Sanitary Value of
Sodium Metaphosphate in Dishwashing
GEORGE 0. HALL Mellon Institute of Industrial Research, Pittsburgh, Pa. CHARLES SCHWARTZ Hall Laboratories, Pittsburgh, Pa.
A bacteriologic study is presented of the sanitational value of sodium metaphosphate in machine dishwashing. Results obtained in restaurants under routine operating conditions indicate the superior sanitary quality of dishes washed with an alkaline detergent containing sodium metaphosphate. This superiority is not attributed to its germicidal action but to its specific function in preventing the formation of films of insoluble alkaline earth salts and soaps. The use of a film-preventive agent in improving sanitation is a novel one.
soluble complexes with calcium and magnesium ions in alkaline solution, is able to prevent completely the precipitation of alkaline earth soaps and salts responsible for film formation. The introduction of sodium metaphosphate as an ingredient of a mechanical dishwashing compound (8) has made possible for the first time the production of physically clean dishes on a commercial scale (6). Inasmuch as alkaline earth films would seem to afford an ideal environment for bacterial growth, it was considered desirable to make a study of the relative sanitary condition of dishes washed with a compound having no specific action (other than precipitation) on the calcium and magnesium ions of water, and of similar dishes washed with a compound containing sodium metaphosphate to ensure complete freedom from alkaline earth deposits. I n brief, the investigation was undertaken to ascertain whether dishes that were cleaner in the physical sense were also cleaner in the sanitary sense.
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N ORDER for dishes to be clean in the physical sense, they must not only be washed free from food debris that soils them, but they must also be freed from insoluble deposits resulting from the interaction between the washing agents Experimental Work (soap, trisodium phosphate, sodium silicate, sodium carbonate, The experimental work may be divided into two parts: etc.) and water containing more than traces of calcium and (a) the determination of the germicidal activity of some magnesium. I n spite of the recognized efficacy of these widely used cleaners by the modified Reddish technic (7) and detergents, they have one failing that is common to all. (b) tests of these cleaners in standard dishwashing machines. When they are added to ordinary water the corresponding alkaline earth soaps and sa1.t~are precipitated. Although The following cleaners were used: a trisodium phosphate preparation (T. S. P.), sodium hydroxide (NaOH), and a soap is not ordinarily an ingredient of commercial dishwashing cleaner that will be referred to as C , a highly alkaline material comDounds. it functions indirectly in mechanical dishwashing. Praitically 'all food debris contins fatty matter which reacts with the alkaline detergent to form alkali soap that is effective until it is precipitated TABLEI. COMPARISON OF GERMICIDAL POWEROF VARIOUS CLEANERS as insoluble, sticky, alkaline earth soap. These 0-o c. 50' C. precipitates are adherent and accumulate as film 1 min. 5 min. 10 min. 15 min. 1 min. 5 min. 10 min. 15 min. and stains on dishes and as thick scale on the Staphylococcus aureus inside of commercial dishwashing machines. Water 11,200 10,870 11,140 7,680 . . . 8,220 7,140 4,860 4,010 3,300 1,500 740 660 0.25% C 3,000 4',080 1,440 4,740 780 380 3,000 648 420 340 I n view of the mechanism of film formation 0.50% C as advanced here, it is apparent that the only way 3.0 NaOH 0 1 2 0 5 0 1 0 0.25% T. S. P. 9,960 9,900 9,720 7,860 6,900 858 300 200 in which the washing of dishes could be accom0.50% T. 9. P. 8,100 2,430 1,920 1,280 2,280 600 240 87 plished without leaving a residue would be to E. coli employ a completely softened water. This con8,740 . . . 7,120 6,810 5,400 . . .0 . . .0 3,000 Water 15 0 0 0 2 0 0.25% C dition is not readily realizable in practice and 1 0 0 0 0 0 0 0 NaOH 0 0 0 1 0 0 0 0 has not seemed necessary to even the most T. S. P. 4,110 3,860 2,280 1,380 198 79 6 enlightened operators of mechanical dishwashing 197 74 52 16 3 5 48 2 0 equipment. B . tuwhosus 6,180 . . . . .3. 5,170 6,420 6,310 4,128 3,600 Recent advances in the chemistry of water Water 185 3 0 0 0 0 0 0 257' C conditioning have led to the development of a 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 new material and a new process (4, 5 , 9) for the 0.25'% T. S. P. :1;$42OH 4,700 1,080 514 310 450 155 47 20 0.50% T. S. P. 115 11 0 0 0 0 0 0 softening of water. Of special interest here is the fact that sodium metaphosphate, by forming 7 7
