April, 1933
INDUSTRIAL AND E N G I N E E R I N G CHERIISTHY
(2) The convection present in a liquid between horizontal plates at different temperatures, suitably heated and properly guarded, is negligible in so far as the transmission of heat is concerned through the liquid. (3) A surface effect is present at the interface of a stationary liquid and a solid during the transmission of heat normal t o the face, and should properly be considered. This surface effect is not t o be confused with the usual film effect encountered with moving fluids.
Therefore, the values of the coefficient of thermal conductivity determined by investigators using thin films tend to be somewhat lower than the true value. A new apparatus is being constructed, incorporating several refinements with which further work will be done with various liquids and solutions.
ACKNOWLEDGMENT The author is indebted to Gordon B. Wilkes, of the Department of Physics, Massachusetts Institute of Technology, for valuable suggestions and constructive criticisms given during the progress of the research. Acknowledgment is also made to Carl G. Selig, of the Department of Physics, for his cooperation in constructing the apparatus. Data on the properties of red oil were kindly supplied by W. H. McAdams of the Department of Chemical Engineering. LITERaTlJRE CITED
AngstrBm, Pogg. Ann., 123, 638 (1864). Barus, Sill. J. (3) 44,1 (1892); Phys.Rev., 2,326 (1892). Bates, 0. K., Doctors Thesis, Mass. Inst. Tech., 1!332. Beetz, Wied Ann., 7,435 (1879). Bottomley, Proc. Roy. Soc. (London), 28, 462 (1879), 31, 300 (1881); Phil. Trans., 172, 537 (1881).
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Bridgman, Proc. Am. Acad. Arts Sci., 59,141 (1923). Callendar, Phil. Trans., 199, 110 (1902). Chree, Proc. Roy. SOC.(London), 42, 300, 43, 30 (1887); Phil. Mag., 24, 1 (July, 1887). Christiansen, -4nn. Physik, 14,23 (1881). Davis, Phil. Mag., 47,972 (1924). DeHeen, Bull. Acad. Belg., (3) 18,192 (1889). Despretz, Pogg. Ann., 46,340 (1839). Goldschmidt, Physik. Z.,12,417 (1911). Graeta, Wied Ann., 18,79 (1883). Guthrie, Phil. Trans., 159, 637 (1869); Phil. Mag., (4) 35, 283 (1868); 37, 468 (1869). Henneberg, Ann. Physik, 36, 146 (1889). Jager, mien. Ber., 29, 246 (1890). Jakob, Ihid., 63, 537 (1920). Kaye and Higgins, Proc. Roy. SOC.(London), A117, 459 (1928). Lees, Phil. Trans., 191;399 (1898). Lorberg, Ann. Physik, 14, 291 (1881). Lundquist, Upsala Universitests Arsskrift, 1869. Milner and Chattock, Phil. Mag., 48,45 (1899). Murray, J., Nicholson J., 1, 165-73, 241 (1802); Gilbert Ann., XIV, 158-83 (1803). Nettleton, Proc. Phys. SOC.(London), 22, 278 (1910); 26, 28 (1914). Kicholson, W., Il’icholson J., 5, 197-200 (1802). Paalzow, P o g g . Ann., 134,618 (1868). Rumford, B., Kicholscn J., 14, 353 (1806). Schleiermacher, W i e d Ann., 34,623 (1888). Smith, IND.ESQ. CHEW,22, 1246 (1930). Thomson, T., IVichoZson J . , 1, 81, 4,529 (1801). Wachsmith, Wied Ann., 48,158 (1893). Weber, H . F., Ihid., 10,103 (1880). Weber, R., Ann. Physik, 115,1047-70 (1903). Winkelmann, Pogg. Ann., 158,481 (1874). RECEIVED August 29, 1932. Taken in part from a thesis submitted in partial fulfilment of t h e requirements for t h e degree of doctor of science, Mechanical Engineering Department, ?rfassachusetts Institute of Technology.
The March of Acidity in Wheat Germ during Storage R. C. SHERWOOD, JOHN S.ANDREWS, W. B. WADE,AND C. H. BAILEY,General Mills, Inc., Minneapolis, Minn.
