Reactivity of Kel-F polymers with organolithium and

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Chem. Prod. Res. Dev. 1983, 22, 303-307

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Reactivity of Kel-F Polymers with Organolithium and Organomagnesium Compounds Neil D. Danlelson,' Rlchard 1. Taylor, Jeffrey A. Huth, Rlchard W. Slerglej, James 0. Galloway, and Joseph B. Paperman Department of Chemistty, Miami University, Oxford, Ohio 45056

Reaction of polychlorotrifluoroethylene(PCTFE or Kel-F) with a variety of organometallic reagents was carried out. For organolithium reagents, the primary reaction was substitution of the chlorine for the organic group. In some cases a significant decrease in fluorine was also observed. Cross-coupling occurred upon reaction of Kel-F with organomagnesium reagents as well; however, loss in fluorine was not evident. The extent of substitution varied according to stoichiometry, solvent, and the nature of both metal and organic reagent. Me,SiLi and Me3SnLi afforded reduction rather than substitution yielding a carboncoated KeCF as had been previously observed with other reducing agents. The results were interpreted in terms of an electron-transfer mechanism. Utility of some of the modified Kel-F polymers for high-performance liquid chromatography was also demonstrated.

Introduction While fluorocarbon polymers such as polytetrafluoroethylene (PTFE or Teflon) and polychlorotrifluoroethylene (PCTFE or Kel-F) generally display excellent stability to most chemical reagents, they have been shown to react with good reducing agents. For example, a solution of sodium metal, naphthalene, and tetrahydrofuran (THF) has been recommended for etching the surface of PTFE before bonding of adhesives (Billmeyer, 1971). Lithium amalgam at elevated temperatures and extended time periods can reduce particles of either polymer to what is essentially carbon (Plzak et al., 1978; Smolkova et al., 1980, Zwier and Burke, 1981). These materials have found utility as column packing materials for high-performance liquid chromatography (HPLC). In an effort to obtain chemically modified fluorocarbon polymers, we undertook a study of the reactions of two different types of Kel-F with a variety of organolithium and organomagnesium reagents and report the results herein. In most cases, alkyl substitution was obtained; however, the organometallics with low reduction potentials did afford reduction products. The results are summarized and interpreted in accordance with a possible reaction mechanism. In addition, utility of two of the functionalized Kel-F polymers as HPLC column packings was demonstrated. Experimental Section Materials. Kel-F 6300 (100-200 mesh), a copolymer of polychlorotrifluoroethylene containing 3 % vinylidene fluoride, was obtained as a gift from the 3M Company (Minneapolis, MN). The polymer was ground with a Pilamec high energy vibration ball mill until the particles were less than 44 pm. Kel-F 6061, 80-100 mesh, (A.M. Plastics, Rockaway, NJ) was jet ground (Alnort Inc., Camden, NJ) until 93% of the particles were less than 44 pm. Light microscopy indicated most of the particles were between 5 and 36 pm. n-Butyllithium (1.6 M in hexane), phenyllithium (1.95 M in diethyl ether), methyllithium (1.4 M in diethyl ether), and methylmagnesium bromide (3 M in diethyl ether) were obtained from Aldrich Chemical Co. (Milwaukee, WI). Vinyl magnesium chloride was obtained from Alpha Products (Danver, MA). The tetradecyl and octyl Grignard reagents were synthesized in our laboratory. Tri-

methylsilyllithium and trimethylstannyllithium were synthesized as described previously (Still, 1976, 1977).

