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2.6 t o 23.2 on the unneutralized, and from 11.7 t o 33.4 on the neutralized cream. Results on one sample of butter from Table I1 and twenty-two samples of salted butter.from Table VI1 agreed very well with those by the original procedure. Another sample of salted butter showed practically no change in t h e ratio by t h e modified procedure after being kept in cold storage for one year. On the other hand, three samples of unsalted butter were examined a t different intervals, and the inorganic PZO5 by the modified procedure was found t o increase very rapidly. I n one sample of unsalted butter from ripened cream t h e inorganic PzOj increased from 0.0261 to 0.0435 per cent on being kept in t h e ice box for I mo. and 1 3 days, TABLE VI1 Per cent Acid Reduced in Cream b y No. Neutralizer1 1 0.0
Times Acidity Alkalinity Ratio: C?0* Butter of Cream Cc. 0.1 h' Inorganic Cc. N Sol. In Was when per P201 per G. Salt-free Washed Churned 100 G. Per cent PzOs Ash 48.2 9.24 20 0.48 1 46.4 9.04 47.7 5.99 Sweet 0.0 5 47.2 5.87 44.7 19.43 23 0.44 0 0.0 46.2 19.87 59.2 18 9.32 1 0.41 0.17 58.4 9.26 43.3 24 12.60 1 0.22 0.18 45.0 12.82 58.0 28 16.06 0.31 1 0.20 58.6 16.22 52.0 34 15.63 0.26 1 0.34 52.3 15.56 72.5 30 13.39 0.21 0.30 I 65.6 12.55 58.8 31 16.79 1 0.29 0.31 16.15 56.3 12.73 72.6 22 1 0.32 0.28 12.73 72.1 16.27 59.6 33 1 0.36 0.24 61.6 16.59 74.2 32 18.07 1 0.38 0.26 68.6 17.39 49.9 40 26.07 1 0.20 0.42 26.13 49.9 39 46.0 23.39 1 0.16 0.47 47.1 2.3. s 9 45.8 46 37.90 1 0.15 0.57 45.6 37.90 54.2 43 23.07 6 0.18 0.52 51.2 22.07 38 22.09 55.1 0.18 6 0.52 53.2 21.76 65.2 43 46.71 1 0.57 0.08 65.0 46.47 66.5 45 46.15 2 0.08 0.57 64.4 45.09 45 0.08 63.3 42.83 3 0.57 63.5 42.95 39 73.8 22.59 0.57 0.35 1 72.7 22.26 41 74.4 22.67 0.57 0.35 2 74.2 22.60 37 22.15 0.57 0.35 3 80.9 21.46 78.1 1 Lime was used as a neutralizer on Samples 5 t o 23, and KOH o n Sam-
..
2 3 4 5 6
8
9 10 11 12 13 14 15 16 17
18 19 20 21 22 23 ple 4
2
T h e determinations of calcium oxide were made b y 3Ir. 0. L. Evenson,
of this laboratory.
