F. A. KULACKI and S. C. KENNEDY Dept. of Mechanical Engineering, The Ohio State University, Columbus, OH 43210

MEASUREMENT OF THE THERMO-PHYSICAL PROPERTIES OF COMMON COOKIE DOUGH

ABSTRACT Despite the volume of research done on food properties, the literature reports little related to the baking industry, particularly confectionary dough. This paper reports results of an experimental study of thermal conductivity, specific heat, density and thermal diffusivity of two dif- ferent types of cookie dough. Thermal conductivity of both doughs were moderately dependent on temperature. Thermal diffusivities of each type dough were evaluated using experimental values of thermal conductivity, specific heat and density. Total experimental uncertainty in the thermal diffusivity was estimated as 0.134 for the AACC formula and 0.15 for the hard-sweet formula dough.

INTRODUCTION MOST MODERN-DAY baking processes are highly mecha- nized, with high productivity rate and low unit cost as proces-, sing criteria. Convective heat and mass transfer to the dough pieces in either continuous through-put or large batch-type ovens generally control the physico-chemical processes in the dough during baking and thus determine overall oven produc- tivity. For optimum design of baking ovens relative to the convective transport processes to and within the dough, accurate knowledge is needed of the thermo-physical proper- ties of the dough in the various stages of baking.

A considerable amount of work has been done on the thermo-physical properties of many food products. Reported information, however, is frequently conflicting, and this often necessitates critical reevaluation and further experimental work. This apparent disagreement could be due not only to experimental uncertainties but also to the experimental methods used. Also, in many cases, the composition of the sample (e.g., content of moisture, fat, protein, etc.) is omitted. If such pertinent data are not reported, property values must be used with a relatively low level of confidence.

Despite the volume of research done on food properties, the literature indicates that very little has been reported on products related to the baking industry. Properties of bread dough and crumbs and various grams are most commonly found, but properties of confectionary dough are scarce.

This paper reports the results of an experimental study of the thermal conductivity, specific heat, density and thermal diffusivity of two different types of cookie doughs, the Ameri- can Association of Cereal Chemists (AACC) formula and a hard-sweet formula dough. These dough formulas were chosen, in part, to provide thermo-physical property data in an area where there is an apparent lack of such information and to compliment the work of Kulacki and Kevra (1975), who pointed out a definite need for thermal conductivity data in their formulation of an analytical model of cookie baking. Furthermore, a large variety of confectionary baked goods, e.g., sweet cookies, is produced today using dough formulas similar to these types of dough.

The major emphasis of the study is on the thermal conduc- tivity measurements. The thermal diffusivity, k/pc,- of each type of dough is evaluated using the experimental values for

0022-1147/78/0002-0380$02.25/O 0 1978 Institute of Food Technologists

the thermal conductivity, specific heat and density. In all cases, the experimental uncertainties in the thermo-physical properties of each type of dough are reported.

APPARATUS & PROCEDURE Thermal conductivity measurements

Numerous methods for the measurement of thermal conductivity have been developed, with most of them best suited to particular ranges of temperature and specific classes of materials. In general, all methods of thermal conductivity measurement can be classified as being either steady-state or transient. In the steady-state methods, the test specimen is subjected to a time invariant heat flux, and its thermal conductivity is determined using measured values of heat flux and temperature gradi- ent. In the transient methods, the temperature distribution in the test specimen varies with time, and the measurement of the rate of tempera- ture change replaces the heat flux measurement. The primary objective in either case is to maintain a controlled heat flow in a prescribed direction such that the actual thermal boundary conditions in the experiment match those assumed in the evaluation of the thermal conductivity.

Investigators of thermal conductivity of foods have used both of the above methods as well as slight variations of them. Kennedy (1975) presents a review of the details of the standard methods of thermal conductivity measurement. Further reviews of measurement techniques and compliations of thermo-physical properties are given by Charm (1971), Reidy (1968), Woodams and Nowrey (1968) and Lentz (1961). Reidy’s work presents a critical review of the advantages and disadvan- tages of the steady and transient methods.

