disinfectants in the dairy industry

CLEW, L. F. L. (1967). J . uppl. Bmt. 30 (l), 117-140.

(Symposium on Chemical Disinfection: Paper XII)

Disinfectants in the Dairy Industry

L. F. L. &EGG

Department of Dairy and Food Science, University of Alberta, Edmonton, Alberta, Canada

CONTENTS PAGE

1. Introduction . . . . 117

2. Chemical disinfection in the milking area . . 119 (a) Udder washing . . . . . . . 119 (b) Treatment of clusters and teats . . . . 119 (c) Commercial application of research . . . 120

3. Chemical disinfection in the milk house and parlour . . 122 (a) Hand washing; the relative merits of combined and separate cleaning

anddisinfection . . . 122 (b) Immersion cleaning . . . . 126 (c) Treatment of pipeline plant . . 128 (d) Bulk milk coolers . . . 132

4. Chemical disinfection of milk processing plant . . . 133 (a) Cleaning ‘in place’ . . . . 133 (b) Containers . . . . . 134 (c) Bacteriophage . 9 . . . . . 135

6. Miscellaneous . . . . 135

6. Conclusions . . . . . . . 136

7. References . . . . . 136

1. Introduction IN THE LAST decade there have been several reviews on disinfectants and their use in the dairy industry (Clegg, 1956~ ; Egdell, 1959; Swartling, 1959; Cousins, 1961; Cuthbert, 1961; Jennings, 1961; Scarlett, 1962; Thiel, 1962; McCulloch, 1966) and i t is not intended here to deal much with historical aspects or the types and characteristios of the materials used. There are a t present 6 general groups of disinfectants which can be used in dairying: (1) chlorine-bearing disinfectants; (2) quaternary ammonium compounds; (3) iodophors; (4) the strong alkalis; (5) the phosphoric acid wetting agents (PAWA); (6) the amphoteric compounds: the last two appear to be effective but are somewhat expensive (Haffer, 1965). It is the intention to discuss some uses made of these materials and the results obtained.

The control of the use of disinfectants and detergents is not the same in all countries. For example in the United States the general type of disinfectant must be approved and the composition of a brand declared. In Germany, detergents and disinfecbants can be approved if desired by the German Agricultural Society (DLG). In England

I 18 L. F. L. Clegg

and Wales there is the Gilbertian situation where a milk producer is not required legally to use a detergent when washing utensils, but he must use a disinfectant that has been officially approved. In Scotland, I understand, chemical disinfectants are still largely disapproved! In Canada, I am bound to say, anything goes-though it usually goes quite successfully.

In dairying as in most other applications of chemical disinfection, the killing of bacteria depends on a number of well known variables. Where equipment surfaces are to be disinfected the nature of the surface and the protection afforded by the soiling film are of great importance. Masurovsky & Jordan (1958), and others, using radiographic methods for studying the removal of micro-organisms from surfaces, concluded that total soil removal from equipment is a practical impossibility. As the residual surface film becomes thicker by bonding of subsequent layers of milk solids to the original monomolecular layer absorbed on the equipment surface, this offers an increasingly favourable site for multiplication of bacteria, although this film may not be visually apparent. Roderig, Clegg, Chapman, Rook & Hoy (1956) pointed out that the chemical composition of milkstone varied according to the hardness of water and the type of detergent used for washing. That this could result in an invisible film was reported and shown by Clegg (1956a, 1962b) (see Plate l(a), (b)). The film shown in these two photographs was not visible before acid treatment and it is of interest that the film was not sufficiently hydrated to be visible or removed until after 30-min treatment with a 4% (v/v) warm phosphoric acid solution which was much in excess of the manufacturer’s directions for use. Many workers, e.g. Hoy & Neave (1955), have shown that old and corroded equipment cannot be adequately disinfected by chemical means. Beyer, Singer, Ziihlsdorf & Meiser (1962) showed that in such equipment Mycobacterium tuberculosis from heavily infected milk could survive. Major (1962) showed that the effectiveness of a number of disinfectants on dairy equipment was impaired in the presence of milkstone.

The protection which such films afford bacteria against chemical disinfection and the difficulties associated with old and corroded equipment led to the recommendation in the more recent official publications on the use of hypochlorites (e.g. Ministry of Agriculture, Fisheries & Food, 1959), of regular weekly heat treatment to penetrate such film and regular (though less frequent) descaling to remove such film. The importance of this has been re-iterated by Hobson, Druce, Griffiths & Thomas (1959).

The realization of the importance of the removal of residual mineral deposits not completely removed by alkaline or non-ionic detergents has led to the recommendation of an acid rinse as part of a routine cleaning procedure (Rather, pers. comm.), the use of phosphoric acid wetting agents (PAWA) and the formulation of iodophors with phosphoric acid to ensure regular acid treatment (Lazarus, 1952 ; Johns, 1954).

In other types of disinfection, skin, air or liquids, such deposits do not interfere but organic matter or mineral salts, particularly the latter, can interfere with some quaternary ammonium compounds (Ridenour & Armbruster, 1948 ; Cousins & Clegg, 1956).

In most instances chemical disinfection is assisted by an increase in treatment temperature but it is important to distinguish this effect from the bactericidal effect of heat alone.

13uct fp 11s

PLATE 2. ( a ) Original immersion cleaning equipmcnt for direct to can equipment (Thiol el al., 1955). ( b ) Lye flooding of pipeline plant showing clusters inverted in trough of caustic solution.

PLATE 3. Connections on pipeline ~ i l an t suitable for lye flooding. ( a ) Plant A ready for milking ( b ) Plant A ready for lye flooding ( c ) Plant I3 ready for milking ( d ) Plant H slide connection open showing connection ~ . i t h milk line.