causing the alkalinity ratio by the modified procedure t o decrease by 1 7 . However, the P 2 0 ~as determined by t h e original procedure without ignition was found t o remain the same. It is evident, then, t h a t as the unsalted butter decomposed some soluble phosphorus compounds were formed, which were converted into inorganic phosphates by ignition and by combining with t h e bases caused t h e alkalinity t o be lower and t h e inorganic phosphoric acid t o be increased. Therefore, the ratio as determined b y titrating t h e acid solution of t h e inorganic salts without neutralization and ignition comes nearer t o expressing t h e true alkalinity-P2Os ratio of the butter. S U 11M A R Y
A method has been devised for obtaining t h e ratio of the alkalinity of the salts t o the inorganic phosphoric
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acid of cream, butter, buttermilk, and milk. This ratio varies within definite limits with milk and cream and is increased in neutralized cream or milk, the increase being in proportion t o t h e amount of neutralizer added. The butter from neutralized cream gave a ratio similar t o t h a t of t h e cream, when i t was churned at an acidity of 0.3 per cent or over. However, when lime was used as a neutralizer and t h e cream churned witt an acidity of less than 0.3 per cent in a few cases t h e alkalinity-P205 ratio was not increased, although the CaO in the salt-free ash in these particular samples was unusually high. This is explained as being due t o the occlusion of insoluble calcium phosphate. A formula is given for calculating the per cent of acid neutralized in a sample of cream or milk which can be applied t o butter when the phosphoric acid content of t h e cream from which t h e butter was made is known. An approximate estimation of the neutralizer in t h e butter can be made without knowing the phosphoric acid content of the cream by taking an average figure. Five samples of cream and the corresponding samples of butter were calculated by the formula t o contain from 0.2 to 1.0 per cent of neutralized lactic acid. Tables are given showing results with twenty-five samples of unneutralized cream, with seven samples of neutralized cream, with nine samples of butter from unneutralized cream, and with twenty-eight samples of butter from neutralized cream. Appreciation is expressed t o Dr. I. K. Phelps, chemist in charge of this laboratory, for valuable direction. THE HEAT OF HYDRATION AND SPECIFIC HEAT OF WHEAT FLOUR By Farrington Daniels, B. H. Kepner and P. P. Murdick WORCESTER POLYTECHNIC INSTITUTE, WORFESTER, MASS, AND THEMAPLE LEAFMILLINGC O M P A N Y , LTD., P O R T C O L B O R N E , O N T A R I O Received April 8, 1920
I n general bakery practice it is conceded t h a t a dough set at a temperature of from 80" t o 82' F. produces bread of the best quality. After a standard period of j hrs. fermentation, a dough set a t this temperature will have a temperature of about 86" F. A lower temperature produces a dough which ferments slowly and a higher temperature one which ferments too rapidly. The two main ingredients in bread are flour a n d water. All t h e other ingredients are so small in comparison t h a t they may be neglected in a consideration of t h e temperature involved. I n making a dough the three temperatures which are vital are those of the flour, the room, and the water. The first two are more or less inflexible, but t h a t of the water is very easily adjusted. Accordingly, no attempt is made t o change the temperature of t h e flour or the room within certain limits, but t h a t of the water is made proportionally higher or lower in order t o make t h e dough of the required temperature. The purpose of the experimental work outlined in this paper is primarily t o establish the relationship of the temperature of t h e flour and water t o t h e re-
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sultant dough and t o compare this relationship with empirical formulas in general use in t h e baking industry. I n ordinary mixtures t h e specific heats of t h e substances and t h e law of mixtures can be applied, b u t in bread making these are not sufficient t o explain t h e empirical formulas and rule-of-thumb calculations which are in use in bakeries. In t h e present investigation i t was found t h a t t h e heat evolved when water acts upon flour is so great t h a t i t must be included in any calculations for dough temperature. EXP ER IM E N TA L