In the present experiments, a single-plate thermal conductivity apparatus was used. This design was chosen owing to its ease of fabrica- tion, operation and adaptability to different test specimen thicknesses and the accuracy of measurements it afforded. In addition, the appa- ratus permitted the preparation of test specimens (described below) in a short period of time. This was considered to be an important aspect of the experimental procedure because it assured that all measurements were made with fresh dough, i.e., with the moisture content specified by the dough formula. In brief, the experimental procedure involved the placement of the test specimen between two plates, one heated and the other cooled, and direct measurement of the steady state heat flux and the temperature difference across the specimen. The thermal con- ductivity was evaluated with Fourier’s law, assuming one-dimensional conduction heat transfer through the test specimen:

k = q/(AT/L). (1)

The thermal conductivity apparatus comprised a stack of plates partially immersed in a constant temperature water bath, which served as a heat reservoir. A schematic of the apparatus is presented in Figure 1. The middle plate contained the dough sample and the top plate, hereafter referred to as the cold plate, was maintained at a constant temperature lower than that of the heat reservoir.

All plates of the conductivity apparatus were 15.875 cm square except the cold plate which was 15.875 cm x 16.51 cm. The plates were tightly held together by bolts on four threaded rods inserted in the bottom plate. The bottom plate was a 0.635 cm thick brass flat and served to establish a uniform temperature across the bottom of the apparatus. Next to the bottom plate was a 0.3175cm thick copper plate which served to further establish a constant temperature bound- ary condition at the lower end of the plates containing the heat flux meter and the test specimen. Adjacent to the copper plate was a 1.095-cm thick Plexiglas plate containing a 5.08 cm x 5.08 cm copper block located in its center. The function of this plate was to induce a uniform heat flow through the central region of the apparatus. A thermocouple well, 2.857-cm deep, was provided in the plate for moni-

380-JOURNAL OF FOOD SCIENCE-Volume 43 (1978)

THERMO-PHYSICAL PROPERTIES OF COOKIE DOUGH. . .

:oring horizontal temperature gradients and heat fluxes. In all of the :xperiments, horizontal heat fluxes were found to be negligible when steady state conditions had been reached. This observation, further- more, gave assurance that the rods which held all of the plates together lid not significantly affect the one-dimensional nature of the heat flow in the vicinity of the test specimen. [The apparatus was designed SO as to minimize horizontal heat flux in the vicinity of the test specimen. No detailed calculations of the effect of the connecting rods on the heat flow field near the test specimen were made. However, the high thermal conductance of the copper blocks bounding the test specimen and the measurement of the temperature difference across the test specimen on its centerline were considered sufficient to eliminate any influence that the connecting rods might have had on the computation of the thermal conductivity using Eq (1). The vertical flow of heat through the connecting rods was considered to be of minimal influence on the calculation of the thermal conductivity for the same reason.]

The next plate in the apparatus was a 0.3175-cm thick Plexiglas plate containing at its center a 5.08 cm x 5.08 cm heat flow transducer (Hy-Cal Engineering “Sensable” Type BI-7). Directly above the heat flow transducer was the specimen plate, which was 0.3175-cm thick Plexiglas with a central cavity of, 5.08 cm x 5.08 cm for the test specimen. This plate also contained a groove along its bottom for posi- tion of a thermocouple on the bottom of the test specimen. Above the test specimen plate was a copper plate similar to that below the plate containing the heat flow transducer. This plate had a groove machined in its lower surface for positioning a thermocouple at the top of the test specimen. Next to this plate was a copper plate which served both to establish a uniform temperature boundary condition at the upper end of the stack and to maintain a unidirectional heat flow through the test specimen.

The cold plate comprised a 1.27-cm thick aluminum block with a double spiral channel machined in it for circulating water. This plate was sealed with a 0.635-cm thick Plexiglas plate. The plate was con- nected to a Lauda Type NB-S15 circulating water bath which supplied water at a constant temperature.

