L A B O R A T O R Y S T U D Y O F F A C T O R S I N F L U E N C I N G

M I L K S T O N E F O R M A T I O N ~

THADDEUS LEWANDOWSKI 2

Pennsalt Che~nicals Corporation, Research and Development Division, Wyndmoor, Pennsylvania

SUMMARY

Through use of an Autotechnicon, an instrument which dips test pieces into and out of a series of solutions, a reproducible method for forming milkstone on stainless steel was developed. Milkstone quanti ty increased with increase of temperature of water used to rinse milk; increasing the drying time before or af ter this rinse did not influence nfilk- stone formation. Interrelated chemical factors involved in producing milkstone were milk, water hardness, chlorine solutions, and cleansers. Wa te r hardness was singly the most important factor and a nonlinear increase of milkstone with increasing water hardness was demonstrated. There was little difference in milkstone amounts formed either with sodium or with calcium hypochlorite; formation of less milkstone with chlorinated trisodium phosphate possibly could be at tr ibuted to water softening caused by hard-water salt precipitation. Of the cleansers tested, least milkstone was produced with an acid, slightly more with a neutral nonionic detergent, still more with one con- taining tr isodium phosphate and pyrophosphate. Comparison of synthetic detergents indicated that a cationic helped produce about twice the milkstone formed with a non- ionic or an anionic. Inorganic cleanser constituents producing clear hard-water solu- tions helped form less milkstone than those producing cloudy solutions.

The w o r d mi lks tone has been used to descr ibe c e r t a i n depos i t s f o r m i n g on d a i r y e q u i p m e n t be ing u t i l i zed in bo th p r o d u c t i o n a n d p rocess ing of m i lk and

milk products. Milkstone has been variously described as any contamination- defying, routine, daily cleansing methods (9), a product resulting from inter- action of heat-precipitated milk film, chemical constituents of the water sup- ply, a n d a lka l i ne c leansers (6, 8), a p r o d u c t of h e a t - p r e c i p i t a t e d mi lk sol ids

a n d va r i ous chemical a n d p h y s i c a l f ac to r s (5), poss ib ly as a th in , meta l l i c - l ike , dense film r e s u l t i n g f r o m i n t e r a c t i o n of heat , a i r , a n d mi lk (3), a n d las t ly , as a r e su l t of i m p r o p e r c l eans ing (1, 2, 5-9). The p r e v e n t i o n or r e d u c t i o n of

mi lks tone f o r m a t i o n poss ib ly m a y be accompl i shed t h r o u g h use of p r o p e r l y f o r m u l a t e d c leansers a n d efficient c l eans ing me thods (1, 4, 7).

L a b o r a t o r y mi lk f i lms have been p r e p a r e d which e l u c i d a t e d the effect of severa l p h y s i c a l f ac to r s on film f o r m a t i o n (3) a n d the i n t e r a c t i o n of m i lk fihns wi th a lka l i ne c leanser i n g r e d i e n t s (6). The l a b o r a t o r y m e t h o d desc r ibed in

th is p a p e r s imu la t e s not on ly t y p i c a l c l eans ing p r o c e d u r e s b u t i nc ludes t r e a t - m e n t of s t a in less s teel w i th w a t e r r inses a n d s a n i t i z i n g compounds , so t h a t the effects of these m a t e r i a l s also could be s t u d i e d a n d eva lua t ed . Thus , b y inclu- s ion of a l l or most of the k n o w n fac tors , a n d b y s t u d y i n g the r e l a t i ve i m p o r t a n c e of each fac to r , i t was h o p e d t h a t a p r a c t i c a l a p p r o a c h to p r e v e n t i o n of mi lks tone

f o r m a t i o n could be f o r m u l a t e d . Based on the r e su l t s ob ta ined , seve ra l possi- b i l i t i es fo r p r e v e n t i o n or r e d u c t i o n of mi lks tone f o r m a t i o n seem feas ib le a n d a re d i scussed l a t e r in th is pape r .