FLouR-The specific heat of flour was determined by packing 3 0 0 g. into small watertight copper cylinders of negligible heat capacity. These were kept over night in a thermostat a t a slightly elevated temperature, and immediately after removal were submerged in t h e calorimeter. Although t h e method involving t h e transfer of a warm body through t h e air. is not accurate, values of 0 . 4 2 and 0 . 4 3 were obtained. These values were checked b y experiments in which flour a t elevated temperatures was mixed directly with t h e water, allowance being made for t h e heat of hydration. By combining t h e data of two such experiments t h e specific heat was made t h e only unknown quantity. The average of all t h e determinations of t h e specific heat of flour was taken as 0 . 4 3 . A s t h e moisture content of these flours was about 13 per cent t h e specific heat of moisture-free flour would be about 0 . 3 4 . Jagol obtained values of 0 . 4 0 t o 0 . 5 3 in laboratory experiments without taking into consideration t h e heat of hydration or t h e heat absorbed by t h e containing vessels. On machine-mixed doughs he obtained values of 0 . 3 0 t o 0 . 4 5 without consideration of the heat of hydration, t h e heat absorbed by t h e mixers, or t h e mechanical heat generated in the mixture. heats of hydration were H E A T O F HYDRATION-The determined experimentally by mixing flour and water in a n adiabatic calorimeter in which cooling corrections were eliminated. The calorimeter was constructed of copper cans surrounded by a water jacket. The temperature was kept within 0.I’ t o 0 . 2 ’ of t h e temperature of t h e dough, as registered by a thermocouple. Heat was introduced into t h e jacket, as required, by t h e operation of a switch in a circuit of a I IO volt alternating current which passed through t h e water of t h e jacket by electrolytic conduction.2 After several experiments i t was found t h a t a mixture of two parts of water t o one of flour was t h e most concentrated which would give satisfactory results. With this proportion t h e mixing was rapid and complete, and t h e temperature attained a permanent maximum in a few minutes. I n more concentrated mixtures stirring was mechanically difficult and a n inconveniently long time was required t o attain t h e maximum temperature. Experiments showed t h a t t h e heat of hydration was practically independent of t h e concentration. I n t h e case of very stiff doughs, however, t h e full heat of hydration was registered only after a considerable time. SPECIFIC HEAT O F
1 2
“Technology of Bread Making,” pp. 5 , 6. J . A m . Chem. Sac., 88 (1916), 1473. Y
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One experiment on a straight grade of flour is sufficient t o illustrate t h e calculations involved. Three hundred grams of flour a t 25.26’ C. were added t o 600 g. of water a t 2 3 . 9 7 ’ C . in t h e calorimeter. T h e temperature rose t o 2 5 . 6 9 ” C. within I O min. and remained constant. The heat of hydration was 1.72’ (600 X 1 . 0 0 (sp. heat water) 3 j (water 0.43’ (300 x equivalent of calorimeter)) 0 . 4 3 (sp. heat flour)) = 1148 cal. Dividing by 3 0 0 gives 3 . 8 3 cal. per gram, t h e heat evolved when flour of this kind is mixed with water. I n a check determination flour a t 2 5 . 6 1 C. and water a t 2 3 . 8 2 O C. gave dough at 2 j . 6 0 ’ C., from which t h e value 3 . 7 7 cal. per gram is obtained. Multiplying by 9/& gives 6 . 8 9 and 6 . 7 8 B. t. u. per pound. The greatest source of error in t h e experiments on t h e heats of hydration lay i n determining the temperature of the flour. A powder-like flour is an excellent heat insulator and a thermometer plunged into it is very slow t o register, and gives t h e temperature in t h e immediate vicinity only, whereas another portion may differ considerably, depending on recent changes in t h e room temperature. Toward t h e last of t h e investigation t h e sample of flour was thoroughly mixed by shaking in a large bottle just before t h e insertion of t h e thermometer. It would have been better t o keep t h e temperature .of t h e room absolutely constant for a long time previous t o the determination or t o keep the flour in a thermostat. I n spite of the difficulty of getting t h e true average’ temperature of t h e flour duplicate experiments checked within 0 .I cal. per gram in nearly all cases. The results are summarized in Table I, where Column 5 gives t h e heat of hydration in calories per gram as determined in June 1916; Column 6 the heat of hydration as given in Column j expressed in B. t. u. per lb.; Column 7 t h e heat of hydration in calories per gram as determined in September 1916, after storag‘e i n glass bottles; and Column 8 t h e heat of hydration a s given in Column 7 expressed in B. t. u. per pound.
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TABLEI-HEAT
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HYDRATION O F WHEAT FLOUR Wet D r y -JuneSeptember GRADE SOURCE Gluten Gluten Cal. B.t u. Cal. B.t.u. Low G r a d e . . . . . . . Manitoba Spr. 3 2 . 0 3 . 3 3 4 . 2 3 7.62 4 . 1 9 7 . 5 4 . . . 4.00 7 . 2 0 3 . 8 0 6 . 8 4 Straight, , . , . . . , , , , . . Manitoba Spr. 2nd Bakers.. . , , , , . . Manitoba SDr. 33 0 12.00 3 . 8 9 7 . 0 0 3.76 6 . 7 7 1st Bakers.. , . . , , , . . . Manitoba Spr. 2 9 . 6 11.33 3 . 6 8 6 . 6 2 3.66 6 . 5 9 1st Clear Snrine.. . , , . . Minnesota Snr. 2 8 . 3 12.06 3 . 6 4 6 . 5 5 3.57 6.41 Fancy Family s p r i n g . . Minnesota Spr. 2 6 . 6 9 . 8 3 3 , 4 1 6 . i 4 3 138 6 :08 H a r d Wheat Clear. .. Kan. Hd. Wtr. 3 0 . 0 10.63 3 . 2 6 5 . 8 7 3 . 2 0 5.76 Patent Hard W h e a t . . K a n . Hd. Wtr. 2 8 . 6 10.00 3 . 3 6 6 . 0 5 3.01 5.42 Winter Wheat Straight Ontario Wtr. 20.0 7.00 3.23 5.82 3.01 5.42
.. . .