Procedure General Methods. To a known quantity of Kel-F 6300 or 6061 in T H F was added a stoichiometric amount of either an organolithium or organomagnesium reagent under an inert atmosphere. The resultant mixture was allowed to react for a given period of time until completion. The reactions were generally terminated by careful addition of acetone or methanol to neutralize any unreacted reagent. The Kel-F particles were then filtered and washed with 100-200-mL portions of hexane, acetone, methanol, water, and finally methanol. Elemental analyses were carried out by either Galbraith or Atlantic Microlab. Infrared spectra were recorded on a Perkin-Elmer Model 680 spectrophotometer. Before column packing, repetitive sedimentation of the reacted Kel-F polymers in methanol was carried out to remove fines. The HPLC columns were packed at 8000 psi using an S. C. hydraulic pump and a Model 9501 slurry packing reservoir (Alltech Associates). The HPLC separations were carried out on a Beckman Model 330 liquid chromatograph. Reactions of Kel-F 6300 with n -Butyllithium. A 7-g amount (0.06 mol) of polymer in the presence of 0.19 mol of n-butyllithium (3:l stoichiometry) was heated at reflux in THF for 30 min. The reaction was repeated twice with 1:l stoichiometry and 0.3:l stoichiometry, respectively. The particles appeared unchanged except for a color change from white to gold-brown. Reactions of Kel-F 6061 with Methyl-, Butyl-, or Phenyllithium. A 7-g amount (0.06 mol) of polymer in the presence of 0.19 mol of the organolithium reagent was allowed to reflux in THF for 30 min. The butyllithium and phenyllithium reactions were repeated as above except that the solvent was hexane. All of the products were gold-brown in color. Reactions of Kel-F 6061 with (CH3),SnLi and (CH3)3SiLi. A 1.16-g quantity (0.01 mol) and 0.01 mol of the organolithium reagent were allowed to react at room temperature for several hours. The color of both products was black. Reaction of Kel-F 6061 with Organomagnesium Reagents. The reactions involving CH3-, C8CI7-, and

0196-4321/83/1222-0303$01.50/00 1983 American

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100

Scheme I % elemental analysis of C, H, o r C1 = wt of element/total wt for 100 monomer units wt of F = 300(19) w t o f Cl=(lOO-X)(35.5) wt of c = (200 + x C)(12) wt of H = X( 1)(H) total w t where X = % substitution of R for C1

6C

-

40

-

20

-

z 0

r_ Y)

U 4. 103

Bi

?-.

-.

\

0 1

1

403r

3000

2000

I

I

1500

1000

400

WAVENUMBER CM-'

Figure 2. IR spectrum of Kel-F 6061 after reaction with phenyllithium.

50

1

c -

40 1 43t2

3000

I

1

>coo WAVENUMBER

1500 (CM - > I

,

1000

c

z ice

Figure 1. IR spectrum of Kel-F 6061.

C,,HB-MgBr were carried out identically with those described for methyl-, butyl-, and phenyllithium except that the reaction time was 1 h. All of the products were goldbrown in color. The reaction involving CH2=CH-MgC1 was similar except that a 1:l stoichiometry was used. The color of the product was light brown. Calculations. For those modified polymers in which the percentages of C, H, C1, and F totaled 94% or higher, it was assumed that substitution of the chlorine for the organosubstituent of the lithium or magnesium reagent was the primary reaction pathway. An estimate of the millimoles of the organic group per gram of polymer could then be made using the calculation in Scheme I. mmol of C g of Kel-F 1000(X) X(F.W. R-Kel-F) + (100 - X)(F.W. Kel-F)

Results and Discussion Reactivity of Kel-F with Organolithium Reagents. Infrared spectroscopy proved to be very useful for the qualitative characterization of the reacted Kel-F. In the region from 3000 to 2800 cm-', no absorption for either Kel-F polymer was noted due to the almost complete absence of C-H bonds in the structures (Figure 1). The bands between 1000 and 1400 cm-' were due primarily to C-F stretches while the strong band at 940 cm-' was assigned to the C-C1 stretch. However, upon reaction of either Kel-F 6300 or 6061 with either alkyl organolithium reagent, strong C-H absorption bands could be seen between 1300 and 1500 cm-' as well as 2800-3000 cm-l. In some cases, a marked decrease in the C-Cl band intensity occurred indicating that reaction of the Kel-F was quite extensive. Absorption bands a t 3059, 3028 cm-', and overtones in the 2000-1660-cm-' range clearly indicated the addition of aromatic groups after reaction of Kel-F with phenyllithium (Figure 2). Two characteristic bands (758 and 697 cm-') confirmed the presence of monosubstituted benzene as well. In general, it was clear that substitution of Kel-F by the organic portion of the organolithium