~
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I.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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containing sodium hexametaphosphate, caustic soda, sodium metasilicate, and trisodium phosphate, evolved through research for use in mechanical dishwashers. Trisodium phosphate was chosen as one of the cleaners to be tested because it is probably the most widely used single material in dishwashing today. Sodium hydroxide, although not employed very widely in dishwashing, is a t present accepted as a germicidal detergent, especially when used in mixtures with trisodium phosphate or sodium carbonate at concentrations of approximately 3 per cent total alkali (1). The germicidal tests of cleaners were run in accordance with the modified Reddish technic, using as test organisms Staphylococcus aureus, B. coli, and B. typhosus. The plate counts obtained are given in Table I. They indicate, as might be expected, that of all the cleaners tested 3 per cent sodium hydroxide is the most uniformly efficient germicidally. T. S. P. and C are not very effective with S. aureus. With B. coli and B. typhosus, however, 0.25 and 0.50 per cent solutions of C are just as good as 3 per cent solutions of sodium hydroxide a t 50" C., and not quite so good a t 40" C. These results, in general, agree with those reported by Devereux and Mallman (3), who found that Calgonite (the material C of this paper, containing sodium hexametaphosphate) had very little effect on S. aurew, but was quite effective on Esch. coli. These investigators, however, reported neither temperature nor concentrations. The general conclusion from the results indicated in Table I may be that, considered from a purely bactericidal aspect, 3 per cent sodium hydroxide is the best of the materials tested.
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Tests in Laboratory Dishwashing Machine EXPERIMENT 1. Forty-five clean Petri dishes were soiled by filling them with ordinary bottled milk. The milk was poured off and the dishes, in the open and uncovered, were allowed to dry for 24 hours a t room temperature. At the end of this time, fifteen of the dishes were washed with water alone in accordance with the technic (8) described previously, . using a 30-second preliminary spray, a 3-minute wash, and two 30-second rinses. Fifteen more of the soiled dishes were washed in a similar manner, using in the wash T. S. P. in a concentration approximating 0.23 per cent. The last fifteen dishes were washed in the same manner, using the same concentration of C. The water temperature was between 55" and 60" C. A t the end of the final rinse, one dish was taken from each set, filled with nutrient agar, covered, and incubated for 48 hours a t 37" C.; then the colonies were counted. The remaining fourteen dishes in each set were resoiled as described and rewashed the following day, when another dish from each set was removed for incubation. This procedure was continued for 15 days in exactly the same way. The average hardness of Pittsburgh city water for this 15-day period was 115 p. p. m. calcium carbonate. The results are presented in Table 11. COUNTS ON DISHES REPEATEDLY SOILED WITH TABLE 11. PLATE MILK AND WASHED Day
Water 23 20 300 75 29 16 20 400 172
T. 5. P. 22 20 57 33 5 10 8 24
37
C 15 15 4 30
3
8 8 12 9
Day 10 11 12
13 14
15 Av.
Water 400 150 ' 30 400 260 200 166
T.S. P. 20 15
6 18 16 6
20
C 3
4 3 9 10 4 9
EXPERIMENT 2. Four Petri dishes were soiled in the same manner by filling with milk, emptying, and drying uncovered a t room temperature for 24 hours. This soiling was repeated for 15days without any washing during the whole period. By
VOL. 29, NO. 4
the fifteenth day each of the dishes was covered with a hard,. dry crust. On the sixteenth day they were washed in the way described, with water, T. S. P., and C, respectively. Each plate was carefully placed in exactly the same position in the machine, so that the water force during each of the washings would be identical. Several large pieces of dried milk residue remained on the water-washed dish. The dish washed with T. S. P. held a few pieces, about 1 sq. cm. in area; the dish washed in C was free from any soil whatever. The dishes. were covered with nutrient agar and incubated for 48 hours, and then the colonies were counted (Table 111). The hardness of the water was 118 p. p. m. calcium carbonate. TABLE111. PLATECOTJNTSON DISHES ACCUMULATIVELY SOILEDWITH MILKAND WASHED Water plate T. S. P. plate C plate
TABLE IV.
