Development of Rancidity in Walnuts

The rancidification of walnuts was promoted by storing shelled kernels at 38°C for 4 weeks. Of six varieties tested, only 'Howard' and 'Chandler' kernels went rancid during this period. Their deterioration was preceded by the appearance of lipase (an oil digesting enzyme) in kernels and an accumulation of free fatty acids in the oil. The implications and limitations of these observations are discussed. The fatty acid compositions of oils from six commercial and two experimental walnut varieties were determined. The implications of these studies for "genetic" reductions in rancidity potential are discussed. OBJECTIVES The objectives of this year's research efforts were as follows: 1. Elucidate the mechanism of rancidity in walnuts and define the molecular entities involved: 2. Develop diagonostic test(s) which would enable industrialists to detect incipient rancidity~ 3. Examine existing walnut varieties with the purpose of providing plant breeders with information which might lead to the production of a walnut which would be more stable with respect to rancidity. PROCEDURE AND RESULTS In order to evaluate the previously mentioned objectives it was necessary for us to generate an appropriate model of the rancidification process in walnuts. Drawing on existing literature dealing with rancidity in other oil seeds, we proposed the following: TAG-PUPA ..J,.. Lipase FFA + Glycerol 1 Lipoxygenase, Heme protein or Inorganic iron + 0 Aldehyde co~pounds, Lipid hydroperoxides Concomitant decrease in FA concentrations R~Kcid products (Volatile aldehydes, etc.) Where TAG-PUFA are triacyl glycerols (TAG) containing polyunsaturated fatty acids (PUFA), and lipase is an enzyme capable of hydrolyzing the ester link between the glycerol backbone and fatty acid thereby generating free fatty acids (FFA). Lipoxygenase, heme protein and inorganic iron represent catalysts, specific (lipoxygenase) and nonspecific (heme protein and iron) which are capable of oxidizing the PUFA to lipohydroperoxides which can then spontaneously decompose to generate rancid products.