CH=CH-CH2-CH=CH-(CH2)7COOHCH3-(CH2)4-CH=CH-CH=CH-CH-(CH2)7COOHO.烷氧基自由基, RO 2,4-decadienal (癸二烯醛癸二烯

10、醛)CH3-(CH2)4-CH=CH-CH=CH-CHOCH3(CH2)6COOHOctanoic acid (辛酸辛酸)RH + O2ROOH1單線態(tài)氧直接加到脂肪酸的雙鍵上 OORHCCHCHHR1OCHRCHCHHOR1 光光敏氧化敏氧化e.g. 亞油酸13-ROOH12-ROOH10-ROOH9-ROOH影響脂肪氧化的因素 不飽和度25時一些脂肪酸的誘導(dǎo)期及相對氧化速率Fatty acid烯丙基數(shù)目誘導(dǎo)期(h)氧化速率(relative)18:00118:1(9)18210018:2(9,12)219120018:3(9,12,15)31.342500 氧氣過氧化自由基的生成依賴于氧的

11、參與 氧氣的去除: 真空包裝和充氮?dú)?溫度As temperature increases, the reaction rate increases食品的低溫保存 Metal catalystsFe、Zn、Cu、 Sn、Ni M + ROOH RO + OH + M+2+-M + ROOH ROO + H + M2+heme compounds (血血紅紅素素化合物化合物) - 食品原料: muscle - 加工: machine - 包裝: materials Aw (水分活度)0.20.40.60.81.0 Aw (water activity)- Aw = 0.1-0.3 With th

12、e increase of Aw, rate decreases initially- Aw = 0.3-0.8With the increase of Aw, rate increase- Aw 0.8With the more increase of Aw, rate slows downSoluble oxygen in water increases, mobility of catalysts increases, more reactive site exposed in lipids molecule Dilution of react substances and cataly

13、sts此時食品中的水分沒有溶解作用，能阻止氧氣同食品接觸Catalytic activity of metal decrease, free radicals are quenched 光和射線- 引發(fā)自由基和促進(jìn)氫過氧化物的降解 食物保存時避光處理抗氧化劑- 延緩自動氧化 天然的抗氧化劑: tocopherols(生育酚), ascorbic acid 合成的抗氧化劑: propyl gallate (沒食子酸丙酯), BHA, BHT自由基清除劑BHA, BHT能提供氫質(zhì)子ROO + AH ROOH + A Radical scavengers VE 抗氧化劑增效劑金屬絡(luò)合劑citrate

14、檸檬酸, EDTA Radical Scavenging -Synergismp1133 脂類在高溫下的反應(yīng)-熱降解與熱聚合 反應(yīng): 氧化、降解、聚合、縮合 產(chǎn)物: 短鏈脂肪酸, 羥基酸, 酯, 醛, 酮、二聚體、三聚體 影響: 品質(zhì)下降, 顏色加深, 碘值和煙點(diǎn)下降、粘度和酸值增加 有強(qiáng)烈氣味Characteristics Fresh oilHeated oil碘價108.9101.3Free fatty acids 0.030.59二?；视?(DG)1.182.73Composition of fatty acids (weight %) 14:00.060.0616:09.909.82

15、18:04.534.4518:1(9)45.342.918:2(9,12)37.029.618:3(9,12,15)2.391.6720:00.350.3522:00.380.38部分氫化的植物油在油炸前后的性質(zhì)變化(195, 80h)Fatty acids, ester, and triacylglycerolsaturatedunsaturatedthermolyticreactionsO2(-attack)O2thermolyticreactionsacidshydrocarbonspropenediol(丙烯二醇)acrolein (丙烯醛)ketoneslong-chainalkan

16、e(烷烴)aldehydesketoneslactones(內(nèi)酯)acylic(非環(huán)) andcyclic dimersvolatile anddimeric productsof autoxidationGeneralized scheme for thermal decomposition of lipids熱降解e.g. tributyrin (三丁酸甘油脂) (180 , 1h, nonoxidative reaction)DGpropane- and propenediol diester丙烷和丙烯二醇二酯oxypropyl ester氧代丙基丁酸酯symmetrical keton

17、e free butyric acidpropane丙烷ethane乙烷methane甲烷 飽和脂肪 無氧條件： Free butyric acid and DG丙烯二醇二酯OH2CC-RRCOOCOHH2COOCRH2CRCOOCH2COOCR+C-ROHO 氧代丙基一酯CH2OOCRCHOOCRCH2OOCRCHOCH2CH2OOCR+OC-ROC-ROacid anhydride酸酐 丙烯醛CHOCH2CH2OOCR+CHOCHCH2RCOOHOC-ROC-ROR-C-ROdecarboxylation脫羧 Thermal decomposition 飽和脂肪 有氧的條件下在碳上形成氫過

18、氧化物R2O-C-C-C-C-R1OR2O-C-C-C-C-R1OR2O-C-C-C-C-R1OR2O-C-C-C-C-R1OOOHOOHOOH 氫過氧化物降解產(chǎn)物 熱解反應(yīng) 不飽和脂肪 無氧氣條件二聚物形成 不飽和 有氧條件 如同常溫下的自動氧化 反應(yīng)速度要快得多 熱聚合反應(yīng) Diels-Alder 反應(yīng)- 共軛二烯烴與雙鍵之間 生成一個四取代的環(huán)己烯 R2R1+R4R3R2R1R3R4CH3OOC(CH2)7-CH2CH3(CH2)3-CH2CHCH=CH-(CH2)4CH3(CH2)7-COOCH3共軛亞油酸熱聚合反應(yīng) 自由基結(jié)合成非環(huán)狀化合物R1-CH2-CH = CH-CH2-R2891011R1-CH2-CH = CH-CH-R2.R1-CH2 = CH2-CH-CH2-R2.R1-CH-CH = CH-CH2-R2.R1-CH2-CH-CH = CH-R2. 烷基自由基產(chǎn)生R1-CH2-CH = CH-CH-R2.R1-CH2 = CH2-CH-CH2-R2.8-R 9-R 自由基與雙鍵結(jié)合R1-CH-CH = CH-CH2-R2891011.+R1-CH2-CH = CH-CH2-R2R1-CH2-CH-CH-CH2-R2.R1-CH-CH = CH-CH2-R28910118910118