T
HE germ or embryo of the wheat kernel has attracted chemical changes may occur on storing which result in inmuch attention as a food product since the discovery of crease of titratable acidity, oxidative changes usually classed vitamins. Wheat germ is one of nature’s richest as rancidity, and, coincidentally, the development of objecsources of vitamins B and E, and a good source of vitamins tionable odor and taste. A and G, in addition to its high content of both fat and Experiments have been conducted t o determine conditions protein. Clinical findings and practical feeding experiments, which would enhance the keeping quality and retard the deas well as animal feeding trials, have shown the value of wheat velopment of objectionable flavor. Increase in acidity has aerms as a sumlement to diets which are unbalanced or de- been followed during storage, in addition to organoleptic ficient in vitamins B, E, and G. In fact the usefulness of tests. The term “acidity” as used in this paper refers to wheat germ as a source of these titratable acidity and s h o u l d v i t a m i n s is so well recognized not be confused with the term Acidity hy the Greek method, which involaes that it is used in its n a t u r a l “rancidity” which is sometimes titration of a n 85 per cent alcoholic extract of form as a component of several loosely used to d e s c r i b e an special foods, and e x t r a c t s of unpalatable c o n d i t i o n due to the material, has been used as a measure of the the germ a r e used i n c e r t a i n various causes. condition qf rejned wheat germ during storage vitamin concentrates. Rancidity is correctly defined at different temperatures. Increase in acidity The demand for wheat germ by Jamieson (8) as the r e s u l t was found to be coincident with the development as a special food has necessiof chemical changes b r o u g h t of objectionable taste and odor. T h e rate of tated a study of t h e k e e p i n g about by the action of oxygen on quality of t h e p r o d u c t under fat, or by e n z y m e s which are increase wus found to be dependent largely upon various conditions of packaging associated with the fat. Jamietemperature. At temperatures qf -lo”, - I ” , and s t o r a g e . As wheat germ son f u r t h e r s t a t e s that “the and 6’ C., the rate of increase was slow, and c o n t a i n s about 10 per cent of presence of free fatty acids either the product retained its original taste and odor fat, a n d 25 t o 30 p e r c e n t of in small or large q u a n t i t i e s protein, and is rich in enzymes, .for seceral months. is no indication w h a t e v e r of
438
INDUSTRIAL AND ENGINEERING CHEMISTRY
rancidity, or that such a product may necessarily become rancid. . . . . . The odoriferous constituents formed in rancid fats are apparently derived chiefly from oleic acid in cases where rancidity is caused by the spontaneous oxidation of fats.”
INCREASE IN ACIDITY OF WHEAT GERMIN SERIESI
Wheat germ is subject to the development of rancidity as well as an increase in titratable acidity, but rancidity is usually delayed for some time, even after the increase in acidity is appreciable. Furthermore, experience here has shown the difficulty of detecting and measuring rancidity by means of chemical tests with greater assurance and facility than by organoleptic tests. Titratable acidity has been followed during storage, since progressive changes in acidity can be determined in advance of the development of flavors that are detected by odor and taste. After certain levels of acidity are reached, further increase is coincident with the development of flavors, which, when pronounced, render the product unpalatable.
Vol. 23, No. 4
used 80 per cent alcohol in determining the acidity of both whole corn and corn germ. They reported that the source of acidity is mainly in the germ, and that the increase during storage is almost entirely in the germ. High temperature was found to cause an increase in degree of acidity in storage. Bailey and Thom (1) studied the acidity of corn meal, finding that 13 per cent moisture was apparently a critical moisture level above which increase in acidity during storage was more rapid and accompanied by greater deterioration of the product. Many studies have been made to determine titratable acidity of wheat flours. The change in acidity during storage was determined by Fifield and Bailey (7) in a series of flours kept a t various temperatures using both the Greek and A. 0. A. C. methods. They also found that the ratio of percentage of acidity by the Greek method to that by the A. 0. A. C. was not constant, but tended to become narrower as the percentage of acidity increased. The rate of increase in acidity rose with the temperature. The character of the acids found in stored cereals has aroused considerable interest. Johnson and Green (9) concluded that the increase in acidity of stored flour can be attributed primarily if not solely to the ether-soluble acids. Determination of the distribution of the acids between ether and water showed that most of the acids were in the ether layer, which indicated to them that, on the average, the fatty acids extracted from wheat flour with ether must have rather long carbon chains. They stated that this increase in acidity can be measured by titrating or determining the H-ion concentration of a water extract, as these acids are slightly soluble in water. A better measure, however, is the determination of the titratable acidity of the alcoholic or the ether extract. Coleman (4) reported experiments with a method for determining the acidity of the fat extracted from wheat and from barley, He used his acidity values as an index to the “soundness” of the grain.
