meal is not more widely utilized industrially is the lack of sufficient specific information concerning the properties of the nitrogenous constituents of the peanut. This type of information is now being made available (5, 4, 5, 7), however, and has focused atten
tion on the need for specifio information concerning the properties of peanut meal which result from controllable processing factors, including temperature, moisture, and duration of the treatment.
The effect of these factors upon the peptizability of the nitroge the extracted protein be substantially undenaturcd. The treat
ments which peanuts receive during commercial hydraulic press
ing for the removal of oil vary in severity, but are usually suffi
ciently drastic to denature the protein considerably and reduce markedly their extractability. In view o f these facts, it will be of economic value to peanut processors to ascertain the processing conditions under which adequate oil removal can be obtained with a minimum of alteration in the physical and chemical proper
ties of the meal proteins.
This investigation is confined primarily to the determination o f the effect o f temperature, humidity, and length of processing treatment on the peptizability of the nitrogenous constituents o f solvent-extracted peanut meal and of flaked raw peanuts.
M A T E R I A L S A N D M E T H O D S
The work was carried out with a Pearl variety of peanuts, grown in 1940 and 1942. These white-skin peanuts were selected because they afford certain advantages over the usual red-skin varieties for many industrial purposes (3). The peanuts of both crops were stored in bags at a Georgia Experimental Farm until they were received at the laboratory, and were then stored in cotton mesh bags at room temperature (25 ° C.). The peanuts of the 1940 crop were received in June, 1941; those of the 1942 crop temperatures and humidities investigated. A steam-jacketed autoclave was used to attain temperatures of 100°, 105°, 110°, and 118° C., and steam was admitted to the chamber containing the samples in order to obtain 100% relative humidity (R .H .).
T o obtain 100% R .II. at 80° C., a glass tray containing water was placed in a small oven, and the temperature was allowed to
(Pearl variety) does n o t differ signifi
can tly fr o m the usu al re d -sk in varieties. Storage o f meal or 70 grams of flaked peanuts were spread in a thin layer on kraft paper to ensure uniform and rapid transfer of heat. The samples taken from the oven and the autoclave were kept at room temperature (25° C.) for 3 days to allow the moisture con
tents to reach equilibrium with atmospheric humidity. The heat-treated flaked peanuts were then solvent-extracted with Skellysolve F and were subsequently freed of residual solvent.
All samples were ground in a Wiley mill and were stored in screw- cap glass bottles until used. Because of variations in the mois
ture content of the ground meal samples, it was necessary to determine the nitrogen content o f each meal at the same time that peptization experiments were carried out.
The procedure for determining the percentage of total meal nitrogen peptized was described previously ( j) . Briefly, 2.5- gram portions of the meal, contained in separate 200-ml. screw- cap centrifuge bottles, were treated with 100 ml. of solvent.
The suspensions were allowed to stand for 3 hours at room temperature with occasional shaking and were then clarified by centrifuging. The pH values of the centrifuged solutions were determined b y a glass electrode, and the total nitrogen was determined on duplicate aliquots by either the semimicro- or macro-Kjeldahl method. The nitrogen values reported in all tables and graphs were calculated on the basis o f total volume o f solvent added in each case.
P E P T IZ A T IO N O F S O L V E N T -E X T R A C T E D M E A L S T he pH-peptization curves for unheated solvent-extracted meals 1 and 2 are shown in Figure 1. They are almost identical;
in addition, there are no significant differences between the white- and red-skin varieties (4). It is obvious, therefore, that the effects observed in the experiments to be described are due to the treatments employed and not to differences in the variety of pea
nuts or in the crop from which the sample was taken.
The fraction o f the total meal nitrogen peptized by water, 0.0025 N sodium hydroxide, and 1.0 M sodium chloride, for the meals obtained after each heat treatment, is recorded in Table I.
It is apparent that there is close correspondence between the data for meals 1 and 2 when water, sodium chloride, and sodium hydroxide are used as peptizing agents. T o simplify presentation of the results, peptization values for meals 1 and 2 with one peptizing agent (1.0 M sodium chloride) are plotted in Figure 2.