30 W

0

1

2

3

R E A C T I O N STOICHIOMETRY

Figure 3. Reaction stoichiometry of butyllithium. Kel-F 6300 as a function of elemental analysis: 70C (0); % H x 10 (0); % C1 (a), % F (m).

reagent was occurring. However, to permit a more detailed comparison between different reactions, elemental analyses of the polymer products were carried out. The effect of reactant stoichiometry was studied for the reaction of n-butyllithium and Kel-F 6300 (Figure 3). A marked increase in carbon and hydrogen and decrease in chlorine and fluorine was noted as the ratio of n-butyllithium to Kel-F increased from 0 to 1. At a higher reaction stoichiometry of 3, the change in elemental analysis had diminished. Essentially a complete loss in chlorine was obtained for the 3:l n-butyl Kel-F product. The fact that all the chlorine atoms could be replaced indicated that the reaction does not occur strictly on the surface of the polymer. Penetration of the organolithium reagent into the polymer probably occurred due to the ability of THF to swell Kel-F (Mark, 1967). The loss in fluorine was not as significant as the loss in chlorine for any of the reacted Kel-F polymers. The total elemental percentages of C, H, C1, and F were 97.8, 92.1, and 84.7% for 0.3:1, 1:1, and 3:l n-butyl Kel-F polymers, respectively. The element not accounted for was probably oxygen. The presence of oxygen on Teflon after treatment with sodium metal has been reported previously (Riggs, 1974). In addition, elemental analysis of 3:l n-butyl Kel-F indicated the presence of oxygen; however, accurate results could not be obtained due to the high percentage of fluorine. The average percent substutition and millimoles of butyl group per gram of Kel-F were 43 f 4 and 3.2, respectively for the 1:l n-butyl Kel-F product. In addition to derivatizing Kel-F, the surface of the polymer was modified after reaction. Electron microscopy of 3:l butyl Kel-F showed that the polymer surface was highly pitted and porous in appear-

Ind. Eng. Chem. Prod. Res, Dev., Vol. 22, No. 2, 1983 305

Table I. Reactivity of Kel-F 6061 with Organolithium Reagents reagent solvent %C %H a CH3(CH2)3

