] ] J J ] j ] ] ] ] ] 1 J -- --- -- DE.VELOPMENTOF RANCIDITY IN WArnUTS L. Carl Greve, Rebecca Damell and John M. Labavitch ABSTRACT 'Ibis year's research was divided into ~ categories; a) ExperiIrents to further understand the biochem:::ial (chemical) mechanisms of rancidity in walnuts and b) Work related to industry practices which might affect the probability of rancidification in walnuts. . ExperiIrents were done to verify the existence of a lipoxygenase enzyne in walnut kernel tissue in agreezrent with last year's proposed mJdel for rancidity developrent in walnuts. Additional experiIrents verified that certain experiIrental varieties of walnuts contain oil of different Irolecular carp:>sition than that of existing ccmrercial varieties. The possibility of antioxident potential in the walnut pellicle was examined. These experiIrents showed pellicle to have considerable antioxident activity and the possibility of varietal differences is nCM being examined. Additionally, experirrents designed to test the use fullness of low oxygen atmJsphere in walnut storage were run. The results of the experiIrents were canplicated by control materials which have failed to go rancid at the time of this writing. A collaboration has been established with Dr. Clyde T. Young (Professor, North Carolina State Uni versi ty) in order to test for walnut production of volatile materials which are indicators of rancidification by specialized gas chranatographic techniques. '!he pw:pose of these experiIrents was the early detection of rancidity in walnuts and the further elucidation of the exact mechanism of the rancidification process. The overall objectives of this project are (as stated last year) the following: 1) Elucidate the IreChanism of rancidification in walnuts and define the m::>lecular entities involved; 2) Develop diagnostic tests for the identification of incipient rancidity in walnuts; 3) Examine existing walnut varieties with the purpose of providing plant breeders the infonnation necessary to produce a walnut less susceptible to rancidity. 102 --- - - -- -- - - - - -- --
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DE.VELOPMENTOF RANCIDITY IN WArnUTS
L. Carl Greve, Rebecca Damell and John M. Labavitch
ABSTRACT
'Ibis year's research was divided into ~ categories; a) ExperiIrents tofurther understand the biochem:::ial (chemical) mechanisms of rancidity inwalnuts and b) Work related to industry practices which might affect theprobability of rancidification in walnuts. .
ExperiIrents were done to verify the existence of a lipoxygenase enzynein walnut kernel tissue in agreezrent with last year's proposed mJdel forrancidity developrent in walnuts.
Additional experiIrents verified that certain experiIrental varieties ofwalnuts contain oil of different Irolecular carp:>sition than that ofexisting ccmrercial varieties.
The possibility of antioxident potential in the walnut pellicle wasexamined. These experiIrents showed pellicle to have considerableantioxident activity and the possibility of varietal differences is nCMbeing examined.
Additionally, experirrents designed to test the use fullness of low oxygenatmJsphere in walnut storage were run. The results of the experiIrentswere canplicated by control materials which have failed to go rancid atthe time of this writing.
A collaboration has been established with Dr. Clyde T. Young (Professor,North Carolina State Uni versi ty) in order to test for walnut productionof volatile materials which are indicators of rancidification byspecialized gas chranatographic techniques. '!he pw:pose of theseexperiIrents was the early detection of rancidity in walnuts and thefurther elucidation of the exact mechanism of the rancidificationprocess.
The overall objectives of this project are (as stated last year) thefollowing:
1) Elucidate the IreChanism of rancidification in walnuts and definethe m::>lecular entities involved;
2) Develop diagnostic tests for the identification of incipientrancidity in walnuts;
3) Examine existing walnut varieties with the purpose of providingplant breeders the infonnation necessary to produce a walnut lesssusceptible to rancidity.
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The objectives of this year's work done in order to canplete or fillvoids in last year's results were the following:
l) Verify last year's results with respect to the unusual oilcanposition of sane experimental varieties;
2) Examine the possibility of the existence of a lipoxygenase enzynein walnut kernel tissue;
3) Test varietal differences with respect to rancidificationpotential; -.
4) Test low oxygen at:m::>spheres as potential handling -procedures forpreventing rancidification;
5) The developrent of special gas chranatographic procedures fortesting the onset of rancidity and to deteDnine the IreChanism ofrancidification.
PR(XE)URES AND RESULTS
Part of the process of understanding rancidity in walnuts involved thegeneration of a model of the chemistry involved in theirrancidification. Last year we proposed such a IOOdel and then proceededto show that at least sane of the canponents we proposed were present inwalnut kernel tissue. That IOOdel is shown below*.
'mG-PUFA
1FFA + Glycerol
1
Lipase
Lipoxygenase, Rene Iron or InorganicIron, and 02
Md~yoos, Upo~d~, etc.
1Rancid Products
We verified the existence of a lipase and showed a correlation between-.-its inducenent and the onset of rancidity last year. We also showed apositive requirement for 02 in the rancidification process. However, wewere unable to find ~ipoxygenase activity in walnut tissue.Upoxygenase enzynes are difficult to isolate for several technical
*TAG-PUFA (Polyunsaturated Triacylglycerols), PUFA (PolyunsaturatedFatty Acids), FFA (Free Fatty Acids).
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reasons, not the least of which is the fact that the products theygenerate tend to poison the enzyrre the m:rnent they are produced.However, we have now deroc>nstrated a lipoxygenase activity in walnuts(Serr) and have thus validated our m:Xlel. We believe, however, thatthis is probably only one mechanism through whichrancidity develops inwalnuts.