OF
. .. .
. ..
From an examination of t h e table i t is evident t h a t t h e heat of hydration of flour depends t o a certain extent on its wet gluten content. Unfortunately there was a delay of a few months between t h e time of milling and t h e experimental determinations, so t h a t the results are not as significant as could be desired. The differences between t h e June and September determinations, although small, point t o a very slight deterioration even when kept in glassstoppered bottles. During t h e same period “Low Grade” stored in an open sack fell from 7 . 6 2 t o 5 . 6 2 B. t. u. per lb.; “First Bakers” in a n open sack fell from 6 . 6 2 t o 5 . 0 8 , and “Second Bakers” when ex-
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posed in a n open sack fell only from 7 . 0 0 t o 6 . 3 0 between July 1916 and July 1917. To study the effect of age on the heat of hydration, samples, of practically t h e same gluten and moisture content, were preserved in airtight, light-tight tins a t the time of milling. The results in B. t . u., of calorimetric determinations made in July 191 7 are given in ,Table 11. TABLE11-INPLUENCE OF AGE ON HEATOF HYDRATION Heat of Heat of HydraHydraDATEOB IVfILLINQ tion DATEO F MILLING tion April 10, 1916 ............ 3 . 8 2 . Jan. 10, 1917 ........ 5 . 7 8 Mar. 10, 1917 ........ 4 . 4 7 4.88 May 10, 1916 June 10 1916 4.02 Apr. 10, 1917 3.98 Sept. 10: 1916 4.77 May 10, 1917... 3.891 Nov. 10. 1916 4.36 Tune 28. 1917.. 3.71 1 This sample was kept in a paper sack.
............ ............ ............ ............
........ ..... ......
There is no marked evidence of decreasing heat of hydration with age under these conditions, where the flour is effectively protected from external conditions. The results seem rather discordant b u t there is a pronounced difference between the winter and summer samples and there is a fair agreement between the same months of the two years. There is almost enough evidence t o lead t o the conclusion t h a t t h e heat of hydration of a flour of constant composition varies inversely as t h e humidity a t the time of milling. Further investigation i s needed t o explain fully these variations, as well as the further change which takes place when exposed t o atmospheric conditions. It appears likely t h a t the rather high heat of hydration of freshly milled flour depends largely on the gluten content, but t h a t the gluten is partially hydrgted on standing. The higher t h e humidity, the greater is this partial hydration. It is so great t h a t freshly milled flour has t o age several weeks, during which time i t is continuously warmer t h a n its surroundings. After this aging period further hydration takes place only when exposed t o external moisture, and t h e extent of hydration depends on the humidity and on the degree of exposure. After exposure of many months t o the atmosphere further exposure has very little effect, even though the heat of hydration may still be considerable. For the calculation of dough temperatures the law of mixtures may be used if a proper correction is made for the heat of hydration. A simple formula may be derived on the basis of the following relation: Heat evolved = wt. flour X ht. of hydration per unit of wt. = (wt. flour X temp. rise flour X sp. ht. flour) (wt. water X temp. rise water X sp. ht. water) Expressed in the corresponding symbols t h e relation becomes: WfHh = WfRfSf WwRwSw (1) For a typical dough of 60 lbs. of water and IOO lbs. of flour, of specific heat 0.43 and heat of hydration 6 . 5 B. t. u. per lb., Equation I becomes: (100 X 6.5) = Rf (100 X 0.43) R, (60 X I ) (2) R, -0.7Rf 11 (3) For t h e special case where the final temperature of the dough is 80' F. the rise in temperature in each
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is equal t o the difference between original temperature of the water = 80 - T, and Rf = 80 - Tf. (3) : T, = -0.7 (80- Tf) 11 (4)
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T, = 1 2 5 - 0 . 7 T j (5) The temperature of the water which will give a dough of 80' F. may thus be found by subtracting 0 . 7 of t h e temperature of t h e flour from 1 2 j. Slight changes in the numerical constants are needed in case the flour does not have a heat of hydration of 6 . j B. t. u. per pound. For a low-grade flour with a heat of hydration of 7 . 6 B. t. u. the formula becomes Tw = 123 - 0 . 7 Tf, while for a winter wheat with a heat of hydration of 5 . 4 B. t. u. i t becomes 1 2 7 0 . 7 Tf. This formula is an "ideal" one and does not hold under practical working conditions, which are complicated by the heat of stirring and the exchange of heat with the room and with t h e mixer. The last two quantities become negligible if the room temperature is close t o 80' F. The heat of stirring, which is large, depends on t h e speed of the mixer. These quantities should be determined experimentally under working conditions. They must be included in a completely general expression, which may be obtained by expanding Equation I as follows: WfHh = WfSfRf 4- WwSwRw- H, - He (6) where H, is the mechanical heat generated in mixing, and He is the heat absorbed from the environment. Combining as before for the typical dough: R~(IOO X 0.43) = WjHh H, H, ( 7 ) -Rw(60 X I > For practical use i t is best t o group the last three terms into one quantity, K, which may then be determined on a large scale under working conditions. It depends on the heat of hydration, the speed of mixing, and the temperature of t h e room, but is constant when these are constant. Substituting K for the three last terms in (7) and simplifying Rw = -0.7 Rf K. (8) T o show how Equation 8 may be used in the bakery, suppose t h a t IOO lbs. of flour a t 70' F., were mixed with 60 lbs. of water a t 76' F., but t h a t t h e final temperature was 84O instead of 80°, as expected from t h e "ideal" formula ( 5 ) because of t h e extra heat due t o mechanical stirripg. The value of K is found b y substituting t h e proper values in Equation 8, thus: (84 - 76) = -0.7 (84 - 70) (9) K = 18 (10) Having determined K, i t can now be used in all cases where the conditions are the same by substituting its numerical value in Equation 8. For t h e special case of a n 80' dough: (80-T ) = - 0 . 7 (80- Tf) 18 (11) T = 118-0.7Tf (12) Solving Equation 12 for t h e example just given i t is found t h a t if the flour is a t 70' the water should be
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CALCULATION O F DOUGH TEMPERATURES
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case (R, or Rj) 80' F. and the or t h e flour: R, Substituting in
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a t 69" t o give a dough a t 80". Equation 1 2 will give t h e proper temperature of t h e water over t h e whole range when t h e bakery conditions are t h e same as when K was determined. As a short cut i t is convenient t o note t h a t t o change the temperature of the dough by I " i t is necessary t o change the temperature of the water in the same direction by I . 7' C., for (100
x
0 . 4 3 .f 60
x
60 X I
I)
= 1.7".