Disinfectants in the dairy industry 119

2. Chemical Disinfection in the Milking Area Two purposes are served by using chemical disinfectants during the milking process ; first, to disinfect the outside of the cow’s udder as it is being washed, with the intention of keeping out of the milk micro-organisms which might affect its keeping quality; second, to prevent mastitis-causing micro-organisms being spread from quarter to quarter and cow to cow.

(a) Udder washing There is little recorded evidence to show that the use of disinfectants in udder washing water exerts a marked effect on improving the quality of milk. One such trial (Kesler, Knodt, Watrous & Williams, 1949) in which plain water was compared with sodium hypochlorite and a quaternary ammonium compound used at concentrations of 200 and 400 p/m, respectively, showed no significant differences between any of the treatments when the comparison was made on a basis of colony count. Another trial (Cowhig & Mulcahy, 1964) comparing sodium hypochlorite a t 600 p/m against plain water showed slight differences in colony count in favour of the disinfectant treatment. Had the comparisons in these trials been made on the basis of keeping quality, significant differences might have been observed.

The use of disinfectants in udder washing to prevent the spread of mastitis does not afford complete protection even when carried out properly. Neave, Dodd & Kingwill (1962) showed that although chlorhexidine proved superior to hypochlorite and iodophor in the disinfection of milkers’ hands contaminated with Staphylococcus uurew, none of these disinfectants was completely effective. Udder cloths may not be effectively sterilized even after soaking in 2% sodium hypochlorite solution for 5 h and can harbour Streptococcus agalactiae for as long as 7 days. The sores on cows’ teats and areas of skin erosion can harbour pathogens which resist chemical disinfection (Neave, Sharpe, Oliver & Dodd, 1959).

(b) Treatment of clusters and teats The dipping of teat cup clusters in a disinfectant solution between use on individual cows has been investigated extensively and this has been shown to effect at least a 90% reduction in numbers of staphylococci with the suspended type of milker where the whole of the milk contact surface, (rubber inflation and metal lid) can be treated (Newbould & Barnum, 1956). These workers concluded that any of the disinfectants they tested whether hypochlorite, quaternary, iodophor or chlorhexadine were all satisfactory providing the proper concentrations and exposure times were used. However, where the long tube milkers were tested, either bucket or pipeline, liners could be contaminated by micro-organisms from milk in the claw piece or the long milk tube (Davidson & Salvin, 1958). The dipping of clusters between cows is made more effective if this is practised in conjunction with the dipping of teats in disinfectant immediately the clusters are removed (Newbould & Barnum, 1956).

Several investigators (Hay, 1941 ; Newbould & Barnum, 1960; Report, 1961,1962) have shown that heat is more effective in reducing numbers of pathogens on teat cup liners. The term ‘teat cup pasteurization’ was applied by Neave and Dodd to indicate a treatment which would destroy pathogens but not necessarily sterilize the liners.

I20 L. F. L. Clegg

The importance of the combined use of efficient udder washing, teat cup pasteurization and teat dipping has been shown by Neave, Dodd & Kingwill (1966) in ex- tensive field trials. Earlier Dodd, Neave & Kingwill (1964), in reviewing the literature on udder disease in dairy cattle, concluded that there had been little if any reduction in the incidence of mastitis infection over the past 40 years and that the general opinion that mastitis was largely a result of poor management was of little help t.0 any dairy producer because precise advice was not available as to how management should be improved. Further these workers stated that in most advanced dairying countries 60% of the milk cows are infected with an average of two quarters/cow and that each cow was subject on average to one new infection each year. While the correct use of some antibiotics can be effective against most types of mastitis, the prevention of new infections seemed a more logical approach than expensive and uncertain therapy once the disease is established.

(c) Commercial application of research Until recently the results of research have been applied only to small numbers of cows in research herds and although evidence was obtained that the conscientious use of udder washing, cluster pasteurization a t 85” for 5 sec and teat dipping (Report, 1960, 1961, 1962) reduced new infections by staphylococci and streptococci, it was not known if such methods would be practical or effective under normal commercial conditions. This is the question that Neave et al. (1966) set out to answer in field trials with groups of c . 700 cows. Two trials were conducted; in the first the experimental hygiene routine consisted of udder washing with chlorhexidine diglu- conate (100 p/m), the use of individual sterile udder cloths or paper towels, pasteurizing the clusters and long milk tubes with water at 85-90” for 5-7 sec before each cow was milked and dipping the teats in chlorhexidine (SO00 p/m) immediately the teat cups were removed.

In the first field trial, 7 herds were put on ‘full hygiene’ described above and 7 herds on a control routine where the udders were washed only with plain water. f i l l hygiene resulted in a 50% reduction in new udder infection.

Because cluster pasteurization was the most expensive part of the ‘full hygiene’ routine and because it was desired to evaluate the effect of a simplified hygiene routine, the second field trial contained in addition to the control and ‘full hygiene’ routines a third routine called ‘partial hygiene’ in which the teat cup clusters were not disinfected between cows. (In the second field trial an iodophor was used at 100 p/m for udder washing and 5000 p/m for teat dipping.) The results of this trial (Table 1) showed that the ‘full hygiene’ routine was only slightly better than the ‘partial hygiene’. The Table includes part of the data of Neave et al. (1966).

Both hygiene routines reduced new infections by more than 50%. The authors concluded that provided the teats and particularly the ends of the teats, where erosion can occur, are effectively disinfected immediately after milking, the pasteurization of teat cups did not greatly affect the incidence of new infections. Thus the authors stress that the partial hygiene routine affords a relatively simple routine to enable the commercial milk producer effectively to reduce the number of new infections in his herd.

Disinfectants in the dairy industry

TABLE 1 The effect of full and partial hygiene routines on clinical mastitis and new infections in 15 herds of lactating cows

(Neave et al., 1966)

Per cent reduction compared with control

Pathogens found New infections < >

Treatment in clinical cases in lactation

Partial hygiene 41 63

Full hygiene 44 64

I 2 I

It would take time to provide the automatic hot water pasteurizing device cheaply and in quantity to many milk producers. The authors state that they are going to test further simplifications of the hygiene routine and if improvement is possible in the effectiveness of teat dipping, the contribution of pasteurization of clusters or cluster dipping may be relatively small in the overall reduction in infection.

The originators of this combined hygiene routine stress that 'the system will not cure existing infections or prevent all new infections in lactating animals and stress the importance of antibiotic therapy during the dry period as this can eliminate > 90% of the streptococcal infections and > 50% of the staphylococcal infections in addition to reducing the number of new infect,ions during the dry period. We are in fact witnessing the biggest advance in mastitis research that has yet occurred. For approximately 22-23 per cow per year any producer who will conscientiously apply the partial hygiene routine using chemical disinfectants can confidently expect to reduce the number of new infections in his herd by at least half, to appreciably reduce the cases of clinical mastitis and to lessen the incidence of teat sores. As a result of experience gained during these trials the experimenters are now able

to recommend the routine shown in Table 2.

TABLE 2 The hygienic milking routine for reducing incidence of mastitis

infection (Neave et al., 1966)

1.

2.

3.

4. Attach cluster for milking.

5.

6.

Wear smooth rubber gloves. Disinfect gloved hands with chlorhexidine or an iodophor (100 p/m).

Examine foremilk with strip cup.

Rinse gloved hands and wash udder in above disinfectant solution using paper towel or sterile cloth.

Rinse gloved hands before machine stripping.

Immediately cluster removed dip each teat in chlorhexidine or iodophor solution (5000 p/m). Treat teat sores with germicidal cream. Glycerol alone or with disinfectant has been found satisfactory.

I 2 2 L. F. L. Clegg

TABLE 3 Potential economy in world milk production of the introduction of

an effective routine of hygiene in milk production

World milk production (1963) 641,400,000,000 lb Assuming mestitis reduces production by New infections can be reduced by

10% 60%

Assuming world cost of milk €1.26/100 lb Then 8.4 x lor1 x 10-1 x (6 x 10-l) x 1-28 x 10-P = 4 x lo8 or

L400,000,000/yeer

It will undoubtedly be several years before the full impact and significance of these research findings is felt in all dairying countries, nevertheless the pobential economic saving to mankind is considerable. It is probably legitimate to make a few assumptions to est,imate this (Table 3). If this value in dairy products were saved annually there could be food supplies available to send to much of the world’s undernourished population.

In this advance in chemical sterilization we must not lose sight of the work of many dairy scientists and in particular to Newbould & Barnum (1960) for their work on disinfection of teats after milking. However the credit of recommending the combination of udder washing, cluster dipping and teat dipping must remain with Neave et al. (1966).

3. Chemical Disinfection in the Milk House and Parlour (a) Hand washing; the relative merits of combined

and separate cleaning and disinfection Although we still regard chemical disinfection as of recent origin as early as 1923 the U.S. Dept. of Agriculture issued a bulletin on the cleaning of milking machines which involved treating with heat or immersion in a solution containing 200 p of available chlorine/m. In 1926 the Milk and Dairies Order (England and Wales) allowed the use of hypochlorite for disinfecting milking machines but not for other dairy utensils. This legislation thus permitted chemical disinfection where it would be least effective!

On the North American continent the early methods of using chemical disinfection called for separate cleaning followed by disinfection with 100 p/m chlorine solution. Although detergent-sterilizers have and still are used extensively the general method advocated is a rinse, detergent wash, rime, immediately after milking with disinfection delayed until immediately before the next milking and applied as a sanitizing rinse (e.g. 100 p of hypochlorite/m) (U.S. Public Health Service, 1965).

In Britain during World War I1 the necessity for the conservation of fuel supplies gave the impetus for further study of chemical disinfection. Until this time such procedures had been rarely practised here because of lack of knowledge and the generally unsatisfactory results obtained. Mattick, Hoy & Neave (1942) published the first satisfactory routine in Britain for the use of chemical disinfectants in dairying ; this was based on experimental findings (Neave & Hoy, 1941) in which they advocated a combined detergent-disinfectant wash preceded by a rinse and followed immediately


by a ‘sterile’ rinse (chlorinated water). This leaflet also was amongst the first to con- tain recommendations on the chemical sterilization of milk cans and milk bottles. These were essentially emergency recommendations and controlled field trials were undertaken by Rowlands (1949) in 1946 and 1947 in conjunction with the (then) Provincial Advisory Bacteriologists of the Ministry of Agriculture. This work provided the foundation for various leaflets on this subject culminating in the present Advisory Leaflet of the Ministry of Agriculture and Fisheries (1954, revised 1959).

The arguments in favour of the combined detergent disinfectant treatment over the separate cleaning and disinfection method made by Clegg (1956a) gave the underlying principles for this treatment as : (1) a contact time of a t least 2 min; (2) a minimum concentration of chlorine of 250 p/m; (3) the use of a combined hot detergent-chlorine wash.

TABLE 4 Comparison of combined and separate treatments by detergents and

disinfectants for dairy utensils ~ ~~ ~~ ~ ~

Separate detergent and disinfectant Combined detergent-

Time treatment disinfectant treatment

After a.m. milking

Before p.m. milking

After p.m. milking

Before a.m. milking

Pre-rinse Warm detergent wash Plain rinse

Cold disinfectant rinse

Pre-rinse Warm detergent wash Plain rinse

Cold chlorine rinse

Pre-rinse Warm detergent-disinfectant

Disinfectant rinse wash

- Pro-rinse

Cold disinfectant rinse

It is worthwhile considering the differences between the combined and separate detergent disinfectant treatments shown in Table 4. The combined procedure should be more efficient because the disinfectant solution is applied hot, and because it is used in the wash an automatic minimum contact time is ensured. Furthermore the fact that satisfactory results can be obtained by the use of the full treatment once a day, and by a simplified treatment in the evening (Clegg, 1955), commends the efficacy of the method. Subsequently, arguments for and against the combined process have been mado by Swartling (1959), Johns (1962), White (1962) and McCulloch (1965).

It is of course well known that the germicidal activity of hypochlorite solution is less under alkaline than under neutral or slightly acid conditions in the laboratory (see Johns, 1934; Costigan, 1937), but in tests designed to simulate practical use, this appeared not to be the case (Cousins & Wolf, 1946; Neave & Hoy, 1947). The latter workers experimenting with dried milk films on stainless steel trays and comparing the efficiency of hypochlorite solutions adjusted with different buffers to pH 7 and 11, showed that not only was the film more readily removed by solutions of higher pH

L. F. L. Clegg

but generally these were more germicidal. These authors explain this result by the more rapid hydration of the films with the solutions of higher pH, thus allowing greater contact between the disinfectant and the micro-organisms in the milk film.

With this information in mind itt might appear difficult to explain why results generally in North America, using separate cleaning and disinfection, seem so satisfactory.

Because of this, the author set up a small field trial to make a direct comparison of the two methods. (At the time this trial was set up (1959) the work of Cousins & McKinnon (1962), which showed little difference between the separate and the combined treatments, had not been published.) In this trial 6 farms were tested over a period of 3 summer months. The technique of the trial was similar to that developed by Clegg, Hoy & Cousins (1959). Three routines were tested on each farm for a period of 4 weeks without the safeguards of heat treatment, etc. However a t the commence- ment of each test period the utensils were descaled, new rubber wa5 provided and all the equipment heat treated. The three routines tested are shown in Table 6.

TABLE 6 Methods of cleaning and disinfection used in farm trial

Treatment of utensils after A r ,

Method a.m. milking p.m. milking

I Separate cleaning + Separate cleaning + sanitizing rinse sanitizing rinse

I1 Combined cleaning and Double strength disinfection disinfectant rinse

I11 Combined cleaning and Double strength disinfection + aanitizing rime sanitizing rinse

disinfectant rinse +

TABLE 6 A field comparison of three routines of cleaning and disinfecting milking equipment

Mean log colony count ( x lO'/article or ml) when utensils treated by

r 7 I I1 I11

Combined detergent Separate cleaning Combined detergent disinfection +

Sample and disinfeotion disinfection sanitizing rime

Cluater rinae 3-60

Buoket rinse 2.28

Can rinse 2-20

1.84 1*03*

3.72

0*73*

0.30t

0.74

Milk e.m. 0.203 0*041* 0*064*

Milk p.m. 0.046 0*017* 0.024

* Significantly different (P = 0.96) from treatment with highest value. t Significantly different (P = 0.96) from both other treatments.


The treatments were randomized on the farms to avoid a bias being introduced by weather, etc. Five of the farms had the fkee-standing type of machine with a long milk tube and one had the suspended type of milker. Each farm was visited twice a week for milk and rinse samples. A summary of the results is shown in Table 6.

Field trials are not normally as clear cut as laboratory tests and the summer when these trials were made (1959) was abnormally cool. However it is obvious that in every instance, Method 111, the combined detergent-disinfectant treatment plus the sanitizing rinse, was superior to the separate treatment. In most instances this difference was significant, but where it was not significant meaningful differences can be observed.

This in not quite so with Method I1 because one result was obviously worse. This result was influenced by the results of one farm only; however all the results on this farm were highly unsatisfactory thus indicating that Method I1 could not deal satisfactorily with these conditions. It is known that some individual metal utensils are much more difficult to disinfect than others (Clegg et at., 1959; Bacic & Clegg, unpublished results) and this may be the reason for this unexpected result.

Further support for the value of the premilking chlorinated rinse has recently been published by Middleton, Panes, Widdas & Williams (1965). Although this work was done on pipeline milkers it has considerable relevance. These workers were able bo show that this treatment improved rinse counts but were unable to show that it improved the bacteriological quality of milk. Laboratory trials on a comparison of these methods are proceeding and preliminary results (Bacic 6 Clegg, unpublished) bear out the findings of the field trials.

Although the theory of chlorine disinfection in alkaline solution is generally against the combined treatment, in practice other factors are of importance. Bacic & Clegg have some evidence but not complete proof which bears out the following explanation. The germicidal action of hypochlorite is reduced by the addition of alkaline detergent and therefore a separate sanitizing rinse after cleaning is a much superior germicide. Although bacteria may be killed by chlorine in a few seconds if unprotected (Riedel & Clegg, unpublished) the time factor is important when dealing with protective film on equipment. With the sanitizing rinse after the equipment is assembled ready for use the treatment lasts up to 30 sec (although some disinfectant solution will remain in contact with part of the equipment for a greater period) whereas with the combined treatment the disinfectant rinse, which takes place immediately after the detergent- disinfectant wash when the equipment is apart, takes a matter of 5 or perhaps 10 min. There is some evidence that this disinfecting rinse is much more active in destroying micro-organisms than is the detergent-disinfectant wash.

On this basis the separate cleaning and disinfection method should be greatly improved by substituting a disinfecting rinse for the plain rinse immediately after washing. There is no doubt from the evidence so far accumulated that the hygienic condition of dairy utensils, subjected to the combined detergent-disinfectant treatment, can be vastly improved by the institution of a sanitizing or premilking disinfectant rinse. Whether this would be better than the separate cleaning and disinfection process with a disinfecting instead of a plain rinse is now being determined by laboratory tests.

I 2 6 L. F. L. Clegg

(b) Immersion cleaning There is little need for detail concerning the development of immersion cleaning because those who are concerned with it are fully aware of it and those who are not will probably be satisfied with a brief statement of the principles. The first published studies of the method were made by Thiel, Clough & Clegg (1955).

Briefly the method, which was an outcome of the wet-storage method of Johns (1933), consists of immersing the milk contact surfaces of a milking machine for the whole of the time between milkings in a 3% solution of' caustic soda. The equipment is dismantled and brushed once a month a t which time fresh immersion solution is prepared. The equipment was originally deRigned for direct-to-can milking equipment. As milk cans are normally washed and sterilized a t the receiving dairy, this meant that the only milking equipment requiring treatment was the clusters (liners, claw piece, long milk tube) and the special vacuum can lid (Plate 2(a ) ) .

Studies on the performance of the method in comparison with steam and hypochlorite were made by Carriera, Clegg, Clough & Thiel(l955) and this showed that on a basis of colony count the keeping quality of milk produced by immersion cleaning could be ranked between that of steam and chemical disinfection. Although the quality of milk was good, the condition of the equipment left something to be desired, as calcium soap and inorganic calcium salts formed deposits on the rubber and metal

Fig. 1. The times required to effect !Byo destruction of B. aubtilia spores at different concentrations of sodium hydroxide at different temperatures (from Whitehouse & Clegg, 1963).

2.6 x lo8 t = c ' . ~ ~ ____ es.rp, where t = time in h to effect 99% destruction of B. aubtilk spores,

C = concentration (yo, w/v), of NaOH, 0 = temperature (OF).

Disinfectants in the dairy industry '27

surfaces and had to be removed a t monthly intervals by acid treatment. Furthermore the bacterial rinse counts of the equipment were above that considered desirable. Experiments with the addition of ethylenediaminetetra-acetic acid (EDTA) to the caustic solution prevented the formation of calcium deposits and improved the bacteriological counts of rinses (Thiel, Clegg, Clough & Cousins, 1966). The effectiveness of caustic soda solutions in killing spores has been shown by Whitehouse & Clegg (1963), the time between milkings being adequate for spore destruction even at low temperatures (Fig. 1).

The effectiveness of materials other than caustic soda including the quaternary ammonium compounds (QAC) and sequestering agents other than EDTA, has been investigated but none so far tested have proved as good as those originally recommended. This unfortunately prevents immersion cleaning being used in countries where aluminium, tinned steel or tinned copper equipment is used instead of stainless steel.

Fig. 2. Diagram of immersion besket in bin to take units of suspended bucket type milking machines (from Whitehouse & Clegg, 1981).

The originators of the method and others, e.g. Whitehouse & Clegg (1961), found the method suitable for bucket milking machines. Other modifications have been made to the method to adapt it for the suspended type of milker (Fig. 2) and to suit local conditions (Clegg, 19628).

Although immersion cleaning did not spread rapidly in Great Britain, i t is now firmly entrenched here and was (after much reluctance) recognized by the Ministry of Agriculture, Fisheries & Food (1961). However it is probable that the method may not receive such widespread acceptiance in areas where the suspended type of milker is most used. There is no doubt that the milk contact surfaces which cannot readily be seen (the claw piece and long milk tube as in the free standing bucket milker) benefit

I 28 L. F. L. Clegg

most from immersion cleaning. The suspended type of milking machine has neither of these parts and therefore will not benefit as much from immersion cleaning. In fact i t is a common practice on the North American continent to remove the liners from the shells of suspended type milkers and with or without brushing drop these into a crock of caustic soda (the strength of which varies between 0.5 and 5.0%) for the period between milkings. This practice under the best conditions is little different from the immersion cleaning of bucket milking machines.

A modified system of immersion cleaning has recently been applied to a Hosier pail milking machine (Egdell, 1959) though published results have not been seen.

(c) Treatment of pipeline plant The convenience of pipeline plant has led to its rapid spread in the major dairying countries. This advance was generally ahead of adequate knowledge of satisfactory methods of disinfection by chemicals; evidence of this in Britain is to be found ina report of a survey by Thornborrow (1960). In fact early machines were not designed for circulation cleaning of chemical disinfectants. This is of course history repeating itself for the same difficulties attended the development of the milking machine. The Ministry of Agriculture, Fisheries & Food in its Advisory leaflet 422 recommended an initial temperature of wash for pipeline of 160°F for 10 min (cf. hand washing at 115°F for 2 min). In Britain the findings of Thiel, Clough, Cousins & Akam (1962) probably more than of any other group have been responsible for plants designed for circulation cleaning.

Workers on early types of plant (Cuthbert, Bird & Bateson, 1954) and recent investigators (Thiel et al., 1962; Egdell & Widdas, 1962) have stressed that chemical sterilization needs to be supplemented by treatment with water a t bactericidal temperature and by periodic desmling. EgdeU & Widdas (1962) found milk solids deposited under gaskets and at the joints of pipes and concluded that these were the sources of microbial proliferation which required a penetration treatment and which chemical disinfectants cannot give with a short contact time. Dyson (1961) stated that increased temperature with hypochlorite is effective in 3 ways: (1) it softens the milk fat and thus assists the detergent ; (2) it increases the bactericidal efficiency of available chlorine (Qlo = 1-3.5); and (3) it lowers the pH of detergents and hypochlorites. The effectiveness of heat has also been stressed by Thornborrow (1960), Cuthbert, (1960, 1961), Snudden, Calbert & Frazier (1961), Whittlestone (1961), Swift, Alexander & Scarlett (1962), Murray & Foote (1963), Clough, Akam & Cant (1965) and Cousins, Clough & Thiel (1966) and is implicit in the dairy regulations in New Zealand (Whittlestone & Phillips, 1965). Inefficient chemical sterilization was found by Thomas (1963) to result in a rapid build up of bacteria, (>2.5 x 1OB/fta) particularly during warm weather. Thiel(l964) stated that the first attempts a t circulation cleaning were done hopefully with cold solutions; later the temperature was raised to 140- 146°F and now none of the manufacturers recommend treatment a t < 170°F. British manufacturers have taken this step rather than use more elaborate techniques of jointing in the pipelines as required by the 3A Standards* in the United States.

* Iasued under the authority of the U.S. Public Health Service, the International Association of Milk & Food Sanitarians and the Dairy Industry Manufacturers’ Association.


An interesting method of cleaning pipeline milking plants without resort to heat was introduced by Whittlestone & Murnane (1962). This involved filling the pipeline and clusters with a foam prepared from the QAC, cetrimide, and a synthetic detergent and leaving this to clean and disinfect milk contact surfaces between milkings. The bacteriological results assessed by swabbing different parts of the plant after deliberate soiling were excellent. Recent work by Peagan & Murnane (1965) has confirmed that the process is effective, but residues of the QAC were left in the plant and these interfered with starter development unless removed by a hypochlorite rinse.

/ c - / - - - 8 1 , i ( I A 0 Bulk milk

Fig. 3. Lay-out of lye flooding of round-the-barn pipeline plant (From:O'Callaghan eC d. 1966). - , Milk and cleaning solution line; - - - - - -, vacuum line. Immediately after milking the clusters (3) are suspended from specially designed racks in the rinsing tank (2) and connected to the milk line by means of a manifold. When the milk hae drained from the line, vacuum cock (6b) is closed, the coupling at B disconnected and the one at A loosened. The coupling B is then connected at C and the coupling at A again tightened. A 1 6 in. outside diam. water hose connected t o a 1 in. water main is inserted in the line at (G) and pushed beyond the connection between the two lines (F). The installation is then rinsed for 6 min by forcing water through the line and clusters after which they are allowed to drain for 16 min. The clusters are next suspended in the lye-EDTA solution in tank (1). A rubber stopper is placed in the line at (6) , the vacuum cock (6a) opened and the vacuum pump switched on. The lye-EDTA solution is drawn by vacuum through the clusters and line to receptacle (7). When this receptacle is half full the vacuum cock (6a) is closed and the vacuum pump switched off. When the line is completely filled sufficient lye-EDTA solution remains in the tank so that the ends of the teat cups are below the surface. Thus, the line and clusters remain flooded between milkings. Immediately before the next milking the line is drained by opening the vacuum cocks at (6c) and (64 . This break in vacuum allows the solution to flow back into tank 1 and the line is drained for 16 min. Each cluster is then taken from the lye-EDTA solution, drained and suspended in the rinsing tank. The vacuum cock ( 6 4 is closed and the line and milking machines rinsed and again drained. The coupling at C is disconnected and the line again connected a t B. The section of line from B to the bulk milk tank is brush washed with a detergent and sanitized with sodium hypochlorite (200 p of available chlorinelm) for 6 min.

J

L. F. L. Clegg

Another method of cleaning pipeline plants without the use of heat known as cauetic flooding, or lye flooding, was reported on by Egdell6 Widdas (1964) and O'Callaghan, Clegg & Vasic (1966). In both instances the system was essentially similar with the solution of caustic soda plus EDTA made up monthly except that Egdell & Widdas (1964) placed the clusters mouth upwards at the height of the pipeline and the equipment was flooded from an overhead tank and drawn back by vacuum before milking, whereas O'Callaghan et al. (1966) inverted the clusters in a trough f l e d with the caustic solution (Plate 2(b) ) which was drawn through the plant by vacuum as can be seen in Fig. 3. Egdell & Widdas (1964) found difficulty in comparing these results with those for circulation cleaning because samples were taken on a different basis. O'Callaghan et at. (1966) made a direct comparison of circulation cleaning and lye flooding on the same plant. A round-the-barn pipeline installation at the University of Alberta Farm was chosen for this study because it was considered that the 240 it of pipeline would present the most difficult challenge to lye flooding. Two types of milking machines were installed in this plant: an in-line filter used in one type of machine was found to give rise to some trouble. With this equipment O'Callaghan et al. (1965) concluded that the rinse counts of equipment from lye flooding were slightly inferior to those from circulation cleaning (140°F for 20 min) but this was not reflected in the milk samples.

It is difficult to make comparisons between results published in the literature on swabbing and rinsing of pipelines because of differences in method. O'Callaghan t Clegg (1964) found considerable differences between the results of swabs and of rinses circulated for 16 min, calculated on the basis of total micro-organisms/pipeline. However, using a combined circulated rinse and mobile swab for the pipeline and adding the counts obtained from this to those obtained by swabbing the clusters the results (Table 7) were considerably superior to the results quoted by Thomas and by McCulloch. In 2 independent. surveys Thomas (1963) reported that 27% and 64% of rinse of pipelines yielded counts > 260,000/fta: in a t,rial lasting over a year on 7 farms McCulloch (1966) showed that the milk lines were the most heavily contaminated part of the pipeline milkers, and arithmetic mean results ranged 69,000-3,960,000/fta.

TABLE 7 Comparison between circulation cleaning and lye jlooding of a

round-the-barn pipeline milker (O'Callug?mn et al., 1966)

Geometric mean of bacterial counts*/ft* or ml for period

Nov. and Dec., 1962 Feb. 1963 Mar. 1963 (Circulation cleaning (Lye flooding) (Lye flooding)

I -,

Sample 14O0+96"F for 20 min)

Pipeline, rinse and swab 380

Clusters total of 4 1760

Total pipeline and cluetem 2130

Milk 8300

1946 1813

27 1 4726

2233 6661

14.440 3600

* From alternate daily resulte.

Disinfectants in the dairy industry 13'

The temperature of the 20 gal circulating rinse used by O'Callaghan et al. (1965) was 140°F initially, dropping to 95°F after 20 min circulation. Thus the rinse would have been > 135°F for c. 1 min. While there would be some slight bactericidal effect from this heat it would be minimal in comparison with treatments with initial temperatures > 170°F. The detergent solution waa not unusual, being 06% trisodium phosphate. Therefore the superiority of these results may perhaps be attributed to the improved type of connection for the two types of machines which allowed the sections of the glass pipeline to be maintained rigidly together. Plate 3 shows the milk outlets connected to clusters or closed for circulation cleaning or lye flooding. One of the connections of this pipeline was recently opened and examined for the fist time in 3 years and swabs taken of the gasket and interior of the line. The gasket and junction of the pipes were free from deposit or film and both swabs were virtually sterile. The ad- vantages of improved pipeline connections were stressed by Quist (1963) though he gives no supporting proof, and Cousins (pers. comm.) regarch crevices provided by joints, gaskets, etc., with poorly fitting pipeline plant as a practically inexhaustible source of micro-organisms when the plant has not been dismantled for a few months.

However very satisfactory results of a pipeline especially adapted for circulation cleaning by Mr. W. H. Alexander have been quoted by Scarlett (1962) (see Table 8). In this case two hot rinses were applied: (1) a detergent-chlorine solution for 20 min at 150°-+1050F and a hypochlorite rinse for 10 min at 150"+100"F. This would provide a treatment at 135" for at least 5 min, and this would have some bactericidal effect.

TABLE 8 Colony counts for plants treated by circulation cleaning (Scarlett, 1962)

Per cent of samples of

5iGrzz? Range of colony counts/ft*

6 and under 49 72 > 6-26 13 12 > 26 38 16

Perhaps the most advanced design of pipeline plant for circulation cleaning ia that of Thiel (1964) (Figs 4 and 5) using jetter cups on the clusters and spreadersinthe weigh jars (frequently a trouble spot in pipeline plants). This plant embodied the ideals of minimum amount of milk contact surface and minimum amount of circulating solution with nevertheless complete coverage of milk contact surface. The disinfection routine was only made satisfactory when Clough et d. (1965) modified the plant for daily circulation of 3 gal water a t 205-212°F containing 2 oz of nitric or 24 oz of sulphuric acid. With this procedure rinse counts consistently < 50,000/fta have been obtained. I$ is important to note that this breatment dispenses wibh the normal pre-rinse with plain water, the removal of milk residues being effectively achieved by the hot rinse and no bake on of milk solids occurs because the hot rinse ia acidified.

L. F. L. Clegg

Milk room Parlour

Fig. 4. Special parlour plant assembled for milking (from Thiel, 1064).

Fig. 6. Special parlour plant assembled for circulation cleaning (from Thiol, 1904).

(d) Bulk milk coolers Once again we have the same story of a piece of dairy equipment being produced and released for commercial use before adequate means for cleaning and sterilization had been developed. Bulk milk coolers on the farm can only be disinfected by chemical means because heat is unsuitable for either the direct expansion or ice bank type of equipment.

When bulk coolers were first introduced it was thought they could only improve the quality of milk because of the efficient cooling and maintenance of milk during storage a t 3638°F. But to be economical the milk from such tanks is collected every other day so that some milk is in the tanks for 48 h.

Several workers, e.g. Johns (1952, 1958, 1960), Orr, McLarty & Baines (1960) and Mourgues & Auclair (1964), have shown that the results of dye reduction tests on milk from bulk coolers may be satisfactory because of inactivation of dye reducers at low temperature, but the colony counts of mesophiles and psychrophiles can be very high if cleaning and chemical sterilization is not satisfactory.

Methods of disinfecting bulk milk tanks, either coolers or pick up vehicles, can be manual, with separate or combined detergent and disinfectant treatment, or mechanical with recirculationof rinses by a spattering device or spray ball (see for exampleMeyers,

Disinfectants in the dairy industry ‘33

1959; Waltere & White, 1961) or by hand operated jets (Kruger, 1965). There is little doubt that specially designed built in cleaning devices with automatic timing and changing of rinses give a more satisfactory result than do either portable devices or manual cleaning (Jensen, Harmon & Hedrick, 1959) and, provided that adequate care is taken to keep the plant free from mineral or protein deposits by the frequent use of acid detergents or a daily acid rinse in addition to the normal detergent routine, low swab counts can be obtained and difficulties with psychrophiles avoided. With either manual or mechanical cleaning, the spraying of a disinfectant on to the wall lid and impeller is sound practice, though it is necessary to ensure that the outlet cock is open at the beginning of the spraying. Chlorinated alkaline detergents have been found to be very satisfactory if brushed on as a paste or slurry when blue ‘protein haze’ or milk film starts to develop. With pick up vehicles a satisfactory routine which may include steaming can be done a t the receiving dairy.

4. Chemical Disinfection of Milk Processing Plant Dairy processing plants present problems for chemical disinfection different from those of farm dairy equipment. The heat processing of milk will induce heavier film than in raw milk plant because of the deposition of calcium phosphates and other inorganic materials which form a mesh in which fat and protein become trapped and not easily removed by normal cleaning. Undor such circumstances greater precautions are required to ensure adequate cleanliness if chemical disinfection is to be effective. In fact different parts of a processing plant may have to be cleaned with different detergents according to the degree of heat used in processing. Thus separate cleaning and disinfection are usually to be preferred.

(a) Cleaning ‘in phce’ Except in the smaller dairies where cleaning of the plant may bo done manually, cleaning ‘in place’ (CIP) is usually practised, because it is more economical and reliable. However, when existing plant is adapted to CIP, difficnltios may ensue becausc of worn or damaged parts. For this reason it is desirable to use equipment especially designed for CIP. (The reader is referred to the excellent manual published on this subjcct by the Society of Dairy Technology (1959)).

The wetting of surfaces of processing plant is frequently difficult e.g., surface coolers, tank lids, bearing surface of valves, etc. With heat treatment, however, this problcm presents little difficulty because heat will be conducted to all such difficult areas. With some types of plant, spray cleaning (Seiberling & Harper, 1957) or fogging is most helpful. Roland (1955) obtained satisfactory results with the use ofan amphoteric substance atomized with steam.

