Chapter 6: Fat composition & stability

1. Chapter 6:Fat composition & stability

2. These are mostly esters of fatty acids and glycerol. Up to 99 percent of the lipids in plant and animal material consist of such esters, known as fats and oils. Fats are solid at room temperature, and oils are liquid.

5. AUTOXIDATION • The unsaturated bonds present in all fats and oils represent active centers that, among other things, may react with oxygen. This reaction leads to the formation of primary, secondary, and tertiary oxidation products that may make the fat or fat-containing foods unsuitable for consumption • The process of autoxidation and the resulting deterioration in flavor of fats and fatty foods are often described by the term rancidity.

6. The autoxidation reaction can be divided into the following three parts: initiation, propagation, and termination. In the initiation part, hydrogen is abstracted from an olefmic compound to yield a free radical. The removal of hydrogen takes place at the carbon atom next to the double bond

7. Once a free radical has been formed, it will combine with oxygen to form a peroxy-free radical, which can in turn abstract hydrogen from another unsaturated molecule to yield a peroxide and a new free radical, thus starting the propagation reaction. This reaction may be repeated up to several thousand times and has the nature of a chain reaction

9. Nutrition labelling Definitions • Compositional requirements in Food Standard Code for polyunsaturated claims and monounsaturated claims • Lipids (fats and oils); • ‘fatty acids, mono- di- and triacylglycerides, phospholipids, sterols (including cholesterol) and lipid soluble pigments and vitamins’

10. ) • Nutritionally important to distinguish between the different forms of fat: • Total fat; total lipid fatty acid, expressed as triacylglycerides • Saturated fat • sum in grams of all fatty acids without double bonds • Monounsaturated fat • cis-fatty acids with one double bond • Polyunsaturated fat • cis, -cis-methylene interrupted fatty acids with 2 or more double bonds • Trans fatty acids • mono- or polyunsaturated fatty acids with trans rather than cis double bonds

11. Double bonds cis and trans • Cis; two hydrogen atoms above double bond • Trans; one hydrogen atoms above and below double bond Coultate, 2002 p75

12. Importance of fat analysis • Health concerns require reporting amount of; • cholesterol, saturated, unsaturated fat and individual fatty acids • Lipid stability, shelf life and safety • oxidation by products such as cholesterol oxide and malonaldehyde exhibit toxicity • Analysis of oils and fats in deep frying • Products composed of modified lipids • not bio-available (sucrose polyester ‘Olestra’) • not contribute same calories per gram • short and medium chain triacylglycerides, Salatrim & Caprenin

13. Characterisation of lipid (triacylglycerides)- general tests used on refined fats and oils • Refractive index • RI decreases as fat saturation increases during hydrogenation • 20-25C for oils and 40C for fats • Melting point • slip melting point • temperature at which a column of fat moves (‘slips’) in an open capillary tube when heated • decreases with increase unsaturation level • dropping melting point • at appropriate temperature, sample flows through a 0.11” hole in sample cup

14. Characterisation of lipid (triacylglycerides)- general tests used on refined fats and oils • Smoke point • temperature at which fat begins to smoke • frying oils should have smoke points above 200C • Flash point • temperature of flash (ignition) on surface of the sample (test flame @ 5C intervals) • Fire point • temperature at which evolution of volatiles is high enough to support continuous combustion • contamination by free fatty acids and residual solvents will lower these points

15. Characterisation of lipid (triacylglycerides)- Solid Fat Index (SFI) & Solid Fat Content (SFC) • Solid Fat Index (SFI) & Solid Fat Content (SFC) • dependent on type of fat, its history and temperature of measurement • measures by; • dilatometer increase in volume as fat melts (SFI) • instrumentally using NMR (SFC) • measures the solid content of the fat • can calculate amount of solid triacylglycerides compared to liquid at a given temperature • important for prediction organoleptic properties

16. Characterisation of lipid (triacylglycerides)-Solid Fat Index (SFI) & Solid Fat Content (SFC) (cont) • SFI% = fat solid volume volume between upper and lower line

17. Characterisation of lipid (triacylglycerides)-Consistency & Spread ability • Includes rheological terms; • plasticity, hardness, creaminess & spread ability of platicised fats • Consistency • distance that a cone-shaped (penetometer) weight will penetrate a fat in a given time and temperature • Spread-ability • force needed to compress fat sample

18. Characterisation of lipid (triacylglycerides)-general tests used on refined fats and oils • Colour • Lovibond method; colour visually compared to yellow and red colour standards • Spectophotmetric method; • sample placed in Cuvette at temperature of 25-30C • absorbance read at four wavelengths and Photometric colour index measured = 1.29 (A460) + 69.7 (A550) + 41.2 (A620) – 56.4 (A670)

19. Oil colour Almond oil Sesame oil Palm oil Sunflower oil Olive oil

20. Lipid characterisation (triacylglycerides)- Iodine value • Measure of degree of unsaturation • iodine value: g’s of iodine absorbed per 100g sample • lipid (and blank) in solvent reacted with an excess of iodine monochloride (Wij’s solution): -CH=CH- + ICl  -CHI-CHCl- + ICl (excess Wij’s solution) (unreacted Wij’s solution) • iodine is then liberated from the unreacted iodine monochloride by adding potassium iodide: ICl + KI  KCl + I2 (unreacted Wij’s solution)

21. Released iodine is determined by titration with standard sodium thiosulphate until no colour: 2Na2S2O3+ starch + I2 Na2S4O6+ starch + 2NaI (dark blue) (colourless) • Iodine value is calculated as follows: Iodine value (g/100g sample) = (B - T) x N x 12.69 sample wt (g) where: B= mean blank titre of sodium thiosulphate, T= sample titre of sodium thiosulphate, N= normality of the thiosulphate ion 12.69 is used to convert from mEq thiosulfate to g iodine