Received for publication September 3, 1957. J This research was dane for Pennsalt Chemicals Corp., at Whitemarsh Research Labora-

tories, and published with their permission. 2 Bonewitz Chemicals, Inc., Burlington, Iowa.

249

250 THADDEUS LEWANDOWSKI

MATERIALS AND 3/IETI-IODS

The instrument used was the Autotechnicon (The Teehnieon Company, New York City), designed primari ly for automatic staining of histological tissue sections. The Autoteehnieon consists essentially of a number of metal arms rotat ing slowly in a clockwise direction while ascending' and descending into va- rious positions, eoming to rest after descent to each position. Type 302 stainless steel strips with a No. 4 polish on one side, 3 in. by 1 in. in size, were cleansed first with an acid cleanser and then with art alkaline cleanser, thoroughly rinse.d in distilled water, and dried. The strips were then dipped into carbon to.trachloride, heated at 100-105 ° C. for :½ hr., cooled in a desiccator for at h~ast ~/z hr., and weighed on an analytical balance. Pour strips were suspended on hooks in pairs, from two opposing arms of the Autotcehnieon, so that the lower 2 in. of the strips were consecutively dipped into beakers containing: (a) 200 p.p.m, available chlorine solutions at room temperature; (b) raw tank- mixed whole milk, initially at 37 ° C. and allowed to cool to room tempera. ture dur ing the test; (c) water at room temperature used as the first, rinse; (d) cleanser solution at 49 ° C., at 0.25% concentration unless otherwise indi- cated, and (e) a second water-rinse at 49 ° C.

The exposure time of each strip to each liquid was 70 see., and the transit time from liquid to liquid was 35 see. This sanitizing-cleansing cycle was repeated 30 times, the milk being replaced with a fresh supply at 37 ° C. after 15 eyeles. The milk was stirred mechanically dur ing each cycle, about 5 min. before the strips were dipped into it, by means of a stainless steel stirrer attached to an arm of the Autoteehnieon.

At the end of the exposure period, the strips were held at room temperature for 10 min. The strips were then treated by total immersion in ethanol, ethyl ether, ethanol, ethyl ether, and petrolemn ether, consecutively, the immersion time in eaeh solvent being 1 min. The strips were dried at 100-105 ° C. for :~z hr., eooled in a desiccator for at least 1~ hr., and weighed, the milkstone quant i ty being determined by weight difference. All of the results shown are the average total amount of milkstone on four strips; at least two runs were made, to give an average figure. The strips were handled with clean, dry forceps throughout the procedure.

Rinses and solutions were composed of, or prepared in, water of 300 p.p.m. hardness calculated as caleimn carbonate, unless otherwise indicated. The water was prepared with calcium chloride and magnesium sulfate with a Ca:Mg ratio of 3:1. Water hardness was determined by the Betz titration method (W. H. and L. D. Betz, Philadelphia, Pa.).

RESULTS

The method just described, for determining amounts of formed milkstone, is limited to s tudy of one condition dur ing a single test, and does not allow the use of parallel control determinations. For this reason, it was necessary to establish the limits of error of the method. Using the same samples of calcium

~V[ [ L K S T O N E F O R M A T I O N 251

hypochlor i te as sani t izer , a n d a commercia l a lka l ine c leanser (Cleanser A,

Table 3) t h roughou t , n ine s ingle tests were done at i n t e r va l s d u r i n g a 2-hr.

period. The results , expressed as to ta l mi l l i g r ams of mi lks tone on fou r s tr ips ,

were: 39, 40, 42, 30, 38, 44, 33, 44, a n d 43. I t m a y be seen t h a t the resu l t s

u sua l l y were reproduc ib le w i t h i n a few mi l l ig rams , b u t t ha t occasional ly a

low figure (30, 33) would be obta ined. I t was ev iden t t h a t for compara t ive

purposes two or more test t r i a l s were necessary, in order to ob ta in a re l iable

average f igure ; therefore, all the resul ts r epor t ed below were der ived on

this basis. Resul t s of a s t u d y of the effect on mi lks tone f o r m a t i o n of r inse t empera -

tu res and d r y i n g time, before or a f t e r the first r inse, are shown (Table 1).

TABLE 1 Relation of temperature of rinses and drying time, before and after first rinse,

to milhstone formation ~

Temperature Time of drying Av. 1st rinse 2nd rinse Before 1st rinse After 1st rinse milkstone

(sec. ) (rag.) Room 49 ° C. 35 35 40 49 ° C. Room 35 35 74 Room 49 ° C. 35 10 32 Room 49 ° C. 10 35 41

The sanitizing-cleansing cycle included calcium hypoehlorite and 0.25% of a commercial cleanser composed of trisodium phosphate, pyrophosphate, carbonate, and an anionic synthetic detergent.

The da ta ind ica te tha t increase of the t e m p e r a t u r e of the first r inse resu l t ed

in a marked increase in the a m o u n t of milkstone. I n c r e a s i n g the t ime of

d r y i n g a f te r the first r inse caused the f o r m a t i o n of s l i gh t ly less milks tone,

b u t an increase in the d r y i n g t ime before the first r inse caused no change in

mi lks tone format ion .

To de te rmine the effect on mi lks tone f o r m a t i o n of each componen t of the

san i t i z ing -c l eans ing cycle, one or two componen t s were rep laced wi th h a r d water .

The resul ts (Table 2) ind ica te t h a t r ep l a c e me n t of cleanser, ca lc ium hypo-

TABLE 2 Sanitizing-cleansing cycle component effect on milkstone formation

Calcium Hard 0.25 % Av. hypochlorite water Milk cleanser b milkstone

(rag.) Yes Yes Yes Yes 40 Yes Yes Yes No c 20 No Yes Yes Yes 17 Yes Yes No Yes 21 No Yes Yes No 5 Yes None~ Yes Yes 5

"None" signifies that distilled water was used for the solutions and rinses, in place of hard water.

b The commercial cleanser used was a mixture of trisodimn phosphate, pyrophosphate, carbonate, and an anionic synthetic detergent.

"No" signifies that the component was replaced with hard water.

25~ THADDEUS LE\VANDOWSKI

chlorite, or milk produced about one-half the milkstone formed with all the components present in the system. Elimination of both ealeium hypoehlorite and cleanser, or replacement of the hard water in the entire system with distilled water and keeping all other components, resulted in very little depo- sition of milkstone.

As water hardness was shown to cause the greatest change of any of the single components of the cycle, a fur ther s tudy of this factor was made. Using (~alcium hypo(dflorite and alkaline Cleanser A (Table 3), it was found that

T A B L E 3

M ill~stone formation with different commercial clean.sers"

Av. Cleanser Cleanser eomposit,ion milkstone

A B C D

(m.q.) Trisodium phosphate, pyrophosphate, carbonate, anionic detergent 35 Polyphosphate, carl)onate, anionic detergent 22 Neutral nonionic detergent 14 Phosphoric acid, nonionic detergent 9

" The sanitizing-cleansing cycle included a commercial calcium hypochlorite compound and a 0.25% concentration of the listed cleansers.

at zero p.p.m, water hardness, 5 mg. of milkstone formed, at 150 p.p.m, hard- hess, 14 rag., and at 300 p.p.m, hardness, 40 rag. was obtained, tt became evident that the amount of inilkstone increased in a nonlinear fashion with increasing water hardness.

A s tudy of the effect of different types of commercial chlorine compounds used with alkaline Cleanser A (Table 3), showed that slightly more milkstone resulted from the use of calcium-free sodium hypochlorite than from use of calcium hypoehlorite; whereas, least milkstone was obtained through the use of chlorinated trisodium phosphate. Actual figures were 38 rag. with sodium hypoehlorite, ;-15 ing. with calcium hypoehlorite, and 30 rag. with chlorinated trisodium phosphate. It should be pointed out that the sodium hypoehlorite solution was clear and that the calcium hypoehlorite formed a cloudy solution; whereas, the chlorinated trisodium phosphate caused a voluminous precipitate to form in the hard water, with a resultant softening of the supernatant. The demonstrated marked effect of water hardness on milkstone formation may, at least partially, explain the low result obtained with chlorinated trisodium phosphate.

