pom212-nuts_labavitch_2005_handout

pom212-nuts_labavitch_2005_handout - Postharvest Biology...

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1 Postharvest Biology and Technology of Tree Nuts Postharvest Biology and Technology of Tree Nuts Labavitch UC Davis Labavitch UC Davis Nuts are marketed in shell or shelled as whole kernel or prepared in a variety of value-added products in various packages that influence their postharvest-life Chemical Composition of Tree Nuts Fatty acids composition of nuts influences their storage potential Comparison among various cooking oils in their fatty acid composition Of the primary California-grown nuts (almonds, walnuts and pistachios), the walnut is by far the most unstable, both in storage and in food products. What is meant by unstable? Can stability issues be addressed by genetic approaches rather than through postharvest management?
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2 As indicated in previous slides, walnuts have a much higher content of polyunsaturated fatty acids in their oils than do almonds and pistachios. The big problem is the content of linolenic acid , an 18 Carbon fatty acid with 3 double bonds. Fatty acids are made in the plastids and transferred to the ER where they are processed in many ways, including being incorporated into storage fats (oils). What is the importance of these fatty acids in the kernels? While some seeds store potential metabolic energy as starch or protein, almonds, walnuts and pistachios store most of the energy that the germinating embryo will need as lipids. These lipids are stored in sub-cellular bodies called lipid bodies or liposomes and the fatty acids that have been synthesized are linked into triacylgycerols (TAGs), complexes of 3 fatty acids and a glycerol molecule. The constant component of a TAG is glycerol ( a 3 carbon sugar alcohol). The variable portion is the nature of the fatty acids that are linked as esters to the -OH groups of the glycerol. TAGs are produced and accumulate in lipid bodies that develop from the ER membranes. Seed oil bodies The oil bodies of seeds may be derived from ER membranes as the TAGs are deposited between the lipid layers of the ER membrane, thus separating the lipid bilayer. Proteins called oleosins coat the droplet, perhaps defining its ultimate size and curvature.
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3 Lipase (an ester-cleaving enzyme) generates free fatty acids (FFAs) and these are metabolized ( β -oxidation) to generate acetyl-Co-A which is utilized in energy metabolism and gluconeogenesis. However, accumulation of FFAs can cause oil stability problems. This is a particular problem when the generation of FFAs is not coupled to the subsequent metabolic use of those FFAs to support seedling energy metabolism. The general impression is that rancidity develops in walnuts pretty much like it develops in other high fat foods like peanuts, fish etc. That is, particularly labile fatty acids (= unstable in the presence of oxygen and fat oxidizing enzymes) like linolenic acid are converted to breakdown products that are chemically reactive. The resulting reaction products are what gives the rancid taste. However
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This note was uploaded on 12/30/2010 for the course POM 212 taught by Professor Kadermitcham during the Spring '05 term at UC Davis.

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pom212-nuts_labavitch_2005_handout - Postharvest Biology...

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