Measuring Lipid Oxidation in Foods

Transcription

1 Measuring Lipid Oxidation in Foods

2 Measuring Lipid Oxidation Most lipid oxidation reactions in food have a lag phase First radical formed Depletion of antioxidant Immediately after lag phase food is rancid since oxidation products have very low sensory Therefore, the lag phase of oxidation is the most important factor relating to food quality

3 Measuring Oxidation

4 Measuring Lipid Oxidation in Foods All measurement techniques has advantages and disadvantages Important to know the limitation of the techniques so data is properly interrupted Best to use two or more techniques to verify results

5 Primary Products of Lipid Oxidation Oxygen Consumption Lipid Hydroperoxide Conjugated Dienes Advantage Measure Early Stages of Oxidation Disadvantage Requires Lipid Extraction Poor Correlation with Sensory nonvolatile

6 Primary Products of Lipid Oxidation Oxygen Consumption Can measure oxygen consumption with specialized probes or by loss of pressure Can be done experimentally in closed vessels Difficult to do on multiple samples Very difficult to do in food packages Microbial growth or other non-oxidation oxygen consumption causes interference

7 Primary Products of Lipid Oxidation Lipid Hydroperoxides Most techniques determine the oxidation potential of extracted lipid using iron or iodine Under certain conditions (e.g. high temperatures or metal reactivity) hydroperoxides will decompose as fast as they form so values are low During later stages of oxidation, hydroperoxide formation is slower than breakdown so concentrations decrease

8 Oxidative stability of spray dried, reconstituted fish oil without antioxidant stored at 37 C breakdown > formation formation > breakdown

9 Primary Products of Lipid Oxidation Conjugated Double Bonds Measure at 234 (dienes) or 278 (trienes) Work well in edible oils Many substances in foods will interfere at these wavelength Poor sensitivity (10 4 ) Will decrease with time as fatty acids decompose

10 2.(b) Radical adducts by EPR radical spin trap* radical adduct t ½ = msec Primary Products t ½ = days Technique known as spin trapping Requires addition of spin trap prior to oxidation Secondary Products

11 2.(b) Radical adducts by EPR Paramagnetic species Zeeman effect: interaction of e - with B field Intensity function of concentration Hyperfine splitting due to interacting nuclei hν energy EPR absorption magnetic field

12 2.(b) Radical adducts by EPR quantitation h AUC

13 2.(b) Radical adducts by EPR vs.

14 2.(b) Radical adducts by EPR Advantages Early predictor Little to no sample prep Fast data acquisition Training can be easy Few moving parts Automation for fluid samples Disadvantages Cost (equipment, traps) Interfering compounds Instability of radical adducts Data interpretation Spin traps are antioxidants! Spin trap (~$160/g) Consumables Tubes

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16 Secondary Products of Lipid Advantage Oxidation Measure Products that Influence Sensory Quality Disadvantage Analysis of Specific Compounds (Volatiles) is Expensive and Time Consuming Nonspecific Assays Subject to Measuring Products not Arising from Lipid Oxidation (e.g. other carbonyls such as carbohydrates)

17 Secondary Products of Lipid Volatile Products Oxidation Headspace analysis Static, dynamic, SPME Analysis can involve sample heating Can measure specific products (e.g. hexanal) or total volatiles Oxidation product concentrations will plateau with time Oxidation products can interact with other compounds (e.g. amines and sulfhydryls)

18 Static Headspace Analysis

19 Solid Phase Microextraction Ormsby, 2005

20 Dynamic Headspace Analysis Scientific Instrument Services, 1999

21 Volatile Analysis in Meats Volatile analysis requires a heat step to drive the volatiles into the heat space Heat step can resulting in cooking of meat and thus promote oxidation Can add antioxidant mixture to control oxidation during volatile development Trolox, EDTA, propyl gallate (Pignoli et al., 2009)

22 Secondary Products of Lipid Oxidation Carbonyls DNPH (dinitrophenyl hydrazine) Forms conjugates with aldehydes that can be measured by absorbance and/or HPLC Anisidine (AV) Measures unsaturated aldehydes especially high molecular weight Can measure nonvolatile oxidation products so is effective for frying oils and distilled fish oils TOTOX = 2 x PV + AV

23 Secondary Products of Lipid Oxidation Thiobarbituric Acid Reactive Substances

24 TBARS in Food Analysis Advantages Very easy and inexpensive Problems Interfering substances

25 Influence of color development technique on the reactivity of nonlipids with TBA Abs 532 nm Unheated, 25C for 20 hr Heated, 100C for 15 min Ascorbate NFDM Sucrose

26 TBARS in Food Analysis Advantages Very easy and inexpensive Problems Reactivity with different oxidation products

27 Fatty Acid Composition Malonaldehyde is only produced from fatty acids with 3 or more double bonds Number of lipid oxidation breakdown products which react with TBA increase with increasing degree of unsaturation A small amount of oxidation in omega-3 can produce more TBARS than a large amount of oxidation in foods containing omega-6 fatty acids TBARS > 1.0 = rancidity; must validate for every food

28 General Considerations for Measuring Lipid Oxidation Avoid use any lipid oxidation markers as a single time point analysis Interfering compounds Products often decrease during the latter stages of oxidation

29 General Considerations for Measuring Lipid Oxidation If you have to use single time point analysis, have lipid oxidation measurements for fresh product Provides a baseline measurement of any interfering compounds in the formulation Differences in lipid oxidation products between unoxidized and stored or processed products more likely reflect oxidation

30 General Considerations for Measuring Lipid Oxidation Use precaution when using lipid oxidation markers to compare oxidation in samples with different fatty acid compositions Use a second measurement of oxidation to verify oxidation Primary vs secondary products Sensory analysis

31 Correlation with Sensory Oxidation markers (secondary products) can be correlated with sensory evaluation of oxidation This correlation is often only good within a single product type and a single type of oxidation product assay If product formulation or fatty acid composition changes, new correlation curves may need to be generated.