A comparison of the amounts of milkstone obtained through the use of various types of eommereial cleansers was made (Table 3). This experiment indicated that the most milkstone resulted through the use of a cleanser containing trisodium phosphate and pyrophosphate (Cleanser A), less milk- stone produeed with a cleanser containing polyphosphate (Cleanser B), still less with a nonionic detergent cleanser containing no hard-water precipitating' salts (Cleanser C), and that least milkstone was obtained with an acid cleanser (Cleanser D).

MILKSTONE FORDIATION 253

In order to elucidate the role of individual cleanser chemicals, different types of organic synthetic detergents were compared. Using the detergents at a concentrat ion of 0.01% and calcium hypochlori te as the sanitizer, the average mill igrams of mi]kstone produced with a lkyldimethylbenzyl ammonium chlo- ride, a cationic, was 33, with an a lkylary l sulfonate (anionic) , 15, and with a nonionic a lkylary l polyether alcohol, 14 mg. Thus, it may be seen tha t the cationic detergent helped produce over twice the amount of milkstone that the anionic or nonionic detergents produced, the la t ter two being about equal.

Invest igat ion of the role of possible inorganic consti tuents of cleansers indi- cated that sodium bicarbonate and sodium hexametaphosphate , at the concen- trat ions used, caused no more milkstone than the control hard water (Table 4).

TABLE 4 Effect of inorganic components of alkaline cleansers on milkstone formation ~

Cleanser solution Av. Inorganic component at 49 ° C. milkstone

None (hard-water control) Sodium bicarbonate Sodium hexametaphosphate Sodium tripolyphosphate Sodium pyrophosphate Sodium bicarbonate plus sodium hexametaphosphate Sodium bicarbonate plus sodium tripolyphosphate Sodium bicarbonate plus sodium pyrophosphate

(mg.) Clear 22 Clear 22 Clear 22 Cloudy 25 Cloudy 33 Clear 24 Cloudy 28 Cloudy 28

~The sanitizing-cleansing cycle included calcium hypochlorite and the listed inorganic chemicals each used as cleansers at 0.075% concentration.

Sodium t r ipolyphosphate and sodium pyrophosphate , at concentrations less than necessary for complete softening of the hard water as indicated by the cloudy solutions, helped to produce more milkstone than the water control, the pyro- phosphate being the worst offender. Combinat ion of sodium bicarbonate and phosphates indicated tha t the least milkstone was produced with sodium hexa- metaphosphate ; whereas, with sodium t r ipolyphosphate and sodium pyrophos- phate more milkstone was formed. I t may be significant that clear solutions of the cleanser ingredients in hard water, indicat ing complete softening or no precipi ta t ion of hard-water salts, produced approx imate ly the same amount of milkstone as the water control, whereas cloudy solutions produced milkstone in marked excess over the control.

DISCUSSION

The present s tudy clearly confirmed the common belief that the fo rmat ion of milkstone is a complex process involving many interre la ted factors. The delineation of the effects of individual chemical factors, such as cleansers and cleanser ingredients, chlorine sanitizers, water hardness, and milk itself, emphasized that each played its role, dependent to various extents on the other materials.

254 THADDEUS LEWANDOWSKI

The most i m p o r t a n t s ingle chemica l f a c t o r i nvo lved in mi lks tone f o r m a t i o n

was shown to be w a t e r ha rdness . As w a t e r h a r d n e s s is a wel l known, a l t h o u g h not e n t i r e l y unde r s tood , en t i t y , i t s p r e d o m i n a n c e in i n f luenc ing mi lks tone for-

m a t i o n s u g g e s t e d seve ra l r a t i o n a l a p p r o a c h e s to the so lu t ion of the p r o b l e m of p r e v e n t i n g or r e d u c i n g mi lks tone depos i t s . A s l i t t l e d i f fe rence could be dem- o n s t r a t e d be tween va r i ous t y p e s of ch lo r ine compounds , modi f ica t ions of these m a t e r i a l s seemed to offer, a t best, on ly a p a r t i a l so lu t ion to the p rob lem.