PREVIOUS STUDIESOF ACIDITY The method of determining acidity used in this study is known as the Greek method, as it has been used by the Greek Government for testing flours that the Greeks import. This method, in which 85 per cent alcohol is used as the solvent, has been discussed and compared with other methods by Brooke (3) and by Collatz (6) and others who reported collaborative comparative tests showing that the ratio of the Greek to the A. 0. A. C. method varied from 1.22 to 1.78, depending upon the magnitude of the acidity value. Available data indicate somewhat better agreement between collaborators in the case of the Greek method. Collatz (6) found the average coefficient of variation 14.51 and 12.84 per cent for the A. 0. A. C. and Greek methods, respectively. There are two distinct advantages of the Greek over the A. 0. A. C. method. Time of extraction is not an important factor in the Greek method, but it is a critical factor in the A. 0. A. C. method. In the A. 0. A. C. method the acidity as determined represents the sum of the native mater-soluble acid-reacting substances plus the acids produced during extraction; in the Greek method only the native alcoholsoluble acid-reacting substances are measured. Schulerud ( l a ) has recently recommended the use of 67 per cent alcohol instead of 85 per cent for flour since the former is more likely t o dissolve all the acid-reacting substances, fatty acids, phosphates, and possibly some amino acids and protein matter. Alcohol as the extracting medium has been used frequently for determining the acidity in cereals. Besley and Baston ( 2 )
INCREASE IN ACIDITYOF WHEAT GERMIN SERIESI1
Martin and Whitcomb (11) made an extensive study of the ether-soluble constituents of flours from three types of wheat. About 90 per cent of the total acids determined by them were water-insoluble, indicating that solvents such as ether and alcohol are preferable to water for extraction in determining acidity. While most of the papers cited here have dealt with acidity in flour, the Greek method of determining acidity is also applicable to wheat germ. EXPERIMENTS TO DETERMIKE ACIDITY This report will cover four series of storage experiments. The commercial wheat germ used in the investigation was produced in a large flour mill, and may be described as granular in form. In the process of manufacture the germs or embryos are not crushed or flaked, but are separated in the form in which they occur in the wheat kernel. The commercial product contains a small amount of bran and granular endosperm, but no fine flour. STOR.4GE
April, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
The product used in the four series varied in content of wheat embryo tissue. A typical proximate analysis of commercial germ containing 81 per cent pure embryo as determined by hand separation and weighing is given in Table I, in comparison with certain determinations in pure embryo. TABLEI. COMPARATIVE ANATAYSES O F COMMERCIAL GERM ASD PUREEMBRYO COVJfERCIhL GERM
% Moisture Protein ( N X 6 . 2 5 ) Ether extract Crude fiber Ash
PURE
EMBRYO
%
%
13.10 30.48 10.12 1.72 4.26
D r y basis 35.40 11.76 2.00 4.95
D r y basis 37.48 16.10 1.40
...
The method for determining acidity was as follows: Weigh 5 grams of ground wheat germ into a 50-cc:. flask, cover with 25 cc. of 85 per cent neutral ethanol, and stopper the flask. Allow to digest for 24 hours with frequent agitation. Remove 10 cc. of the supernatant liquid with a pipet and titrate with 0.1 N sodium hydroxide, using tincture of curcuma as indicator. Report results in percentage of sulfuric acid, each cubic centimeter of 0.1 iV sodium hydroxide being equivalent t o 0.0049 gram or 0.245 per cent sulfuric acid. The end point is sharp when flour is titrated, but the alcoholic extract of wheat germ is yellow, increasing the experimental error in determining the end point. For germ samples either 0.05 N or 0.1 N sodium hydroxide is preferable, whereas 0.02 N is better for flour. In this investigation a microburet graduated in 0.01 cc. was used in titrating. The wheat germ was packed in one-pound, sealed tins, either in vacuum (25 to 26 inches), carbon dioxide, or nitrogen, as indicated subsequently. At approximately 30-day intervals sealed cans were opened and sampled for acidity determinations and organoleptic tests. I n the four storage series the cans of wheat germ were placed in constant-temperature chambers a t several different temperatures. Temperatures of -lo", 0", i o ,22", 29", and 35' C. were employed. The low temperatures were used to determine how long the wheat germ could be kept in fresh condition; the elevated temperatures-namely, 29 " and 35" C.-were chosen to accelerate the changes which occur during normal storage. Storage experiments are great,ly shortened by using the two highest temperatures cited. SERIES I. Table I1 and Figure 1 show the results of series I; acidity is expressed as percentage sulfuric acid in commercial wheat germ which contained i 5 per cent of pure embryo. The cans were stored a t 29" * 1.0" C. (84" F.). Packing in vacuum was decidedly better than in carbon dioxide. TABLE11. RESULTSOF SERIESI STORAQE PERIOD Days 38 67 105 129 157 185 222 248 2x0 308 346 ~~