CH3(CH2)3-

phenylphenylCH,( CH,),SGb

THF HEX THF

HEX

THF THF

21.2 29.3 42.9 30.5 21.4 33.9 25.5

%

c1

30.6 22.8 0.0 21.0 29.9 5.7 14.3

0.0 1.7 4.8 1.3 0.5 2.6 1.7

%F 48.2 (bd) 41.2 28.5 41.2 4 9 . 2 (bd) 27.5 43.8

% Si

nd nd

nd nd nd nd < 0.1

total 95.0 76.2 94.0 69.7 85.4

Reaction stoichiometry 1:l;bd = by difference; nd = not determined; a Elemental analysis data for unreacted Kel-F. THF = tetrahydrofuran; HEX = hexane. 100 r ance. A surface area increase from about 1.5 m2/g for unreacted Kel-F to 14.4 m2/g for the reacted Kel-F was determined. KO Reaction of Kel-F 6061 with a variety of organolithium I I1 reagents was carried out and the elemental analyses are p BO reported in Table I. The extent of reaction based on the + t 5% C1 data were similar for both butyl- and phenyllithium Ez 40 reactions in THF. In addition, essentially no loss in c fluorine was indicated for these Kel-F products. However, r the methyl Kel-F product indicated a substantially greater 20 loss in C1 as compared to the butyl and phenyl Kel-F products. A definite loss in fluorine and the presence of oxygen was found for this polymer product as well. Al4000 3000 2000 1500 1000 400 though phenyllithium is a dimer in THF while n-butylWAVENUMBER 1 C M - I ) lithium and methyllithium are tetramers (Brown, 1970), Figure 4. IR spectrum of Kel-F 6061 after reaction with CI4Hz9steric factors could still account for the greater reactivity MgBr. of methyllithium. As indicated by the 3 1 butyl Kel-F 6300 product as well as the methyl Kel-F 6061 product, a subTable 11. Reactivity of Kel-F with stantial decrease in the fluorine percentage did not occur Organomagnesium Reagents until most of the chlorine had been displaced. The rereagent Kel-F % C % H % C1 % F total activity of butyllithium in THF was about 2-3 times lower with Kel-F 6061 than Kel-F 6300. The percent substituCH, 6300 42.5 3.4 1.0 47.6 94.5 tion and mmol of butyl-g of Kel-F 6061 were 23 f 1and CH,6061 35.5 1.8 12.3 48.3 97.9 W,,6061 55.7 6.2 4.9 32.7 99.5 1.8, respectively. The corresponding values for phenyl C,'lH,,6061 53.4 5.9 4.6 32.7 96.6 Kel-F 6061 were essentially the same. The lower reactivity CH2=CH-a 6061 21.6 1.6 15.2 n d b of Kel-F 6061 is due to the fact that this polymer has a higher degree of crystallinity and therefore is greater in a Reaction stoichiometry 1:1. nd = not determined. hardness. The crystalline form of Kel-F 6300 has been reported to have an R value of 92 whereas the R value for the surface of Kel-F 6061 after reaction with butyl- or crystalline Kel-F 6061 is about 110 (Deehan, 1981). phenyllithium was also markedly etched and pitted. Reactions of butyllithium and phenyllithium with Kel-F Surface area values of 1.6 and 3.5 m2/g were found for 6061 were also carried out in hexane instead of THF. The Kel-F 6061 before and after reaction with n-butyllithium. reactivity of Kel-F with butyllithium in hexane was subThe extensive carbonization of the Kel-F particles due to stantially greater (about 3 times) than in THF. On the the (CH3)3SnLior (CH3)3SiLireagents resulted in a uniother hand, phenyllithium and Kel-F in hexane only reform etching and cracking of the surface. acted to a very slight extent due to the poor solubility of Reactivity of Kel-F with Organomagnesium Reagphenyllithium in hydrocarbon solvents (Brown, 1968). The ents. The infrared spectrum of Kel-F 6061 after reaction greater reactivity of butyllithium in hexane is somewhat with the CI4HzsGrignard reagent is shown in Figure 4. surprising due to the likelihood of more extensive aggreThe presence of the strong C-H adsorption in the region gation ( h t h a m and Gibson, 1963a). However, it has been from 3000 to 2800 cm-l again indicated that alkylation of reported that direct reduction of an alkyl halide by nKel-F by organomagnesium reagents was likely. Elemental butyllithium is facilitated in a hydrocarbon solvent analysis data of Kel-F with various organomagnesium (Eastham and Gibson, 1963b). This would explain the reagents are indicated in Table 11. As shown by the inalmost complete loss in chlorine and substantial loss in crease in carbon, alkylation of Kel-F by all the Grignard fluorine for the butyllithium reaction in hexane as shown reagents except vinyl MgCl occurred. The loss in chlorine in Table I. indicated that some reaction had taken place for the latter Both Me3SnLi and Me,SiLi reacted with Kel-F or reagent. No significant loss in fluorine was noted for any Teflon almost instantly to form carbonized products. of the reacted Kel-F products in contrast to that observed Although some chlorine and fluorine were lost, essentially for organolithium reagents. Comparison of the methyl no functionalization occurred as indicated by the low 5% Kel-F 6300 and 6061 products again showed the greater Si in Table I. Carbonization was also the result upon reactivity of the softer polymer, Kel-F 6300. Essentially reaction of Kel-F or Teflon with Li amalgam (Zwier and complete substitution of the chlorine for methyl was calBurke, 1981). However, these reactions generally required culated for the 6300 polymer. In general, the extent of at least several days under vacuum at about 100 "C. Elfunctionalization of Kel-F 6061 decreased with increasing emental analysis and the light tan color of the product chain length of the organomagnesium reagent. The indicated that no reaction occurred between (CH3)3SiCmmol/g for CH,-, C8H1,-, and C,,H29-Kel-F were 9.2, 4.4, H2Li and Kel-F. and 2.5, respectively. The mmol of CH3/g of Kel-F 6061 As with Kel-F 6300, electron microscopy showed that were similar regardless of whether CH,Li or CH3MgBr was y1