300 colonies 75 oolonies 9 colonies
PLATE COUNTSON DISHES SOILEDWITH B. prodigioSUS-INOCULATED MILKAND WASHED Five water late8 Fjve T.9. plates Five C plates
138 colonies 142
8
9
TABLE V. PLATECOUNTSON DISHESSOILEDWITH MOUTH WASHINGS AND INOCULATED MILKAND THENWASHED Day 1
2 3 4 5 6 Av.
Water 160 279 317 10 4 58 136
T. €3. P. 25 19 5 3
2 50
17
C 30 10 4 2 2 10 10
On the first two dishes the presence of pieces of unremoved soil made an accurate count impossible; therefore these data are to be regarded as minima. The dish washed with C was perfectly clean and hence the figure is accurate as given. EXPERIMENT 3. Sterile whole milk was inoculated with B. prodigiosus. This organism ordinarily withstands a temperature of 60" C. and is easily differentiated from others because of its red color. Sterile Petri dishes were soiled with 1.5 cc. of the inoculated milk and allowed to dry for 24 hours. A count of the bacteria present in the milk, made a t the time of soiling by the dilution-plate method, showed that each plate was inoculated with over 54 million bacteria. Naturally the bacteria multiplied rapidly in the incubator. which brought the inoculum on each plate into the billions. One plate a t a time was washed in the manner described, and each plate was carefully placed in the same position in the machine. A total of five plates was washed with each of three materials. These plates were then covered with nutrient agar and incubated for 48 hours, when plate counts were made. The hardness of the water used was 120 p. p. m. calcium carbonate. Average results are given in Table IV. Such counts would of course include any organism in t h e rinse water; but counts on sterile dishes run through the same procedure with water alone failed to develop colonies, showing the absence of organisms in the water. EXPERIMENT 4. Petri dishes were soiled with milk inoculated with washings obtained by rinsing the mouth with sterile salt solution; 1.5 cc. of the inoculated incubated milk was added to each dish, which was allowed to dry for 24 hours. The dishes were then washed as described. The average hardness of the water during this experiment was 124 p. p. m. calcium carbonate. After washing, the dishes were filled with nutrient agar and incubated for 48 hours, after which time the colonies were counted. The results are given in Table V.
APRIL, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
It is interesting to note that on the fifth day the plates were rinsed in sterile distilled water. On the sixth day the plates were not rinsed a t all. EXPERIMEKT 5. Forty Petri dishes were soiled with milk that had been previously inoculated with B. subtilis. The dishes were dried in an incubator a t 37.5" C. for 24 hours. Ten of them were washed in 3 per cent sodium hydroxide in the manner previously described; another set of ten was washed in 0.25 per cent C; a third set was washed once in 3 per cent sodium hydroxide, followed by another washing in 0.25 per cent C; the remaining ten dishes were washed in 3 per cent sodium hydroxide. The average hardness of the water during this experiment was 85 p. p.m. calcium carbonate. The plates were covered with nutrient agar and incubated for 48 hours, when the colonies were counted. The results shown in Table VI were obtained. Tables I1 to V indicate results which are not surprising. These findings show that the metaphosphate cleaner, C, yields dishes that are cleaner biologically than those washed with either water or T. S. P. under similar conditions of soiling and procedure. Figure 1 presents graphically the relation between the results of the various experiments described. The dishes washed in plain hot water had on them considerable numbers of bacteria. Those washed with T. S. P. had fewer colonies, and those with C had still less, as might be expected from the results given in Table I and from a knowledge of the cleaning properties of this material. If the numbers of colonies counted on plates washed in all four of these experiments with each of the three materials are averaged, we find that the average number of colonies on the water-washed dishes was 185, on the T. S. P.-washed dishes, 63, and on the C-washed dishes, 9. If we arbitrarily assign to the water a value of 0 per cent bateria removal and destruction, we find that the T. S. P. removed or killed 66 per cent of the bacteria and that C removed 95 per cent. When viewed in conjunction with each individual experiment, this averaging is indicative, at least in a broad sense, of the difference in results obtained with these materials. TABLE VI. PLATECOUNTS ON DISHES SOILED WITH B . subtilisINOCULATED MILKAND WASHED Washed Washed Washed Washed
Treatment of 10 Plates once in 0 . 2 5 7 C once in 3 GaOH once in 3 9 NaOH, once in'0.24% C twice in 3% NaOH
Av. Count
1 16 2 4