Av. daily increase
ACIDITYAS % HzSOh I n vacuum I n Cot 0.289 0.345 0.561 0.706 0.712 0.846 0.925
0.414 0,559 0.723 0.747 0.884 0.856 1.200 1.236
1.138 1.140
1,232 1.280
0,00286
0.00326
0.635
ncm
I. .....
1 .....
xno
SERIES 11. These experiments contained leans packed either in vacuum or nitrogen and stored a t two temperature levels. The commercial wheat germ contained 55 per cent pure embryo. The results in Table I11 and Figure 2 show a steady increase in acidity a t 29" C. (84" F.) and a very slow change a t -10" C. (14" F.). Acidity increased eight times faster a t 29" C. than a t -10" C. The nitrogen and vacuum curves have 'the same trend, but a t 29" C. the nitrogen
439
packs showed slightly higher values. Organoleptic tests were slightly but consistently in favor of germ packed in vacuum under comparable storage conditions. T.4BLE 111. RESULTSO F
SERIES
11
ACIDITYAS % HsSOa
c
STORAGE
A T -10'
C.
AT 22'
4~ 2 g 0
C.
-
c
PERIOD I n vacuum I n nitrogen I n vacuum I n vacuum I n nitrogen Days 0 30 67 94 124 159 192 217 268 304 338
0.086 0.125 0.125 0.129 0.117 0.134 0.140 0.140 0.132 0.178
0.086 0.118 0.110 0.141 0.111 0.118 0.140 0.157 0.122 0.183 0.147
0.086 0.154 0.206
daily increase
0.00030
0.00032
0.00179
...
... ... ... ... ... ... ...
...
0.086 0.179 0.240 0.337 0.341 0.421 0.432 0.528
0.179 0.245 0.326 0.363 0.446 0.493 0.541
...
...
... ...
0.086
...
...
AV,
0.00204
0,00210
,4t -10" C. the product was maintained in a fresh, palatable condition, with no evidence of undesirable flavor for 338 days; a t 29" C. the first traces of objectionable flavor were apparent a t 90 days, and a t 120 days the product had a distinct objectionable flavor. The vacuum pack was better than the nitrogen.
INCREASE IN ACIDITY OF WHEATGERMIN SERIESI11
SERIES111. Since the nitrogen used in these experiments proved to be less desirable than vacuum, it was decided to start another series of storage experiments, using a new supply of nitrogen. A new tank of water-pumped nitrogen was secured, and series I11 was started under the same conditions as series 11, except that the commercial germ in series I11 contained 75 per cent pure embryo. The results are shown in Table IV and Figure 3. TABLEIV. RESULTSOF SERIES111 ACIDITY.4s H B Q OP 7 STOR.AQE A T -10' C. .AT 22' C. AT 2 3 ' C . PERIOD I n vacuum I n nitrogen I n vacuum I n vacuum I n nitrogen Days 0 0.135 0.135 0.135 0.135 0.135 0.149 0.172 0.201 28 0.189 0.126 0.126 56 0.234 0 : 203 0,244 0.131 0.149 93 0.321 0.280 0.215 0.129 0.154 0,337 0.324 123 0.165 0.165 0.422 157 o:i69 0.386 0.142 0.178 0.411 187 0.414 0,277 0.150 0.160 0.490 218 0.510 0.142 0.160 0,611 248 0.561 0,220 0.191 0,597 281 0.640 0.183 0.627 315 0.686 0.201 c
daily increase
AV,
0.00021
0,00015
0.00076
0.00156
0,00175
The rate of increase in acidity a t 29" C. was of the same order as series 11, and again the acidity of the germ packed in nitrogen was higher than that packed in vacuum. At - 10" C. the rate of increase was very slow, and the difference between the two packs was not significant. Organoleptic tests again showed that the germ packed in vacuum was slightly superior in flavor to that packed in nitrogen both a t 29" and - 10" C. The germ was still in fresh condition after approximately 300 days a t - 10" C. SERIESIV. The first three series of storage experiments showed that increase in acidity was a function of temperature,
1hU
410
S T II 1 A 1, A \r 1) E N G 1 Y . :b 15 I