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I

I

A

1 1

0

PHENYL KEL-F

KEL-F

I

I

I

30

60

90

I

0

10

20

i

1

MINUTES

Figure 5. Separation of aniline (A), N-methylaniline (MA), and N,N-dimethylaniline (DMA) on a 20 cm X 4.2 mm column packed with phenyl Kel-F 6061. Mobile phase: (80% 0.1 M KH,PO,, pH 8.0)-20% CH,CN; flow rate = 1.9 mL/min.

used. The good reactivity of Grignard reagents with Kel-F could be due to the absence of aggregation effects in THF (Negishi, 1980). Again the (CH3),SiCH2MgC1reagent reacted poorly with Kel-F, indicating probable steric hindrance for trimethylsilylmethyl compounds. The surface of Kel-F 6061 after reaction with CH,MgBr was not changed significantly with regard to pitting and pore formation. Apparently the reactivity of organomagnesium compounds is sufficient to functionalize but insufficient to alter greatly the polymer surface area. Chromatographic Studies. Organolithium or organomagnesium derivatized Kel-F 6061 polymers were found to be useful as HPLC column packings. Because Kel-F is very hydrophobic and stable toward mobile phases of extreme pH, it offers some definite advantages over reverse phase silica HPLC packings. A comparison of unreacted Kel-F with phenyl Kel-F for the separation of aniline, N-methylaniline, and N,N-dimethylaniline is shown in Figure 5. Figure 6 shows the separation of three other aromatic hydrocarbons on a HPLC column packed with CI4HmKel-F 6061. Again this separation was not possible using unreacted Kel-F and an identical mobile phase composition. The increase in separation capability of both modified Kel-F packings was probably the result of the increased surface area on which adsorption of the solutes may occur. In addition, the CI4Hz9chains may be of sufficient length to allow some partitioning of the solutes with the alkane layer. A narrower particle size range of the Kel-F packings would undoubtedly lead to sharper peaks and better resolution. Conclusion Reaction of Kel-F was demonstrated using either organolithium or organomagnesium reagents. Substitution of chlorine with an alkyl or phenyl group (R) was usually found for Kel-F 6061 after reaction with either compound class. A reaction mechanism for this process involving letransfer can be proposed as

I

T

I

0.008 a u

, 10

20

30

Time (min.)

Figure 6. Separation of p-nitroaniline (l),toluene (2), and phenanthrene (3) on a 15 cm X 4.1 mm column packed with C14Hn,Kel-F. Mobile phase: 90% methanol-10% 0.01 M tetrabutylammonium hydrogen sulfate; flow rate = 0.7 mL/min.

In general, the presence of fluorine will retard SN2reactions (Chapman, Levy, 1952). In addition, the polymer backbone should inhibit backside attack and preliminary experiments with good nucleophiles that are poorer reducing agents (NaNH2 and H202)showed a lack of reactivity. Radical anion species have been generated previously and found to exhibit some stability in fluorocarbon halides (Wang and Williams, 1980). It also should be noted that phenyllithium affords clean substitution which is incompatible with a 2-electron transfer. A 2-electron transfer mechanism is likely to be the case for the carbonization reactions involving Kel-F 6061 and Me,Si-Li or Me&-Li.