22. Iodine value is used to; • characterise oils • follow hydrogenation process during refining • indicate unsaturation during lipid oxidation • Iodine values for: • primarily saturated fatty acids have low value • butter fat, 26 to 38 • highly unsaturated vegetable and fat oils have high values • sunflower oil, 129 to136 • cod liver oil, 137 to 166

23. Lipid characterisation (triacylglycerides) -Saponification value • Saponification; degradation into glycerol and fatty acids by treatment with alkali

24. Lipid characterisation(triacylglycerides)-Saponification value • Saponification value = amount of alkali (KOH in mg) needed to saponify 1g of lipid • it is an indication of the mean molecular weight of the triacylglyceride • divided by three gives an approximate mean molecular weight of fatty acids

25. Add excess alcoholic KOH to lipid sample & heat to saponify fat • Back titrate un-reacted KOH with HCL using phenolphthalein as indicator Saponification value = (B - S) x N x 56.1 sample wt (g) where: B = blank titration, HCL S = sample titration, HCL N = normality of the HCL 56.1 = molecular weight of KOH

26. Lipid characterisation (triacylglycerides)-Free Fatty Acid • Free Fatty Acid (FFA) • % by weight of a specified fatty acid • Acid value • mg of KOH (NaOH) necessary to neutralise the free acids (acid value) in 1 g of fat or oil • used to measure free fatty acids hydrolysed from triacylglycerols • May convert FFA to Acid Value via conversion factor • % FFA (as oleic) x 1.99 = Acid Value

27. Titrate with NaOH %FFA (oleic) = ml alkali x N of alkali x 282 x 100 sample wt (g)

28. Measurement of extent of lipid oxidation • Key definitions: • rancidity • lipid oxidation (autoxidative rancidity) • lipolysis (hydrolytic rancidity) • Why measure rancidity? • characterizes level of chemical deterioration of lipid in food • predicts end of shelf life when unacceptable flavours and odours make the product unacceptable to the consumer

29. Ways to measure rancidity • peroxide value • hexanal content • thiobarbituric acid (TBA) test • sensory evaluation

30. Characterisation of lipid (triacylglycerides)-Peroxide value • Defined as milliequivalents (mEq) of peroxide per kg of fat • Fat dissolved in glacial acetic acid-isooctane • Excess potassium iodide added • reacts with peroxides liberating iodine ROOH + 2KI  ROH + I2 + K2O • Solution titrated with standardised sodium thiosulphate using starch indicator 2Na2S2O3 + I2 + starch  Na2S4O6 + 2NaI + starch (blue colour) (colourless)

31. ) • Peroxide value calculated by: Peroxide value (mEq peroxide/kg fat) = (S - B) x N x 1000 sample weight (g) S = sample titration, ml B = blank titration, ml N = normality of Na2S4O6solution • Measures transient initial products of oxidation • highest values at mid stages of oxidation • disadvantage, 5g lipid sample size required • fresh oils peroxide values well below 10 mEq/kg • rancid taste noticeable when peroxide value is 20 to 40 mEq/kg

32. Characterisation of lipid (triacylglycerides)-Hexanal determination • An aldyhide produced during lipid oxidation (C6H12O) • Method; • sample of vapour collected (after heating) from above food sample (headspace) in sealed container • sample analysed by GC to identify & quantify hexanal • no need to extract lipid • Hexanal levels correlate well with sensory evaluation of rancidity

33. Characterisation of lipid (triacylglycerides)- Thiobarbituric acid (TBA) test • Oxidation products of unsaturated fatty acids such as malonaldehyde give red-coloured reaction products with thiobarbituric acid (TBA) • malonaldehyde distilled off (water medium) to eliminate interfering substances • measures final stages of autoxidation • correlates more closely with sensory evaluation of rancidity than does the peroxide value test

34. Characterisation of lipid (triacylglycerides)- Thiobarbituric acid (TBA) test (cont) • Weight of the sample is combined with distilled water and mixed. • pH is adjusted to 1.2 and transferred to a distillation flask • An aliquot is combined with TBA reagent in a boiling water bath for 35 min. • Absorbance of the solution determined at 530 nm using standard curve absorbance readings • Converted to milligrams malonaldehyde per kg of sample

35. Characterisation of lipid (triacylglycerides)- Measurement of oxidative stability(cont) Nielsen, 2003 p 240

36. Characterisation of lipid (triacylglycerides)- oxidative stability • Lipids vary in their susceptibility to rancidity due to; • inherent properties • level of unsaturation • presence of natural antioxidants • external factors • added antioxidants • processing and storage conditions • Accelerated test; artificially hasten lipid oxidation by exposing to; • heat, oxygen, metal catalyst, light, enzyme

37. Oil Stability Index (OSI) & Active Oxygen Method (AOM) • OSI determines rancidity induction period by; • bubbling air through lipid held at 110-130C • trapping acidic volatiles (formic acid) in deionised water • conductivity of deionised water is measured, giving an indication of rancidity • AOM is similar to OSI except induction period is measured by either; • peroxide value • sensory evaluation of rancid odour

38. Oxygen bomb • Measurement of time for onset of rapid disappearance of oxygen • sample is placed in heavy walled container • oxygen pressurises container to 100 psi • container placed in boiling water bath • Induction period determined by; • measuring time for rapid drop in O2 pressure • corresponds to rapid absorption of oxygen by the sample

39. Sensory evaluation of rancidity • Approximately 30 trained panelists assess off- flavour of oil or fat by marking on hedonic scale such as: