Oils & Fats - FST663 Chapter 1 PDF

OILS & FATS Topics in Oils & Fats: 1. Rancidity – Photooxidation, Flavour reversion, Enzymatic oxidation. 2. Frying – changes during frying, stability of frying oils/fats. 3. Novel Oils & Fats – including Fat Replacers. 1 Learnin...

OILS & FATS Topics in Oils & Fats: 1. Rancidity – Photooxidation, Flavour reversion, Enzymatic oxidation. 2. Frying – changes during frying, stability of frying oils/fats. 3. Novel Oils & Fats – including Fat Replacers. 1 Learning Objectives Able to explain and differentiate various types of oxidative rancidity reactions (photooxidation, flavour reversion and enzymatic oxidation) in oils and fats. Able to explain the mechanism of frying, changes in physical and chemical properties of the oil. Able to describe the accelerated tests for measurement of oil stability. Able to describe the various novel oils and fats. 2 RANCIDITY DEVELOPMENT IN OILS & FATS RANCIDITY Definition: Rancidity is the development of disagreeable flavour in oils/fats which are produced through specific chemical reactions Types of Rancidity: i. Hydrolytic Rancidity ii. Oxidative Rancidity  OXIDATIVE RANCIDITY a) Common oxidative rancidity, known as Autooxidation (FST603) b) Photooxidation - New c) Flavour reversion - New d) Enzymatic oxidation - New 5  b) Photooxidation Photooxidation = Light induced oxidation (visible/UV light) Occurs from the reactivity of an excited state of O2, known as singlet state oxygen (1O2) Normal O2 or ground state O2 = triplet state oxygen (3O2) When 3O2 is converted to 1O2, energy (E) is taken up (at 92 kJ/mol) In the excited state, oxygen is much > reactive. The reactivity of 1O2 1500 x greater than that of 3O2 6 Sensitizers Production of 1O2 requires the presence of a sensitizer Cmpds that act as sensitizers are widely occuring food components: Chlorophylls, Mb, riboflavin (vit B2) & heavy metals A Sensitizer is activated by light & can: i) react directly with substrate (called type I sensitizer) OR ii) activate O2 to the singlet state (called type II sensitizer) In both cases, unsaturated FA residues are converted to Hydroperoxides Temp has an impt effect on photoxidation rates, but freezing does not completely prevent the R 7  Most Sensitizers promote type II oxidation Rs. In type II Rs: i) the sensitizer is transformed into the activated state by light ii) the activated sensitizer then reacts with oxygen to form singlet oxygen, 1O2 hν sen sen* (activated state of sensitizer) sen* + 3O2 sen + 1O2 The 1O2 can react directly with unsaturated FAs, as follows: 1 O2 + RH ROOH (Hydroperoxide / HP) 8  Singlet O2 attack on oleate (18:1) produces 2 HPs, while linoleate (18:2) yields 4 HPs Photooxidation has no Induction period, but the R on 18:1 can be quenched by carotenoids that can effectively compete for the 1O2 & bring it back to 3O2 Thus, carotenoids are widely used as quenchers. Another quencher is ascorbyl palmitate. Phenolic antioxidants do not protect oils/fats from photooxidation (eg flavonoids, BHA, BHT etc.) Photooxidation affects our palm oil industry if oil palm fruits are not harvested at the mature stage ie. the oil palm fruits still contain chlorophylls (which act as sensitizers) Note: combination of light & sensitizers is also present in many foods displayed in transparent containers in brightly lit supermarkets. Eg: light- induced flavour deterioration of milk fat & SB oil (corn oil is least susceptible to singlet O2 attack) 9  O2 Uptake versus Time IP = Induction Period IP = Period of time before O2 uptake 18:3 becomes appreciable & 18:2 before odours become 18:1 detectable O2 uptake time 10 IP  c) Flavour Reversion Term used to describe the development of objectionable/ off flavours in oils containing 18:3 (linolenic A) SB oil & other fats/ oils containing 18:3 (eg. fish oil, rapeseed oil) show the reversion phenomenon when they are exposed to air Reverted flavour is a particular type of oxidised flavour that develops at comparatively low levels of oxidation The off-flavours may develop in oils that have a PV of 1 or 2 (other oils may not become rancid until the PV reaches 100) Flavours usu. associated: grassy, fishy, painty, raw bean/beany 11  Origin of flavours: Volatile oxidation pdts resulting from the terminal pentene radical of 18:3: CH3−CH2−CH=CH−CH2− Flavour descriptions used Crude, Processed & Reverted SB oil: State of SB oil Flavour Crude Grassy, beany Freshly Processed Sweet, pleasant, nutty Reverted Grassy, beany, buttery, melony, tallowy, painty, fishy 12 Egs of Reverted aldehyde flavour cmpds (obtained from the breakdown of HPs): 3-cis-hexenal – pronounced green bean odour. 