Those reagents which seem to afford carbonization products exhibit two basic characteristics. First, they are good reducing agents, capable of further reducing the radical anion. Secondly, they are sufficiently nonnucleophilic so as to minimize product formation from other modes. Both Me,SiLi and Me2SnLi are substantially more powerful reducing agents than the other organometallic reagents (Alnajjar and Kuivila, 1981; Stork et al., 1980; Still and Mitra, 1978). Both of these reagents were found to cause extensive carbonization of Teflon. However, no reaction was observed when Teflon was allowed to react with either organolithium or organomagnesium reagents. A carbanion p to fluorine should eliminate rapidly, too quickly to allow substitution (Johncock, 1969; Adcock and Renk, 1979). We feel a sequence of reduction/elimination cycles is responsible for the carbon-forming reactions. Acknowledgment The authors thank T. K. Wilson of Miami University for assistance with the electron microscopy. Grant support to N. D. Danielson and R. T. Taylor from the donors of the Petroleum Research Fund, administered by the American Chemical Society for this research was gratefully appreciated. One of us (N.D.D.) also wishes to thank the Research Corporation for support of this work. Funds for

Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 307-312

the purchase of the Perkin-Elmer Model 680 spectrometer were provided in part by the National Science Foundation through Grant TFI-8022902. Registry No. (CH3)3SnLi,17946-71-3;(CH3)@Li,18000-27-6; MeBr, 74-83-9;C8H1,Br, 111-83-1;CI4HBBr,112-71-0;CH2=CHC1, 75-01-4; Kel-F 6300, 9010-75-7; n-butyllithium, 109-72-8; methyllithium, 917-54-4; phenyllithium, 591-51-5;aniline, 62-53-3; N-methylaniline, 100-61-8; NJV-dimethylaniline, 121-69-7; p nitroaniline, 100-01-6;toluene, 108-88-3;phenanthrene, 85-01-8.

Literature Cited Adcock, J. L.; Renk, E. B. J. Org. Chem. 1979, 4 4 , 3431. Alnajjar, M. S.; Kuivlia, H. G. J. Org. Chem. 1981, 4 6 , 1053. Blllmeyer, F. W. “Textbook of Polymer Science”, 2nd ed.; Wlley-Intersclence: New York, 1971; Chapter 7. Brown, T. L. Acc. Chem. Res. 1968, 7 , 23. Brown, T. L. Pure Appl. Chem. 1970, 2 3 , 447. Chapman, N. 6.; Levy, J. L. J. Chem. SOC.1952, 1677. Deehan, D., personal communication, 3M Company, Nov 1981. Eastham, J. F.; Gibson, G. W. J. Am. Chem. SOC. I963a, 8 5 , 2171.

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Eastham, J. F., GJbson, G. W. J. Org. Chem. 1963b, 28, 280. Johncock, P. J. Organomet. Chem. 1969, 19, 257. Mark, H. F.; Gaylord, N. G.; Blkales, N. M., Ed. “Encyclopedia of Polymer Science and Technology”; Wiley: New York, 1967; Vol. 7, p 181. Neglshi, E. “Organometallics In Organic Chemistry”; Wlley: New York, 1960; VOl. I, p 93. Plzak, 2.; Dousek, F. P.; Jansta, J. J. Chromatogr. 1978, 147, 137. Riggs, W. M.; Dwight, D. W. J. Electron Spectrosc. Relat. phenom. 1974, 5 , 447. Smolkova, E.; Zlma, J.; Donsek, donsek, F. P.; Jansta, J.; Plzak, Z. J . Chromatogr. 1980, 791, 61. Still, W. C. J. Org. Chem. 1976, 4 7 , 3063. Still, W. C. J. Am. Chem. SOC. 1977, 9 9 , 4836. Still, W. C.; Mitra, A. Tetrahedron Lett. 1978, 2659. Stork, T. R.; Nelson, N. T.; Jenson, F. R. J. Org. Chem. 1980, 4 5 , 420. Wakefield, B. J. “The Chemistry of Organollthium Compounds”; Pergamon Press: Oxford, 1974; p 22. Wang, J. T.; Williams, F. J. Am. Chem. SOC. 1980, 102, 2660. Zwler, T. A.; Burke, M. F. Anal. Chem. 1981, 53, 812.