The experiments on dishwashing thus far described show merely that the use of C leads to more sanitary dishes, but they do not provide an explanation of this improvement. C may give better dishes, either because it effects physically cleaner dishes than T. S. P., as has been pointed out, or because it is a better germicide than T. S. P., which may be gathered from the data in Table I. Experiment 5, which was a comparison between 3 per cent sodium hydroxide solution and 0.25 per cent C solution, indicates that in actual washing operations the cleaning ability of a material is just as important as (if not more important than) its germicidal power. Three per cent sodium hydroxide solution has for some time been regarded as an efficient germicidal cleaner and is proving very useful in the field of beverage bottle washing. The results in Table I indicate, too, that this material is a better germicide than any of the others tested. Yet in experiment 5, where Petri dishes were soiled with milk inoculated with B . subtilis, the comparatively low concentrations of C that were used gave results far superior to those obtained with 3 per cent caustic solutions. Owing to the fact that the actual bactericidal efficiency of sodium hydroxide is higher than that
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of C, it must follow that the dishes washed with 3 per cent caustic were not so clean as those washed with C. This result, therefore, is attributed to the fact that sodium hydroxide is inferior to C as a detergent, rather than to the fact that it leaves a residue, because sodium hydroxide in water of the character of Pittsburgh water would lead less to film formation than the other commonly used alkalies. This reasoning would not necessarily apply to a water containing bicarbonate
RESULTS OF TABLE=
RESULTS ff TABLE I It
RESULTS OF TABLC
nr
RESULTS Of TA0LEP
AVLRAW5 RESULTS
hardness. At present, work is being continued for the purpose of determining whether in continuous large-scale operations, such as exist, for example, in dairies, the commonly used 3 per cent alkali solutions may be replaced with more dilute solutions of C with consequent improvement in results
Tests in Restaurants These experiments were performed in two large restaurants, each of which was equipped with a modern mechanical dishwashing machine. Both machines were typical of those usually met in first class restaurants, and the operating conditions of the two establishments were believed to be average and representative. Thorough examinations of the machines indicated that they were in good operating condition before the test period was started. No changes in procedure or operation were suggested by the investigator, because, for the purpose of this work, the results of ordinary practice were desired. At the time the work was begun in restaurant A, there was being used a common dishwashing material composed almost entirely of trisodium phosphate. The machine used in restaurant A was a single-tank machine of the automatic conveyor type with direct-connected rinse. During wash and rinse the dishes were sprayed both from above and below. The average temperature of the wash water was 60" C., that of the rinse water was 82" C., and the average time of exposure to the hot rinse was one minute. Ten to fifteen dishes were collected daily and at random as they emerged from the dishwashing machine; they were carefully placed in previously sterilized paper bags and brought back to the laboratory; they were removed from the bags and on them were poured 35 cc. of nutrient broth. The dishes were immediately covered with previously sterilized pie tins and placed in an incubator for 2 hours. At the end of this time 1 cc. of broth from each dish was plated on agar, and after an incubation period of 48 hours the colonies were counted. When two weeks had elapsed, C was substituted for the trisodium phosphate cleaner and the same technic continued. No other changes were made. I n so far as could be determined, all conditions remained the same as they had been when T. S. P. was used. C-washed dishes were collected for 2 weeks and treated as just described.
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The machine used in restaurant B was a double-tank machine of the manual push-through type. The average washing temperature was 54' C., that of the rinse was 65' C., and the average duration of exposure to the rinse was 1.5 to 2 minutes. In this restaurant, which was using C when the work was started, dishes were collected for 2 weeks, after which time a T. S. P. cleaner was employed for 2 weeks. The results are as follows: Restaurant
A
B
I
No. of
Plates Tested 120
150 180 153
Colonies/Cc. of Broth T. S. P. C 19
..
..7
..8
.. 1
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(4) Gilmore, B. H., Oil & Soap, 12, No. 2,29 (1935). (5) Hall, R. E., U. 5.Patent Reissue 19,719 (Oct. 8, 1935); U. S. Patent 2,035,652(March 31, 1936). (6) Purdy, A. C., Marine News, 21, 83 (1936). (7) Reddish, G. F.,Am. J. Pub. Health, 17, 320 (1927). (8) Schwartz, C., and Gilmore, B. H., IND.ENG. CHEM.,26, 998 (1934). (9) Smith, G.W., Am. Dyesty#' Repi?., 23, 161 (1934). RECEIVED October 19, 1936. Contribution from the Calgonieing Fellowship a t the Mellon Institute of Industrial Research.
