Lipid oxidation is one of the major reactions limiting the stability of freeze-dried foods. Water has an inhibitory effect on the reaction. Studies conducted in model systems containing cellulose, methyl linoleate, and cobalt resulted in confirmation of the hypothesis that the effects of water are due to: 1) hydrogen bonding of hydroperoxides with a resultant delay in the onset of the rapid bimolecular decomposition of hydroperoxides and 2) inactivation, through hydration, of metal catalysts, such as cobalt. Studies were also conducted on a more complicated model system consisting of egg albumin, linoleate, and myoglobin, and on freeze-dried salmon. The results obtained are in agreement with those observed in the simpler model systems.
Bibliographical noteFunding Information:
The potentially high organoleptie and nu-~ritioaal qu~llity of freeze-dehydrated foods is limited by several chemical and physical de-teriorative processes. These processes may occur during processing or in storag(' and are s~r(mgly influenced by vario~s fi~ctors such as ten~pcrature, moisture content, and the presence of oxygen. This paper discusses some recent work pertaining to oxidation of ]ipids, a deteriorative mechanism of primary import',nee in the storage stability of dried foods. Iff the case of foods, oxidation of lipids is usually initiated by peroxidation of unsalurated fatty L., ,II*.1 .,~ ;1 dtlts.~ " it results in a munber of undesirable changes in flavor, color, and mltritm.ml ° I J value, as indie'~ted in TaMe 1. In addition, Iipi~t oxidation products participate iv. reqe-lions with nonlipid constituents and can produce in. foods more complicated effects th:m ~hose observed in pure lipid systems. For instt~nee, hydrol)eroxides and other oxidation products may react with proteins to produce, mMesirable textural properties by causing protein aggregation t'~ or seission (A. Zirlin and M. t(nrel, unpublished results). Oxidation products of fa.tty acids nmy also parlieipate in nonenzymatic browning, as indicated in Table 2. This typ.e of nonem, ymati,: browning in dehydrated foods is I)a t'tieularl.v~ m" ~por~.a' nt because it can occur at moisture contents well below those optimal * Presented at the Symposium on Freeze-Drying, Sooiety for Cryobiology, AugusL 8, 1966, Bo~ ton, Massachusetts. t'This study has been supported in part by U. S. Public I-Iealth Service Research Grant EF 00376-04 from the Division of Environmental Engineering and Food Protection and by Pro~ect No. 41-609-66-46, Aerospace Medical Division, Air Force Systems Command, United States Air Force, for Milch the authors are grateful. This manuscript is eontribu~t.ionNo. 1031 from the Department of Nut,rition a%d ]Yood Science, Mass:tchusei.ts Institute of Te~ihnology.