The AACC formula dough and the hard-sweet formula dough were prepared according to standardized procedures. A list of the ingredients is presented in the Appendix. All ingredients were weighed on a Sartouius Selecta Rapid 2009 balance. The accuracy of this instrument was f 10Ag. For both types of dough, all ingredients were weighed to within + 0.002g.

After the dough had been prepared, a piece of it was placed in the cavity of the specimen plate directly on the heat flux meter and a small sheathed thermocouple lying flat against the heat flux meter. The dough was spread to fill the entire cavity and flattened to remove excess dough. With a small Plexiglas rod (2.54 cm diam x 7.63 cm), the dough was rolled with each stroke of the rod moving diagonally across the cavity. The excess dough which accumulated in the comers of the cavity was removed. The thickness of the specimen and the flatness of its surface were then checked using a 7.94 cm x 5.39 cm x 1.91 cm Plexiglas block. The 5.39 cm x 1.91 cm surface of this block was pressed on the dough surface in several positions. The proper specimen thickness was indicated when very slight indentations from one edge of the cavity to the other was observed. The average time for these speci- men preparations was 10 min.

Once the test specimen was prepared, the upper conduction plate was assembled above the specimen plate and a sheathed thermocouple was inserted in the groove on the bottom of this plate such that its junction was at the center of the test specimen on its upper surface. The upper copper plate and the cold plate were next added to the stack, and aU other thermocouples were mounted in place. (All thermo- couples were copper constantan and were made from 36 ga wire.) The entire apparatus was then allowed to reach thermal equilibrium (approx 5 min were sufficient) and then placed in the water bath. Data were recorded immediately after immersion of the plate assembly as a check for normal operation. In most cases, the system reached steady state within 60 min after immersion. When steady state was reached as near as possible to a preselected average test specimen temperature and temperature difference across the test specimen, several readings of heat flux and temperature difference were taken. The average duration of a run, from dough preparation to last data recording, was approximately 2 hr. For this reason, a new batch of dough was always prepared for each run to ensure that the dough properties from run to run would not be appreciably different.

Additional details of the design and construction of the thermal conductivity apparatus and the experimental procedure are presented by Kennedy (1975).

Constant Temperature Water

r Cold Plate

Fig. ?-Thermal conductivity apparatus.

Specific heat measurements The technique developed for the specific heat measurements was

based on the method of mixtures. The primary factors underlying the choice of this method were the ease of operation of the apparatus (to be described below), its suitability for working with dough samples containing water, and the fact that test samples could be prepared to conditions similar to those of the thermal conductivity measurements. In brief, the technique consisted of heating a known volume of water in a calorimeter and immersing the test specimen at a lower temperature, rather than heating the specimen and immersing it in the water, as in the usual application of the method of mixtures. The specific heat of the dough was evaluated, using a gross energy balance, from the equa- tion,

m,c,AT, - QL Cd = (2)

‘% ATd

The experimental apparatus comprised a 24.13 cm x 24.13 cm x 15.24 cm insulated block having a central cylindrical well 7.62 cm diameter x 10.16 cm in which a 400-ml beaker would be tightly in- serted. The block was made of three 24.13 cm x 20.32 cm x 5.08 cm Fiberglas layers. A similar layer was used as a removable cover for the block.

The test capsule consisted of a copper tube, 2.54 cm i.d. x 0.158 cm wall x 5.39 cm, and two rubber stoppers for closing both ends of the tube. One of these stoppers allowed axial penetration of a sheathed thermocouple probe at its center. Temperature measurements of the water and test specimen were made with sheathed copper-constantan thermocouples.

