372 FOOD SCIENCE APPLICATIONS 



it is evident that group tests involving a determination of several differ- 

 ent substances, each of which contributes to odor and flavor, may be 

 more reliable than specific tests. 



Group Tests for Freshness. These group tests include (a) total vola- 

 tile base^^ in which ammonia, trimethylamine, and other basic substances 

 are aerated or distilled from a mildly alkaline fish extract, collected in 

 standard acid and determined, (b) total volatile acid^ in which formic, 

 acetic, and other volatile acids are steam distilled into a standard base 

 and determined, (c) volatile reducing substances^ in which permanganate 

 reducing organic substances are aerated from neutral fish press juice or 

 extract and the volatiles passed through standard alkaline permanganate 

 at room temperature and determined, (d) pH, a measure of accumulated 

 acids, bases, or both. 



Of these tests, that for total volatile base is quite generally applicable ; 

 it usually gives reasonably good correlation with organoleptic freshness, 

 and it can be carried out without the use of special equipment. Of the 

 above listed tests it has the most advantages and if a single test is to be 

 applied for freshness, except when special circumstances pertain, it is 

 generally the author's choice. Following is a brief description of a simpli- 

 fied volatile base method ^^: 



Blend 40 grams fish in a liquidizer (such as a Waring Blender) with 

 100 ml ethanol for 5 minutes. Centrifuge and decant into a 250 ml volu- 

 metric flask. Wash the precipitate twice with 25 ml portions 60 per cent 

 ethanol, decanting the wash solutions into the volumetric flask after 

 centrifuging each. Make to mark and transfer to a 500 ml Kjeldahl flask. 

 Add 4 glass beads, 0.5 ml capryllic alcohol, and 5 gm powdered borax. 

 Connect immediately to Kjeldahl distillation equipment and distill 100 ml 

 distillate into 50 ml 0.05 N hydrochloric acid. Rvin a blank at the same 

 time using 60 per cent ethanol in place of fish extract. Back-titrate excess 

 of acid using methyl red as indicator. Calculate results as mg volatile 

 base nitrogen per 100 gm of fish. 



Total volatile base values for different degrees of freshness of fish ex- 

 pressed as above generally are about as follows: fresh fish, 12 or less; 

 slight decomposition but entirely edible, 12 to 20; borderline edibility, 

 20 to 25; badly decomposed and inedible, above 25. 



Total volatile acid gives as accurate correlation with freshness of fish 

 as any test (including total volatile base). It is a somewhat tedious 

 determination to carry out, requiring considerable attention to keep the 

 steam distillation rate constant. The volatile reducing substances test 

 requires special equipment and in the hands of some operators fails to 

 give reproducible results. A measurement of pH is unreliable for most 

 species of fish because end products of spoilage of both alkaline and acidic 

 nature tend to neutralize each other. With shellfish, particularly oysters 