Received for review April 12, 1982 Revised manuscript received October 27, 1982 Accepted November 16, 1982

Recovery of Proteins from Whey R. R. Zall,‘ D. J. Dzurec,+ and J. H. Chen Depatiment of Food Science, Cornell University, Ithaca, New York 14853

By mildly heating milk to 74 O C for 10 s (thermalized but not pasteurized) prior to cooling and storing on dairy farms, it was possible to move some of the whey proteins to the casein fraction. Two types of bonding, disulfide bridges and calcium linkages, are partially responsible for binding some of the whey proteins to casein, so when cheese is made there is more protein available. I n addition, blanching or thermalizing reduces the amount of casein lost in making cheese, thereby resutting in hwer yields. Yields appear to increase approximately 5 % in cottage cheese, 4 % in cheddar cheese, and 2 % in quarg cheese.

Introduction The predominant protein in milk is casein. It exists in a colloidal form called a micelle. This micelle is composed of the three major types of casein: a,0,and K. The micelle is held together by several forces: hydrophobic, electrostatic, and hydrogen bonds (Webb et al., 1978). Other proteins exist outside the micelle in the serum phase of milk. These are primarily whey proteins, of which 0-lactoglobulin and a-lactalbumin are the most common. Soluble caseins also exist outside the micelle in the serum, primarily as 0-casein. Milk proteins are in a dynamic state even while still in the mammary tissue of the cow. When milk is harvested, milk proteins undergo many changes. When milk is cooled, caseins become more soluble and migrate out of the micelle (Downey and Murphy, 1970). When milk is heated, whey proteins are denatured and tend to associate with one another and with casein (Smits et al., 1980). The general trend is for the amount of casein to increase with increasing temperature of heat treatment and for the amount of whey protein to decrease with increasing temperature (Ramos, 1978). Many studies have shown that the association of casein and whey proteins occurs after a heat treatment, but there have been uncertainties in their explanation of the mechanism or bonds involved between the proteins. Di*Address inquiries to this author at the Department of Food Science, Food Science Annex, 147 Riley-Robb Hall, Cornell University, Ithaca, NY 14853-0364. ‘Oberlin Farms Dairy, Inc., 310 Chester Street, Painesville,OH 44077. 0196-4321183f 7222-0307$07.50/0

sulfide bridging, calcium linkages, hydrophobic bonding, and hydrogen bonding have been shown to be involved in these associations (Smits and VanBrouwershaven, 1980). However, in these studies, milk was heated to high temperatures, higher than 85 “ C , and for long periods of time, longer than 5 min. An early study (Harland and Ashworth, 1945) showed that heat treatments equal to 80 “ C for 45 min or higher would denature 93-95% of the whey proteins. Later studies have shown the relatively heat labile whey proteins to be completely denatured by the following heat treatments: 77.5 “C for 60 min, 80 “C for 30 min, and 90 “ C for 5 min. The thermal stability of whey proteins in increasing order is: immunoglobulins, serum albumin, alactoglobulin, and 0-lactalbumin (Hillier and Lyster, 1979; Fox, 1981). High-temperature heat treatment (>85 “C) to milk is known to increase cheese yield, but textural quality is impaired since fragile curd and excessive fines are produced (White and Ray, 1977). Storage of milk prior to pasteurization leads to poor quality cheese. At the same time, yields are decreased. Therefore, by tradition, heat treatment and storage of milk used for cottage cheese manufacture is kept to a minimum (Emmons and Tuckey, 1967). A storage period prior to processing allows psychrotrophic bacteria to grow. This bacterial growth negatively affects milk quality and cheese yield. To overcome this problem, Zall and Chen (1981) showed that milk could be heat-treated, cooled, and then stored on farms before pickup. The practice would be beneficial to fluid and cultured milk quality and was deemed to be so after testing the idea in a study over a two-year period. By heating milk to 74 “ C (165 O F ) for 10 s, psychrotrophic 0 1983 American Chemical Society