The experimental procedure comprised the following: A beaker was inserted into the insulated block and filled with water approximately 10” C above the experimental test temperature and allowed to remain in place until it reached thermal equilibrium with the block. During this time, another beaker was filled with 350 ml of water and heated to a temperature 5°C higher than the initial water temperature desired in the experiment. The mass of water in the beaker was recorded. At the same time, the test capsule was filled with dough. Care was taken to insure that no air pockets, or voids, existed in the dough. The net mass of dough in the capsule was then determined and recorded. A thermo- couple was inserted into the dough through one of the rubber stoppers. Next, the temperature of the insulated block was measured, and when it and the temperature of the water in the test beaker were the same, the test beaker and test capsule were placed in the block. A thermo- couple was immersed in the beaker and the cover was placed on the insulated block. A thermocouple was also mounted in the insulated block to measure the ambient temperature above the test beaker and

Volume 43 (19781~JOURNAL OF FOOD SCIENCE- 381

capsule. When the test capsule and water reached thermal equilibrium, their temperatures were recorded, as well as the ambient and block temperatures. Density measurements

The density of both dough types was determined by weighing a known volume of dough. All of the density determinations were made at 2S°C, and the test specimens for these measurements were prepared from fresh batches of dough in a manner similar to that used in the preparation of the test specimens in the thermal conductivity measure- ments. Thus, the density measurements were made for dough essential- ly at its “rest” state.

RESULTS & DISCUSSION THE MEASUREMENTS of thermal conductivity of the AACC and hard-sweet formula doughs are summarized in Table 1 and graphically presented in Figures 2 and 3. For each run, four readings of the heat flux and temperature difference across the test specimen were made. The standard deviation of the thermal conductivity within a run never exceeded 6% of the mean and was typically on the order of 1% of the mean. The standard deviation of the test specimen temperature within a run never exceeded 1% of the mean. Variations from run to run, however, produced the standard deviation in the means as indicated in Table 1. The variance reported for the tempera- tures in Table 1 is primarily that which arose from the diffi- culty in achieving a predetermined average temperature in the test specimen during a given run. Thus, where more than one

Table l-Thermal conductivity data for the AACC and hard-sweet formula dough

w,i-K, No. of No. of

Dough type Mean S.D. Mean S.D. runs readings

AACC 297.5 299.4 302.6 304.9 307.1 313.5 318.1 325.5 331.7 336.9

HS 297.9 303.4 306.1 308.6 314.6 319.5 325.8 330.6 337.3

0.5 0.5 0.8

- 0.3 0.3

-

0.5 - -

0.7 0.4

- 0.4 0.3 0.3 0.3 0.01 0.06

0.385 0.400 0.417 0.439 0.423 0.394 0.376 0.311 0.314 0.305

0.380 0.423 0.414 0.342 0.304 0.305 0.302 0.295 0.325

0.080 0.033 0.059

- 0.071 0.092

-

0.030 - -

0.025 0.024

- 0.011 0.012 0.033 0.031 0.029 0.049

4 11 5 20

11 44 1 4 9 36 5 20 1 4 4 16 2 8 2 8

5 20 4 16 1 4 3 12 4 16 3 12 4 16 3 12 3 12

Table P-Specific heat data for the AACC and hard-sweet formula doughs

Dough type

AACC

HS

T (K)

302.8 308.5 310.9

303.0 309.5 312.0

kJ,kcg-K)

2.835 2.841 3.128

2.420 2.809 3.182

Average

2.94 f 0.17

2.804 + 0.38

run is indicated, the standard deviation in the mean specimen temperature is due primarily to a pooling of the data (Kenn- edy, 1975).

From Figures 2 and 3, it can be seen that for both types of dough, the thermal conductivity first increases with tempera- ture, then drops to a relatively constant value at higher tem- perature. A possible explanation for this behavior is that some nonhomogeneity may have developed at the higher tempera- tures; and at the higher temperatures, different rates of expan- sion of the various ingredients could have caused the compon- ents to separate. Nevertheless, significant separation of the dough components was not observed, and only about 1% of the mass of the test specimen was lost by out-migration of liquid between the surfaces adjacent to the test plate. The increase of the thermal conductivity of water with tempera- ture could also account, in part, for the initial increase in the thermal conductivity of the dough. However, the thermal conductivity of water is strictly an increasing function of temperature for the ranges of temperature of interest in this study. Thus the decrease in the thermal conductivity at the higher temperatures cannot be explained entirely by consid- ering the behavior of the water component.