The sodium chloride peptization values are considered the best and simplest measure of protein denaturation, since the degree of peptization in sodium chloride solution is less affected by varia-625
626 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 36, No. 7
& C orresponding values for meal 2 , unheated:
water, pH 6.77, 9 0 % ; N aO H solu tion , pH 7.75, water, pH 6.95, 88.6 % ; N aO H solu tion, p H 8.08, reatments were carried ov
were solven t-extra cted before peptization measurements were made.
9 3 .4 % ; N aC l solu tion, p H 6.55, 9 2 .8 % . H ea t treatm ents were carried out on flaked raw peanuts in the nonprotein nitrogen values, indicating, as would be expected, that in the absence of water no protein degradation takes place.
In hydrochloric acid suspensions at pH 4.20 to 4.25 of the heated peanut meals described, significantly less nitrogen is peptized than in 1.0 M trichloroacetic acid suspensions.
This unusual behavior is noteworthy, in
asmuch as nitrogenous substances which are soluble in 1.0 M trichloroacetic acid are gener
ally considered to be nonprotein in character, this difference in solubility is widely used analytically to determine the relative amounts of protein and nonprotein nitrogen in mix
tures. The occurrence in urine of a protein soluble in trichloroacetic acid solution was recently reported (2).
P E P T IZ A T IO N O F IIY D K A U L IC -P R E S S F .D M E A L S
The practical significance of the results presented above are apparent when compared with similar data on samples of meal taken during a carefully controlled 3-day mill run in which 330 tons of peanuts were crushed
■for oil (S). The flaked peanuts were heated for approximately 15 minutes in each stack of a four-stack cooker. The temperature of jieptization values obtained with the other solvents investigated (Table II). It is obvious that, in 1.0 M sodium chloride-meal suspensions, changes in pH similar in magnitude to those found in the experiments reported in Table I have an almost negligible effect upon the amount of nitrogen peptized.
The peptizability of the nitrogenous constituents of heat- treated solvent-extracted meal 1 does not differ materially from that of heat-treated flaked peanut meal 2, except, possibly, after treatment at 110° C. and 100% R.H . for 1.5 hours where slightly tion of peanut protein is attained rapidly. B y contrast, soybean protein shows considerable denaturation after 2.5-hour heating at 120° C. and 0 % R.H . and also after 2.5-hour heating at 80° C.
and 100% R .H . (1).
The results in Figure 2 indicate that, after prolonged heating of peanut meal and meats, there is a slight but gradual increase in the amount of peptizable nitrogen. This increase is probably due to protein degradation. The amount of nitrogen peptized by 1.0 M trichloroacetic acid (nonprotein nitrogen) was determined
for several of the meals which had been heated for 2.5 hours at - x - x - , 1940 c r o p ; —0—0—, 1942 c r o p
July, 1944 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 627 meals were examined with respect to amount of nitrogen peptized by 1.0 M sodium chloride solution. These data indicate that the three meals obtained from the controlled mill run were pro
duced under milder cooking conditions than any of the other meals except 0-143. It is also evident that, in general, the heat treatments routinely employed in a majority of the peanut oil mills are insufficiently controlled to prevent rather drastic dé
naturation of the meal protein, as indicated by markedly de the best example of what can be consistently produced by careful control in an oil mil!, as far as the quality of the protein is con
cerned. Although the oil content of this meal is relatively high (9 % ), the peptization properties o f the meal compare favorably with those of solvent-extracted meal. When these facts are considered, together with the results in Table I, it is logical to conclude that both temperature and. moisture were carefully controlled in the production of meal 0-143 and that satisfactory oil removal was accomplished with a minimum of protein dé
naturation.
The present work and previously published results (7) point to the conclusion that critical denaturing temperatures for peanut protein are above 118° C. (dry heat) and above 8 0 ° at 1 0 0 % R .H . Since there appears to be little need for excessively high temperatures and moisture contents to rupture the oil cells of the peanut, it would seem advisable to employ in the mill the lowest moisture content and lowest temperature compatible with ade
quate oil removal. In this manner the meal proteins can be pre
served in a substantially undenatured state and thus be available for wider industrial utilization.
AC KN OW LEDGM EN T
The authors wish to express their appreciation to Alva F. Faust and R. H. Robinson of the Analytical, Physical Chemical and Physical Division of this laboratory for making a considerable number of the nitrogen determinations reported in this paper.
L IT E R A T U R E C IT E D