With early CIP routines the use of hypochlorite solutions a t low temperatures was recommended to avoid corrosion, but with the introduction of corrosion resistant stainless steel and glass, temperatures >140”F were possible. Smith (1957) recommended 120°F for cold wall tanks, 150°F for pipeline, and 170’F for heat exchanges, but higher temperatures have been found more effective, and Jennings (1961) reported a Q,, of 1.6 for the removal of cooked-on milk films with NaOH solutions within the rangc of 115-180°F.

'34 L. F. L. Clegg

Maxcy & Shahani (1961) showed that by leaving very low concentrations of hypochlorite (1.0-2-6 p/m) in the system overnight, virtually sterile conditions were achieved so long as there was residual chlorine in the solution. This result compared very favourably with that of an average of 1.3 bacterial8 in2 following circulation with a 0.67 yo alkali solution a t 170°F for 30 min and both results compared favourably with the standards of 100/8 in2 (18OO/ft2) recommended by American Public Health Association (1960).

Much has been done recently with radioactive labelled soiling material and bacteria incorporated into soil deposits for measuring the removal of such deposits from plant surfaces (Jennings, 1961) and ib has been suggested that such techniques might replace existing and sometimes doubtful bacteriological methods. However, labelled bacteria from surfaces which have been disinfected but inadequately cleaned would presumably still be radioactive even when dead.

(b) Containers (i) Cam

Little need be said about milk cans. The Society of Dairy Technology's memorandum on can washing (1962) embraces the principles underlying mechanical can-washing but does not refer to chemical disinfection; this aspect is adequately described by Advisory Leafleb 422 by Ministry of Agriculture & Fisheries (1959). However, a rapid method for disinfecting cans after washing and immediately before use has been described by Gibson (1957) which uses steam and 200 p of hypochlorite/m and gives adequate disinfection in 2-3 sec.

(ii) Bottles Much has been learned aboiib the disinfection of milk bottles since the classical

work of Hobbs & Wilson (1943) who pointed out one of the main problems of botble

c . _ E - E .- I-

Fig. 6. The times to effect 99% destruction of B. subtilia spores at different temperatures and with different caustic solutions possible in conventional bottle washers (from Clegg, 19666).

Disinfectants in the dairy industry I 3 5

washing, that of re-contamination after the hot (145°F) detergenb wash. Because bottles must be cooled down by stages of never more than 40°F to avoid breakage, a tempering warm rinse is necessary before the final cold main rinse, and this warm rinse is usually at a bemperature most conducive to microbial multiplication. This subject is dealt with by Resuggan (1957) in his book on the cleaning and sterilization of bottles. Various chlorinating devices for the warm rinse have been used successfully, e.g. Tucker & Evans (1964).

Attention has been given in recent years to the treatment of bottles to render them sterile for aseptic bottling. Franklin & Clegg (1956) examined the possibility of using conventional bottle washers with temperatures up to 180°F and concentrations of caustic soda up to 2 6 % but they came to the conclusion that under such conditions the degree of sterility required for aseptic bottling could not be obtained (Fig. 6). These results were confirmed by Labots, Galesloot & Stadhouders (1962) although they were of the opinion that a new washer which they designed and which rinsed bottles with sterile water could sterilize most of even the worst soiled bottles. How- ever, the superiority of heat, over chemicals must be acknowledged in some circumstances, and although early efforts a t producing sterile bottles with 'flash heating' resulted in a high breakage rate the problem has now been solved a t the National Institute for Research in Dairying (Anon, 1966).

The aseptic filling of Tetra Pak cartons has now been made possible by a development of the Alpura Co. in Switzerland, who combined heat and hydrogen peroxide to sterilize the paper as it is being formed into cartons (Davis, 1965).

(c) Bacteriophage The control of airborne phage particles in cheese making by the use of chemical disinfectants as aerosols has been of great economic importance to the cheese industry and was pioneered by Wolf, Nichols & Ineson (1945) who showed that concentrations of sodium hypochlorite yielding 0.003-0.02 p of available chlorine/m gave satisfactory destruction of airborne phage.

More recent work has confirmed these findings and shown the superiority of chlorine containing compounds over the iodophors, the PAWA and the QAC, in that order (Sing, Elliker & Sandine, 1964a, b, c). These workers showed that an aerosol of 0.048 p of available chlorinelm achieved a phage inactivation of > 99.999%.

5. Miscellaneous I t is difficult to cover all aspects of chemical disinfection in dairying. It, is probably reasonable to exclude the accidental addition of disinfectants to milk, but the controlled addition of such materials as hydrogen peroxide (World Health Organization, 1962) for the preservation of milk and the addition of such antimicrobial agents as nisin (Hirsch, McClintock & Mocquot, 1952) to various products including the non- acid cheese to prevent the development of Clostridium spp. could perhaps come within this realm. While these aspects should not be lost sight of, i t would be impossible to do then justice within the limits of this review.

L. F. L. Clegg

6. Conclusions In almost every development of new dairy equipment we have seen a pattern as follows :

(1) equipment produced without adequate knowledge of cleaning and sterilization ; (2) trouble experienced as result of commercial use of new equipment,; (3) bacteriologists called in to solve problem. In a few instances bacteriologists have been called in a t the beginning of a new

development, e.g. immersion cleaning and aseptic bottling, but these are 6he exception. We now have sufficient sad experiences of the outcome from the failure to consult bacteriologists that there should be no question that any new piece of equipment for the direct handling of dairy or other food products should be designed so that it can be satisfactorily cleaned and sterilized. This means consultation with bacteriologists at the drawing board stage. Failure to do this must be expensive and could even be disastrous.

Finally, it is gratifying to note what a high percentage of the workers mentioned in this review are members of this Society.

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