1st perceptible reversion flavour found 2-trans-hexenal – green, grassy 2-trans-nonenal – rancid 2-trans-6-cis-nonadienal – cucumber flavour 13 d) Enzymatic oxidation (by lipoxygenase) Old name for lipoxygenase = lipoxidase Enz. catalyses oxidation of unsaturated oils/fats Enz. mainly found in legumes, soybeans,other beans & peas. Peanuts, wheat, potatoes & radishes contain a smaller amt of this enz. 2 types of enzymes: o Type 1 lipoxygenase catalyses only FFAs with a high stereo- & regioselectivity. o Only reacts with oils/fats in food only after FFAs have been formed by lipase action 14 Type 1 lipoxygenase o Enz is highly specific & attacks cis-cis-1,4-pentadiene grp, (at optimum pH ~9): 6 7 8 C1 −CH=CH−CH2−CH=CH− (methyl end) cis cis o Type II lipoxygenase is 43 ppm, discard oil *Polar Compounds = All non-TG components of oils, the majority of which are secondary oxidation pdts such as aldehydes, ketones, epoxides, dimers and hydrolysis pdts such as FFAs, MGs and DGs **ACM = Surfactants that are formed from the oxidative reactions in the oil. ACM surfactants are formed naturally during oil degradation reactions through the combination of FAs with metal ions 38 4. NOVEL OILS & FATS a) Modified Oils New types of oils modified through plant breeding & genetic engineering Oils with different FA composition are developed to produce oils with: i) improved nutritional props ii) improved oxidative stability iii) improved physical & technological props 1st oil modified was Canola Oil with improved oxidative stability ie low-linolenic canola oil (previously low-euricic acid) Also: low-linolenic SB oil, high oleic sunflower & safflower oils 39 b) Fat Replacers Substances that are meant to replace fat (9 kcal/g), either completely/partly The ideal fat replacer will create the attributes of fat & significantly reduce the fat & calorie content of food They may consist of: 2. Protein/CHO 1. Fat-like substances that cmpds that mimic are not absorbed/partly OR absorbed by humans the gustatory qualities of fats Olestra (type 1 above) Olestra is a sucrose polyester with 6-8 acyl (FA) grps derived from SB, corn, cottonseed or sunflower FAs It is not absorbed by the human digestive system & thus yield NO calories 40 Olestra (a mixture of hexa-, hepta-, and octaesters. R = FA, C12-C20) Stachyose fatty acid polyester 41  Olestra is a type of Sugar Esters (SE)/Sugar fatty acid esters (SFAE) SFAE = Sugars or related cmpds esterified with fatty acids by means of chemical/enzymatic methods i) Low *DS (1-3): hydrophilic, digestable, absorbable. ∴ used as surfactants/emulsifiers ii) High DS (4-14): lipophilic, non- digestable, non- absorbable. ∴used as non-calorie fat replacers *DS = degree of substitution Sucrose monostearate (emulsifier) 42  Application of Sucrose fatty acid ester ® (Olestra ) As a frying medium in savoury snack foods Can also be used in baked pdts, as dough conditioner or in food flavours Approved by US FDA in Jan 1996 Packets of food containing Olestra® must be labelled 43  CARBOHYDRATE-BASED FAT REPLACERS Carbohydrate Polymers: consists of modified food starches (also acts as anti-staling for baked goods), dextrin & maltodextrins. created from cereal, grain and/or starches such as corn, potato & tapioca. also include polydextrose such as Litesse. needs more liquid to be added. a gelling reaction occurs that provides a thick, creamy mouthfeel.  CARBOHYDRATE-BASED FAT REPLACERS Hydrocolloids: Gums, gel and fibres Provides thickness Stabilising, emulsifying and/or providing structure to foods 2 common foods – pureed prunes and apple sauce Add bulk and flavor in baking  CARBOHYDRATE-BASED FAT REPLACERS Polyols (sugar alcohols or bulking agents) Provide volume or bulk Eg.Sorbitol, isomalt, lactitol, xylitol, mannitol, maltitol and hydrogenated starch hydrolysates Characteristics – sweeteners, viscosity, crystallization, absorb moisture, laxative In low-calorie foods (