MJre important to the prevention of rancidity in walnuts was thedeteD11ination that at least two experi1rental walnut varieties in Dr.Gale McGranahan's seedling block possess nuts with unusual oilcatq?Ositions as canpared to existing carmercial varieties. Nuts frannumbered varieties 77-12 and 76-21 were again tested this year for thedegree of unsaturation of their oils (Degree of unsaturation = [C 18: 2]+ [C 18:3] /total oil = DUS). The tendency of any oil, or oil nut, to gorancid is dependent on its DUS. The nore polyunsaturated oil presentthe greater the tendency for rancidification. C 18: 2 and C 18: 3 are thepredaninant polyunsaturated fatty acids in walnut oil. The average DUSfran canrercial cultivars fran the UCDvarietal block was 86% + 5.6%*.Last year, seedlings 77-12 and 76-21 had DUS values of 49 -and 68,respectively. This year they had DUS of 61 and 72, respectively.Whether this increase in DUS represents a trend because of maturation orwas envirorurentally induced can only be guessed at present. This willbe evaluated by arulUal analyses.
It was suggested by industry representatives that a nore ccmpleteevaluation of the existing ccmrercial cultivars be carried rot this yearwith respect to oil canposition. Last year we examined 6 varieties;Hartley, Ashley, Howard, Vina, Eureka and Chandler. This year weexamined 19 varieties (see footnote) and found that all had essentiallythe same oil canposition. .
An important finding which developed out of our research regarding thelipoxygenase was that there are naturally ocurring antioxidants in bothstored and genninating walnuts. These antioxidants are located in thepellicle of stored walnuts and can be found in the fleshy tissue ofgenninating nuts. It is believed that the antioxidants are phenolic innature and efforts are underway to deteDnine whether there are varietaldifferences in concentrations or types of these canpounds.
A set of experi1rents to deteDnine the effect of low oxygen content inthe storage atIrosphere on the rancidification process in walnuts hasgiven us mixed results. Experiments where half kernels were storedeither in air or a nitrogen atIrosphere containing approximately 1.0% 0were carried out. '!he nut halves were stored in a static environment o~50°C and 100% relative humidity to (preStm1ably) accelerate the onset of
*Cultivars examined in 1986 were; Payne, Franquette, Mayette, Early,Erhardt, Eureka, Pedro, Idaho, Waterloo Vina, Ashley, Amigo, Howard,Chandler, SUnlund, Chico, Tehema, Serr, and Westside.
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rancidity. Nuts fran Chandler and Hartley varieties were examined. Nutmeats were examined organoleptically every fourth day for a four-weekperiod. No lots which were stored in the low oxygen atIrospheres wentrancid during the four week storage period. However, only half of thecontrol lots (four 50 gram lots of each variety) were found to be rancidafter four weeks of storage in air. This inability to initiaterancidity in stored walnuts under nmrerous conditions has been adifficulty with this project fran the beginning. This fact and otherdata have led us to believe that there may be handling factors which areimportant to the inducement of rancidity in walnuts. - In palm nutsmechanical injury is an important ParaIreter in the initiation ofrancidity in the whole fruits. Because of this we intend""t.o examine therole of bruising in the onset of rancidity in walnuts. Data fran theseexperinents should be available by the time of the Walnut ResearchConference.
Dr. Clyde Young (North Carolina State University) has had considerablesuccess in correlating the onset of rancidity (and other flavor defects)in peanuts with the em:mation of trace aIOOunts of volatile chemicalsfran the nuts. Our collaboration with Dr. Young should enable us toevaluate stored nut neats for incipient rancidity with sane degree ofcertainty. However, the need for special aparatus in order to perfonnthese procedures may limit the application of this analysis. Data franthese exper:iIrents will hopefully be available by early 1987.
OONCLUSIONS
After tYIO years of research it appears that there are several factorsthat contribute to rancidity in walnuts. It is probably that there area mnnber of nechanisms that lead to "Off Flavors" in walnut nut neats.In other oil nuts (peanuts) the canpounds responsible for these "OffFlavors" are different dependent on the chemical mechanism which hasbeen induced. The situation may be the same in walnuts.
Certainly a major factor in rancidification of walnuts is the degree ofunsaturation of walnut oil. The finding that certain seedlings (77-12and 76-21) produce nuts containing oil with a reduced DUS is a Il'Ostpranising observation for the long tenn solution to ~ancidity.
The finding that both lipase and lipoxygenase activities are present inwalnut kernel tissues may be important to future breeding programs.Certai.n+Y& screening programs could be established to test for reducedlevels of these activities in walnuts thus, potentially, leading to areduced probability of rancidity.
At present perhaps the Il'Ost interesting observations we have made withrespect to the short tenn solution for rancidification of walnuts instorage relates to our finding that in our hands walnuts are notparticularly susceptible to rancidity. This leads us to believe thatwhile it is certainly true that walnuts can be made rancid there must besane set of factors related to industrial practices that potentiate thetendency for walnuts to go "bad". Our observations that walnuts containsignificant aIOOunts of natural antioxidents suggest that these fruits
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should be resistent to oxidative processes in general. Because of thiswe believe that research in the area of mechanical injury may be veryrewarding. We will direct considerable research effort in thisdirection.
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