If one discounts the data for AACC formula dough for T > 318K, the average thermal conductivity is 0.405 * 0.022 W/m - K for 297.50 < T < 318K. A similar treatment for the hard-sweet formula dough gives an average thermal conduc- tivity of 0.390 + 0.037 W/m - K for 297.9 ,< T < 308.5K. It is believed, however, that the data of Figures 2 and 3 do indicate a reasonable trend for the temperature dependence of the thermal conductivity. At the higher temperatures, one would expect some change in the physical and chemical composition of the dough, just as in actual baking (although in the present study, the volumetric expansion of the dough was restricted). The tendency in baking is for the product to undergo a decrease in thermal conductivity relative to that of the dough; and it may be expected that “unbaked” dough will show a lower thermal conductivity as its temperature approaches a level where significant physio-chemical changes occur.

Specific heat measurements are summarized in Table 2. The values obtained in this study are higher than those indicated by Reidy (1968) for a similar dough and by Kulacki and Kevra (1975).

Density measurements indicated a density of 1252.3 k 17.6 kg/m3 for the AACC formula dough, and 1286.6 * 8.8 kg/m3 for the hard-sweet formula dough.

The thermal diffusivity of both dough types was evaluated using the thermal conductivity data represented in Table 1 and the average of the specific heat data of Table 2 for each dough type. It was felt that the lack of a large data base for the specific heat did not permit any conclusions on its tempera- ture dependence. The thermal diffusivity of both dough formulas is presented as a function of temperature in Figure 4. Error estimates

Experimental errors in the thermal conductivity results are due to instrumentation errors, geometrical uncertainties, and deviations from the assumed one-dimensional nature of the heat flow through the test specimen. Similar sources of un- certainty exist for the density and specific heat data. In this section, estimates will be put on the experimental uncertain- ties in each of the thermo-physical properties for each type of dough.

From Eq (l), the fractional uncertainty in the thermal conductivity is the sum of the fractional uncertainties in the heat flux, the specimen thickness and the temperature differ- ence across the specimen. There are two primary sources of error in the heat flux. The inaccuracy of the heat flux meter is 0.5% of full scale. Radial heat losses, computed using measured temperature differences and literature values for the thermal conductivity of Plexiglas, are less than or equal to

382-JOURNAL OF FOOD SCIENCE- Volume 43 (1978)

3.4% for the AACC formula data and less than or equal to 5% for the hard-sweet formula data. Geometrical uncertainties in the specimen thickness are due to errors in specimen prepara- tion and the error in positioning of the thermocouples. It is estimated that the total uncertainty in the specimen thickness are due to errors in specimen preparation and the error in positioning of the thermocouples. It is estimated that the total uncertainty in the specimen thickness amounts to no more than 2%. Uncertainties in temperature difference is estimated to be less than 2%, including the uncertainty in associated instrumentation.

Thus, the total fractional uncertainties in the thermal con- ductivity measurements are,

5 0.074, AACC

and 2 0.09.

It may be noted that these uncertainty levels cannot com- pletely explain the variance observed in the thermal conduc- tivity data, especially in the data for the AACC formula dough. It is possible that slight variations in dough composi- tion and test specimen density (as prepared) would account for some additional variance.

It is estimated that the experimental uncertainty in the measured specific heat is on the order of 5%, which includes the uncertainties in the temperature measurements, the mass of dough, the mass of water, and the heat loss. The un- certainty in the density measurement is estimated to be of the order of 1%.

Upon combining the uncertainties in k, p, and cd, one finds that the total uncertainty in the thermal diffusivity values of the two dough types are,

s 0.134,

sa and F 0 HS

50.15.

0.44 - 0.44 - '. ' '. ' I r I '_ I r I '_

0.42 0.42 ' ' - - . .

0.40 0.40 - - 0 0 0 0

. . 0.38 0.38 - - Y Y

0.36 0.36 - -

0.34

0.32 0 .

