Food Chem lecture 5

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Food Chem lecture 5
2012-02-12 20:26:43
Food Chem Lecture

Food Chem Lecture 5
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  1. What is lypolysis?
    Hydrolysis of the ester linkage between glycerol and the fgatty acid in the triglyceride molecule
  2. What causes hydrolysis of the ester linkage?
    • 1. Thermal Stress- heating of oils (frying)
    • 2. Deliberate chemical action- eg. saponification with NaOH
    • 3. Enzymatic action
  3. How does heating speed up lypolysis?
    Heating increases the kinetic energy of the system- straining the covalent bonds, this can result in the random breaking of the ester linkage
  4. How does a release of free fatty acids affect the oil?
    Release of free fatty acids cause a reduction in the smoke point of the oil- fire hazard- air pollution
  5. How do you make soap?
    Simply reat triglycerides with strong NaOH or KOH
  6. How is saponification used in the refining of oils?
    Saponificatio is used i a controlled ashion in the refining of oils- where oils are washed with dillute base to remove FFA- tricky, because excess base produces mroe FFA
  7. Which enzyme is reponsible for lypolysis?
  8. Where is lipase found?
    Found in every living system
  9. How does one reduce lipolysis during extraction?
    Can minimize this reaction due to naturally present lipase by heating the crush to denature the enzyme and its activity- helps, but never 100% successful
  10. What is the purpose of refining an oil?
    All extracted oils will cotnain some free fty acids, which later have to be removed by refining to make the product acceptable from the standpoint of flavor and temperautre stability
  11. Why is the presence of free fatty acids in most fats and oils generally not noticeable organoleptically?
    Because fatty acids >C10 are relatively flavorless and odorless
  12. Why is lipolysis organoleptically apparent in milk fat?
    Because some of the free fatty acid released are short chain, volatile and odoriferous
  13. What is oxidative randicity?
    Rancidity resultig from autoxidative reactions
  14. Why are milk and butterfat partiularily susceptible to lipolytic rancidity?
    Because lipase works best at the water/fat globule interface
  15. Why is lipase not very active in raw milk?
    Because the membrane bound on fat globules
  16. How does homogenization affect lipase?
    Homogenization increases the surface area of the fat globules and activates the enzyme
  17. What is autoxidation?
    Autoxidation is a free radical reaction of unsaturated fatty acids with molecular oxygen
  18. What is the overall result of autoxidation?
  19. What is rancidity?
    A nebulous term, describing a wide variety of undesirable off flavors and odors associated with oxidized fats and oils
  20. What can extensive autoxidation lead to?
    Extensive autoxidation leads to losses in functionality, a decrease in nutritional value, and the oil may ultimately become toxic
  21. What are the 3 steps of any autoxidation reaction?
    • Initiation
    • Propagation
    • Termination
  22. What are the two requirements for autoxidation to begin?
    • 1. A hydrogen must be abstracted from the fatty acid chain
    • 2. Molecular oxygen must be present
  23. What are some ways in which a hydrogen can be abstracted from the fatty acid chain?
    • Light or ionizing radiation (hv)
    • Kinetic energy (heat)
    • Presence of metallic cations
    • Enzymatic Catalysis (lipoxygenase)
    • Reactive singled oxygen produced by photo sensitizers such as chlorophyll or myoglobin
    • Combinations and permutation of 1-5
  24. What happens to free radicals in the absence of oxygen?
    In the absence of oxygen, free radicals quickly recombine to neutralize themselves
  25. What is propagation?
    Propagation is the reaction of a free radical with oxygen
  26. What is the key to self propagating nature of the autoxidation reaction?
    The key to the self propagating nature of the autoxidation reaction is the fact that the peroxy radical has a preference for terminating its free radical state by abstracting a hydrogen from another fatty acid
  27. How is the free radical chain reaction terminated?
    When two free radical react, a neutral, unreactive species is formed and the free radical chain reaction terminates
  28. What is a peroxy radical capable of?
    • Reacting with another free radical (Terminating)
    • Abstracting a hydrogen from another fatty acid (propagating)