Water Hardness P. p. m. CaCOa
68.6 58.7 25.6 46.6
These experiments show that, under the particular conditions obtaining in each place, dishes washed in C were bacteriologically cleaner than those washed with T. 8. P. The results in each of the two restaurants are quantitatively different, and this difference may be ascribed to differences in the washing procedure a t each place. No change was made in the actual technic of washing other than change in cleaning material. In restaurant B the work was done carefully, the dishes were racked as they should be, and the washing was neither careless nor hurried. In restaurant A less care on the part of the washman yielded less satisfactory results. But in both restaurants, in spite of rather marked differences in the conditions existing, brighter, cleaner dishes were secured with C than were obtained with the other materials. The superior sanitary quality of the dishes washed with the sodium metaphosphate preparation does not proceed from any specific germicidal action but must be attributed to its effects in facilitating both the washing action of the alkaline detergent and the rinsing. Thus the dishes are not only freed from soil, but they do not accumulate any organic or inorganic film that results from precipitation during cleansing. Sanitary regulations governing machine dishwashing vary in different localities. The sanitational value of hot alkaline detergent solutions is well recognized. It is also recognized that, in machine washing, dishes may be suitably disinfected by exposing them to hot rinse water for a sufficient period. As an alternative, when an adequate supply of hot water is not available, the use of an auxiliary germicide, usually a compound of chlorine ( 2 ) ,is specified. I n hand dishwashing, in which low temperatures and low alkalinities prevail, the use of an auxiliary germicide is well justified, especially in public eating places. Wherever absolute sterilization is demanded, it may be more readily obtained by the use of germicidal solutions when sodium metaphosphate is used in the washing process because of the nonexistence of protective films. Thus more effective and economical use of the germicidal solution is made possible. Film formation in dishwashing is a source of great concern to health and restaurant officials because such films form an ideal environment for bacterial growth. It has been demonstrated that the utilization of the film-preventive function of sodium metaphosphate represents a distinct improvement in dishwashing sanitation. Moreover, the dishes washed present an unusually attractive appearance, owing to their extreme physical cleanliness. Thus the practical removal of dishwashing film represents a contribution to both sanitational and esthetic progress.
Literature Cited (1) Buchanan, J. H., and Levine, Max, American Bottlers of Carbonated Beverages, Educational BUZZ.l (rev.), 17 (1929). and Yongue, N. E., Am. J. Pub. Health, 26, 237 (2) Cumming, J. G., (1936). (3) Devereux, E.D., and Mallman, m7.L., Ibid., 26, 165 (1936).
Removal of Fluorides from Natural Waters by Defluorite H. GLADYS SWOPE AND ROBERT H. HESS Kansas State Board of Health, Lawrence, Kans.
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N 1926 the cause of the mottled enamel of teeth was definitely traced to the water supply (iW). Kehr (IO) made a survey in 1930 of the occurrence of mottled enamel in Kansas: the results of this survey are shown in Figure 1. Not until 1931 was the causative agent (fluoride) in the water supply found (3,16). Much has been written in these last five years regarding the destructive power of fluorides on teeth. We are now greatly interested in the removal of the cause since there are areas in Kansas where all fresh water supplies contain fluorides. Some research has been undertaken on the removal of fluorides from water (2,8, il, 13). Chetopa, Kans., obtains its water from a well 1320 feet deep. The water is soft and has a good taste, but it contains 6 parts per million of fluoride. All of the children who have
Between 174 and 186 gallons of water containing 6.4 p. p. m. fluoride can be treated at the rate of 12 gallons per hour before the effluent contains 2 p. p, m. fluoride when filtered through 10 pounds of Defluorite. After regeneration with 5 per cent sodium hydroxide and neutralization with 0.35per cent hydrochloric acid, between 228 and 237 gallons can be treated. At the rate of 5 gallons per hour only between 100 and 107 gallons can be treated. Tap water, to which 13 p. p. m. of fluoride had been added, was passed through a filter regenerated with caustic solution.