A more f r u i t f u l a p p r o a c h seemed to be (me d i r e c t e d at the p r o p e r fo rmu- la t ion of d a i r y c leansers . The use of p o l y p h o s p h a t e s in a m o u n t s sufficient to

p r o d u c e c lear h a r d - w a t e r solut ions , in p lace of t r i s o d i u m phospha te , py rophos - pha te , or poss ib ly o the r h a r d - w a t e r p r e c i p i t a t i n g sal ts , was p a r t i c u l a r l y no tab le in r e d u c i n g the a m o u n t of mi lks tone fo rmed . I t shou ld be rea l i zed t h a t a l imit ,

bo th chemica l a n d economical , can be r eac he d in the a m o u n t of w a t e r ha rdnes s t h a t m a y be so f t ened b y means of p o l y p h o s p h a t e s (or o rgan ic seques te r ing agen t s ) i n c o r p o r a t e d in a lka l i ne cleansc~rs. I t is poss ib le tha t , in v e r y h a r d wa te r , p r o p e r l y f o r m u l a t e d a lka l ine c leanse r s m a y be used in c o n j u n c t i o n wi th

a chemica l or p h y s i c a l w a t e r - s o f t e n i n g process. The use of the l a t t e r process, however , has l i m i t e d a p p l i c a t i o n because of space a n d cost cons idera t ions . A more p r a c t i c a l so lu t ion seems to be the u t i l i za t ion e i the r of n e u t r a l d e t e r g e n t c leansers c o n t a i n i n g no h a r d - w a t e r p r e c i p i t a t i n g sa l ts or of ac id c leansers , as

c leansers of bo th t y p e s m a r k e d l y r e d u c e d mi lks tone fo rma t ion .

I t was d e t e r m i n e d also t h a t the t e m p e r a t u r e of r inse wa te r s p l a y e d a l a rge role in mi lks tone fo rma t ion . The r e su l t s r e p o r t e d here p a r a l l e l the common

field expe r i ence tha t , i f m i lk is r i n sed f r o m e q u i p m e n t su r f aces w i th ho t wa te r , more mi lks tone fo rms t h a n wi th a co ld -wa te r r inse. No evidence for a nmrked effect on mi lks tone f o r m a t i o n by d r y i n g of mi lk films a t room t e m p e r a t u r e was f (mnd.

REFE]~ENCES

(1) ELLIKER, P. R. l)raetical Dairy Bacteriology. 1st ed. McGraw-Hill Book Co., New York. 1949.

(2) HA~DINe, H. G., aND TR.EDLt~tt, H. A. Detergents for Dairy PIants and Methods of Their Evaluation. Food Technol., 1: 478. 1947.

(3) JOHNSON, J. J., AND ROLAND, C. T. Study of Dairy Cleaning Problems. I. l~ilms and Deposits on Hot-Milk Equipment. J. Dairy Sci., 23: 457. 1940.

(4) JO]=INSON, J. J., AND ROLAND, C. T. Study of Dairy Cleaning Problems. II . Effectiveness of Alkalies in Removing Heat Deposited Milk Solids and Butterfat Films. J. Dairy Sci., 23: 463. 1940.

(5) PARKEI~, ~/~. E., AND JOHNSON, A. H. Notes on the Prevention and the Removal of Milk- stone. Froe. Intern. Assoc. Milk Dealers, 23: 5. 1930.

(6) PIPER, P. E. Milkstone: I t ' s Double-Trouble if Not Nipped Fast. Food Inds., 22: 87. 1950.

(7) RO~ADtIOUSE, C. L., AND tIENDEESON, J. L. The Market Milk Industry. 2nd ed. McGraw- Hill Book Co., New York. 1950.

(8) SCIIWARTZ, C. Detergents in the Dairy Industry. J. Milk Teehnol., 4: 258. 1940. (9) SH~,;RE, L. HOW to Prevent and Remove Milk Deposits. Food Inds., 14: 44. 1942.