Another variable lies in t h e fact t h a t in thick doughs t h e flour may not have time t o becomcfully hydrated in the mixing process. If such is the case the heat evolved up t o t h e time of setting the dough will be less t h a n t h a t given in the tables used in the formulas. However, in t h e formula for practical use (Equation 8) this source of error is absorbed in the constant K. C O M P A R I S O N W I T H E M P I R I C A L RULES
It is of interest t o examine empirical rules which are now in common use and compare them with the formulas just given. The simplest one averages the temperature of the flour and the water t o give the final dough temperature. If this is t o be 80°, 2 X 80" minus t h e temperature of t h e flour gives the required temperature of the water. This is possible as a n approximation because t h e heat capacities of the two chance t o be roughly equal, 60 X 1.0and I O O X 0 . 4 2 . An improved formula averages the temperature of t h e room with t h a t of the flour and t h e water, thereby making a n approximate but wholly empirical correction for exchange of heat with the environment. Tables' made up from this rule and copyrighted in 1 9 1 2 have been widely used. When t h e room temperature is close t o 80" F. t h e latter rule has no advantage over t h e former. Neither takes into consideration the heat of hydration, and the error is shown by t h e fact t h a t if t h e flour and water, both a t 80" F., are mixed in a room at 80' with negligible heat of stirring, t h e dough temperature for a straight grade. of flour will not be 80" as given by the rules, but nearly 84' F. A third formula corrects for the heat of hydration by arbitrarily subtracting I O " from the sum of the temperatures of the flour and t h e water. Thus if the dough is t o be So", 1 5 0 ( 2 X 80 - IO) is the quantity from which the temperature of the flour must be subtracted t o obtain the proper temperature for the water. By a fourth rule 12' is subtracted from the sum of the temperatures of the flour, the room, and the water. A comparison of the different rules is shown in Table 111, where Column I gives t h e temperature of the flour, Column z the temperature of the room (calculated only for 80" and 70°), and the succeeding columns the temperature of the water which will give an 80" dough according t o the formulas already discussed. It is evident from a comparison of the different col1 Washburn-Crosby
Dough Sheet F. 1.
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umns t h a t there is a wide discrepancy in the results obtained by t h e various rules which are used by bakers. It is common practice t o have the water too cold and bring the dough u p t o the proper temperature by longer or shorter operation of the mixing machine. I n the case of high-speed mixers, particularly, a rapid rise in the temperature results from t h e mechanical heat generated. TABLE 111-CALCULATION OF WATERTEMPERATURES FOR AN 80' F. DOUGH
FLOUR E. 60 60 62 62 64 64 66 66 68 68 70 70 72 72 74 74 76 76 78 78 80 80 82 82 84 84 86 86 88 88 90 90 92 92
ROOM (Tw=160 ( T w = 2 4 0 ( T u r ~ 1 5 0(Tw=228 ( T w = 1 2 5 "F. -Tf) -Tf-Tr) -0.7 Tf) -Tf-Tr) -1Y 80 100 100 90 88 83 110 .. 98 70 80 98 98 88 86 8i:6 ... 70 108 96 96 86 80 96 84 8012 106 70 94 80 94 94 84 7818 82 104 .. 70 ... 92 92 82 80 92 80 7;:4 102 90 70 90 80 80 90 78 76:O 88 70 .. 100 .. 80 88 76 74:6 88 78 86 98 .. 70 ... 80 86 86 76 74 7j:2 84 70 ... .. 96 80 84 84 74 72 7i:s 82 ... 94 .. 70 80 82 70 70: 4 82 72 80 .. .. 70 92 80 80 80 70 68 69:O 70 .. 90 .. 78 78 78 68 80 66 6j:6 .. 88 76 70 ... 80 76 76 66 64 66:2 74 70 .. 86 74 64 62 64:8 80 74 .~ 72 84 .. 70 80 72 72 62 60 6i:4 .. 70 82 70 80 70 70 60 58 62:O 80 70 68 80 '$8 68 5s 56 60:6 70 ... 78 66
...
.. ..
... ..
... ... ...
..
..
All the figures except those in Column 7 are obtained b y purely empirical rules. Although based on scientific experiment, the figures in Column 7 hold only for "ideal" conditions in which the room temperature is close t o 80' F. and the heat generated by mechanical mixing is negligible. For practical use in bakeries the formula for "ideal" conditions must be modified as shown in Equations 7 t o 1 2 with the help of data taken under actual working conditions. SUMMARY
I-The heat of hydration of various wheat flours has been found t o range from 7 . 6 B. t. u. per lb. in a low grade t o 5 . 4 B. t. u. per lb. in a winter wheat flour. For a straight grade, such as has been used chiefly in bakeries, 6 . 5 B. t. u. per lb. was found t o be a good average. 2-The heat of hydration decreased on exposure t o the atmosphere, but did not change appreciably with age when completely isolated from the air. 3-The true specific heat of wheat flour was found t o be 0.43. +-Both specific heat and heat of hydration must be considered in any calculation of dough temperatures. 5-Empirical rules for dough temperatures have been discussed and a scientific formula has been proposed, together with suggestions for its application t o the baking industry.
