F r o m the extensive literature reviewed it is evident that the sensory judgment, particularly that b a s e d on the olfactory sense, has b e e n and still is the most frequently and widely used means for the evaluation of
98 L I O N E L F A R B E R
the state of freshness of fish, shellfish, and their products. I t is also the reference criterion against w h i c h t h e usefulness of other methods is judged. Nevertheless, everyone concerned with freshness evaluation realized long ago the limitations inherent in the use of the senses as measures of quality, and the n e e d for an objective and quantitative yardstick with which to compare the subjective results o f the sensory tests was recognized. Methods to accomplish this purpose fall into two possible categories: ( 1 ) freshness tests to detect the early stages of the progressive deteriorative process up to t h e formation of the first percep
tible odors not occurring in the original material, and ( 2 ) spoilage tests to detect and measure those changes in the transition stage from fresh to spoiled that are associated with the presence of undesirable and unpleasant odors, mainly resulting from bacterial actions which render the product unacceptable. At the present time not one of the reviewed methods is a c c e p t e d as a true freshness test, that is, one which would enable the determination, while the sample was still judged usable, of the degree of c h a n g e w h i c h h a d occurred in the muscle constituents since removal of the fish or shellfish from the original environment. T h e report b y F ä r b e r and L e r k e ( 1 9 6 1 ) on the use of an incubation procedure along with the determination of substances volatile in air at ambient temperatures offered a means of assessing freshness not possible other
wise. R e c e n t work on the changes in the nucleotides of fish muscle during the early stages of storage before any off-odors are detectable offers a more hopeful possibility for a true freshness test. F u t u r e studies on a large n u m b e r of species stored under diverse conditions will show whether this approach will develop into a generally applicable procedure for estimating the freshness of a sample as compared with its condition on removal from its normal habitat.
T h e chemical methods proposed to date for quality evaluation depend mostly on the presence of one or more products of degradation of the muscle constituents, e.g., volatile nitrogenous bases, volatile acids, indole and skatole, hydrogen sulfide, and carbonyl compounds. Since the forma
tion of these products, largely volatile and odoriferous, is the result of bacterial action, and since the bacterial flora of fish and shellfish m a y vary, chemical tests b a s e d on the presence of a single compound or type of compound have proven unreliable and less sensitive than the sensory judgment. T h e only chemical tests which can b e expected to have a more general applicability and to give results correlating with the sensory evaluations are those b a s e d on the determination of the aggregate of the volatile constituents in a sample. T h e measurement of steam volatile substances b y Mayrhofer ( 1 8 9 8 ) , Issoglio ( 1 9 1 6 ) , Strohecker et al.
( 1 9 3 7 ) , and Tomiyasu et al. ( 1 9 5 1 ) was an approach toward this goal.
This procedure has the d r a w b a c k that it determines not only t h e sub
stances which are volatile in air ( t h o s e mainly responsible for the odor of a s a m p l e ) but also higher boiling compounds volatilized in steam as well as volatile products arising from the breakdown of nonvolatile precursors. T h e determination of substances volatile in air at ambient temperatures (e.g., see F ä r b e r , 1 9 6 3 ) does not have this disadvantage, more closely approximates the olfactory test, and affords a measure of the odor intensity of a sample relative to that of a reference standard, such as very fresh material. This procedure has the further advantage that it can b e used for all types of marine products, both raw and proc
essed, including those preserved b y salting, pickling, smoking, and can
ning. This test, together with the more restricted determination of tri
methylamine in white-fleshed marine fish species and in shellfish, of all the available spoilage tests, gives the best correlation with sensory judg
ment and has the greatest range of application.
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