0 0.3 L-4

s 93 303 313 323 333 343 T

Fig. Z-Thermal conductivity of AACC for- Fig. 3-Thermal conductivity of hard-sweet Fig. 4-Thermal diffusivity of AACC and mula dough. formula dough. hard-sweet formula doughs,

THERM0PHYSICAL PROPERTIES OF COOKIE DOUGH. . .

Concluding remarks The results of this study indicate that the thermal conduc-

tivity of both the AACC and the hard-sweet formula doughs are moderately dependent on temperature, each showing a maximum at about 293K and a decrease on the order of 25% for temperature up to 338K. If one considers restricted ranges Of temperature, kAACC = 0.405 * 0.022 W/m-K for 297.5 < T < 31gK; and kHs = 0.390 +_ 0.037 W/m-K for 297 < T =Z 308.5K. The experimental uncertainty in the thermal conduc- tivity was estimated to be 0.074 for the AACC formula dough and 0.09 for the hard-sweet formula dough. The thermal diffusivities of each of the dough types were evaluated using experimental values of thermal conductivity, specific heat, and density. The total experimental uncertainty in the thermal diffusivity was estimated to be on the order of 0.134 for the AACC formula dough and 0.15 for the hard-sweet formula dough. It is felt that the data obtained in this study will be useful in analysis of the convective heat and mass transfer processes in the baking of confectionery dough products, and in particular, the analytical modeling of the baking process within the dough. The later area represents, we believe, a challenging and fruitful problem for future investigation.

NOMENCLATURE

Symbol

L L m

& T (Y P

Subscripts d

:ACC HS

Description Units Specific heat kJ/kg-k Thermal conductivity W/m-K Specimen thickness Mass kmg Heat flux W/m2 Heat loss J Temperature K Thermal diffusivity m2/s Density kg/m3

Pertaining to the dough Pertaining to water Pertaining to the AACC formula dough Pertaining to the hard-sweet formula dough

0.38 A 0.38- A F

y y 0.36 0.36-

0.34 1

0.34 -

A 0.32 0.32-

AA A 0.30 0.30 - AA A A A

0.2 % ' ' I ' I ' a 0.23-

93 93 303 313 323 333 343 303 313 323 333 343 T

I .20 0 A

m II2 0

:,F , , , ;;,,,;, :,I 293 303 313 323 333 343

T

Volume 43 (1978/-JOURNAL OF FOOD SCIENCE- 383

APPENDIX

AACC formula dough: Ingredient wt%

Sugar 27.5 Floura 46.8 Shortening 13.5 Salt, sodium bicarbonate, and amonium bicarbonate 1.1 Watera 4.1 Dextrose solutionb 7.0

Hard-sweet formula dough: Ingredient

Sugar Flourc Shortening Invert syrup Water Salt, sugar spray dried whole eggs, nonfat dry milk solids, sodium bicarbonate and amonium bicarbonate

18.7 56.5

9.9 3.1 8.5

3.2

a The flour and water ingredients were adjusted according to the mois- ture base of the flour, which for the present study was 12.6%.

b The dextrose solution contained 8.9g dextrose dissolved in 150 ml water.

c The flour contained 12.6% moisture.

REFERENCES

Charm, SE., 1971. “The Fundamentals of Food Engineering.” Avi Publishing Co., Inc., Westport, CT.

Kennedy, SC. 1975. Experimental measurement of thermo-physical properties of baking dough. M.Sc. thesis, The Ohio State University.

Kulacki, F.A. and Kevra, J.M., 1975. Experiments on coupled heat and mass transfer in the presence of a point source electrostatic field. The Ohio State University Engineering Experiment Station, Report No. 2. Project EES 439X.

Lentz. C.P. 1961. ThermaI conductivity of meats, fats. g&tine gels and ice. Food Technol. 15: 243.

Reidy. G.A., 1968. 1. Methods for determining thermal conductivity and thermal diffusivity of foods. 2. Values for thermal properties of foods gathered from the literature. Thesis, Dept. of Food Science, Michigan State University.