  29. Why is the net effect of autoxidation an accumulation of peroxides over time?
    There is a greater probability of another hydrogen being abstracted
  30. What are the primary oxidation end products produced?
    • Hydroperoxides (ROOH)
    • Peroxides (ROOR)
  31. Are peroxides associated with off flavors and odors?
    No, they are only a precursor
  32. What compounds are respomsible for off-flavors?
    Complex mixtures of low molecular weight aldehydes, alcohols, ketones and short chain fatty acids
  33. What are the two hydroperoxide breakdown mechanisms?
    • Monomolecular reaction
    • Bimolecular reaction
  34. When does the monomolecular mechanism occur?
    Monomolecular mechanism occurs while the hydroperoxide/peroxide concentration is relatively low
  35. When does the bimolecular reaction occur?
    The bimolecular reaction predominates as the peroxide concentration becomes relatively high
  36. What is the basic breakdown of a monomolecular reaction?
    • R-O-O-H--> R-O∙+OH∙
    • Formation of Alkoxy and hydroxy radicals
  37. How does an alkoxy radical generate an aldehyde?
    • Alkoxy radical decomposes further, undergoing a variety of reactions to produce an aldehyde
    • 1. unstable strucutre- the alkoxy radical steals an electron from the R2 chain and donates an electron to form the double bond- an aldehyde is formed plus the release of a free radical
    • 2. the R2 free radical produced is capable of perpetuating the autoxidatio reaction
    • 3. Concurrent with the formation of an aldehyde, the chain fatty acid is split, resulting in two compounds of lower molecular weight
  38. What happens when an aldehyde is produced?
    The aldehyde formed is not necessarily stable and may subsequently be oxidized or reduced, depending on the environmental conditions
  39. How is alcohol generated from a monomolecular reaction?
    Steals a hydrogen from R3H to form the alcohol and once again a free radical is formed to carry on the reaction
  40. How is a ketone formed from a monomolecular reaction?
    In this case a hydrogen is abstracted from the radical chain and the reaction is a termination step
  41. What is the general formula for a bimolecular reaction?
    2R-O-O-H --> R-O∙+R-O-O∙+H2O
  42. What happens as the concentration of hydroperoxides increases?
    As the concentration of hydroperoxides increases, they react with themselves, the net result being the formation of both alkoxy (R-O∙) and Peroxy (R-O-O∙) radicals
  43. What happens when peroxy and alkoxy radicals are formed?
    The peroxy radicals prefer to abstract a hydrogen form a fatty acid, while the unstable alkoxy radical can also decompose, leading to the formation of aldehydes, ketones, alcohols or carboxylic acids, which includes chain scission
  44. What must be present in order for hydroperoxides to accumulate?
  45. What is a common test for the determination of the oxidative status of oil?
    Peroxide value
  46. What does a peroxide value actually measure?
    Method actually measures only the hydroperoxides which are considered primary oxidation products
  47. How does is titration used to measure the oxidative status of an oil?
    The amount of iodine released from potassium iodide in an acidic environment is proportional to the ROOH levels and can be measured by titration with standardized sodium thiosulfate
  48. How is PV (peroxide value) defined?
    PV is defined as milli-equivalents (mEQ) of peroxide oxygen per 1000 grams of fat
  49. Why are PV values correlated with sensory data?
    To provide one with a calbrated objective indicator of acceptbility
  50. Why is there no off flavor associated with the induction period?
    Because thee hydroperoxides are largely being formed, not broken down
  51. What is a thiobarbituric acid test (TBA test)?
    • TBA test looks at one of the many common secondary breakdown products for example malonaldehyde
    • The assumption made is that the quantities of malonaldehyde produced ar directly propertional to the degree of oxidation and that this measure can be correlated to the flavor defects found in a fat or oil
  52. How does the TBA test work?
    Malonaldehyde reacts wth 2 molecules of thiobarbituric acid (TBA) to produce a red complex which can readily be assessed spectrophotometrically
  53. Why is acid used in the TBA test?
    In foods, due to its high reactivity, malonaldehyde is often bound to proteins and has to be released using acid to assay for it
  54. Why must malonaldehyde tests be coupled with sensory evaluation?
    Malonaldehyde formation is a function of the lipid source- thus one has to carry out sensory evalutation with a taste panel and correlate the results
  55. What is anisidine value test?
    Reaction of p-anisidine with aldehydes, their reaction adduct measured in the UV