Woodams. E.E. and Nowrey, J.E.. 1968. Literature values of thermal conductivities of foods. Food Technol. 22: 494.

MS received 6/18/11; revised D/21/77: accepted 9/30/71.

The authors appreciate the donation of computer time by the In- struction & Research Computer Center at The Ohio State University.

PEANUT BUTTER QUALITY. . . From page 374 -

products. The imitation products had the benefit of lowest cost but lacked the typical qualities associated with products which contained enough peanuts to conform to the Standard of Identity for peanut butter. The unstabilized products had the highest cost but were rated poorest in sensory quality attributes.

Average prices (cents/oz) of stabilized, smooth peanut butter produced by national, chain store and “other” manu- facturers are shown in Table 8. Smooth butters produced by national manufacturers had the highest average cost per ounce (4.85@), chain store brands were intermediate (4.336), and “other” brands were lowest (3.93~). The chain store brands were somewhat less variable in cost than the national or “other” brands.

Average prices (cents/oz) of stabilized, crunchy peanut butter produced by national and chain store manufacturers are also shown in Table 8. Results showed that there was no sig- nificant difference in average prices of crunchy butters pro- duced by the two groups of manufacturers. Chain store brands, however, had greater variation in prices than national brands. Conclusions

Results of this study have shown that the quality of stabilized peanut butter was preferred over that of unstabilized or imitation products. Composition, quality attributes and price varied not only by manufacturer (national, chain store or “other”) but also by brand. In most cases, national brands received significantly higher sensory quality ratings; their price was also higher than chain store or “other” brands. Con-

sumers, therefore, have available to them a range of peanut butter products from which to choose, with choices being dependent upon preference for certain quality characteristics, nutritional attributes or cost savings.

REFERENCES

AOCS. 1970. “Official and Tentative Methods of the American Oil Chemists’ Society.” Methods Ab 2-49 and Ab 4-50.

Anonymous. 1975. “Peanut Butter, Standard of Identity.” The Almanac. E.E. Judge & Sons. Inc.. Westminster. MD.

Carruthers, D.R. and DeHaas. H. 19?1. The efficiency of utilization of protein from peanut butter sandwiches and dried miIk solids by the growing rat. Univ. Mass. Agr. Expt. Stat., Research in the Life Sciences 19(6): 1.

Federal Register. 1975. Peanut spreads: Proposed common or usual name. 40(212): 51052.

FAO. 1970. Amino acid content of foods and biological data on pro- teins. Food and Agriculture Organization of the United Nations. Rome, Italy.

Freeman, A.F., Morris. N.J. and WiIIich, R.K. 1954. Peanut butter. USDA AlC-370.

Jones, D.B. 1931. Factors for converting percentages of nitrogen in food and feeds into percentages of proteins. USDA Circular No. 183.

Morris, N.J. and Freeman, A.F. 1954. Peanut butter. 4. The effect of roasting on the palatability of peanut butter. Food Technol. 8: 3’71.

Roberson, S., Marion. J.E. and Woodroof. J.G. 1966. Composition of ~commercial peanut butters. J. Amer. Dietetic Assoc. 49(3): 208.

USDA. 1977. Peanut stocks and processing. United States Dept. of Agriculture, Washington, D.C.

WiIIich, R.K., Hall, AS., Morris, N.J. and Freeman, A.F. 1952. Peanut butter. 1. Roasting, cooling, blanching and picking of peanuts. Food Technol. 6: 71.

Woodroof J.G. 1973. “Peanuts: Production, Processing, Products.” The AA Publishing Co., Westport, CT.

Msreceived 4/30/11; revised l/21/77; accepted 1125177.

The authors gratefully acknowledge the technical assistance of Msrgree E&or. Eunice Parks and Katie Stewart and the statistical analysis by J.C. Elrod and G.O. Ware.

384-JOURNAL OF FOOD SCIENCE-Volume 43 (1978)