STERILIZATION METHOD

[0001] The present invention is directed to a method of sterilizing a material, said method comprising the steps of: a) providing a sterilizing composition comprising (i) peracetic acid and (ii) a stabilizer selected from the group consisting of citric acid, isocitric acid, aconitic acid and propane-1,2,3-tricarboxylic acid; b) introducing such sterilizing composition into a hot gaseous stream to produce a peracetic acid vapor; and c) contacting such peracetic acid vapor with the material to be sterilized. The use of such an organic acid stabilizer results in an unexpected reduction in the amount of residue deposited on the heating surface employed to vaporize the sterilizing mixture.

[0002] The necessity of sterilizing surfaces for health and sanitary purposes has long been recognized. Effective sterilization processes are needed for a variety of purposes including aseptic packaging, medical instrument sterilization, biocidal vector environmental remediation, fumigation, vessel sterilization, food stuff treatments, and others.

[0003] Among the compounds employed as a sterilizer for such uses is peracetic acid (“PAA”), also called peroxyacetic acid. In order to prolong the shelf-life of PAA compositions, stabilizers are typically added. These stabilizers act as chelators for dissolved metal cations that can disrupt the peroxide bond.

[0004] The use of vapor phase peracetic acid to sterilize surfaces is described in US Patent Application 2010/0196197. This publication discloses the use of peracetic acid stabilized with phosphonic acid or acid stabilizers such as Dequest 2010, 1-Hydroxyethylidene -1,1,-diphosphonic acid. While such process is effective to sterilize a variety of different surfaces, it has been found that when such phosphonic acid stabilizers are employed the surface of the heating element used to vaporize the peracetic acid solution tends to become covered with residue over a period of time. This build-up of residue requires that such surface be periodically cleaned, a process which can be time consuming and expensive.

[0005] It would therefore be highly desirable to possess a method for using vapor phase peroxyacetic acid as a sterilizing agent which method did not require the relatively frequent cleaning of the heating surfaces of the equipment employed.

[0006] While PAA compositions comprising citric acid have been disclosed in the past, such compositions have been employed in liquid aqueous sanitizing applications only. Thus, for example, WO 2008/079999 discloses sterilizing compositions comprised of PAA, citric acid or a salt thereof, and salicylic acid or a salt thereof, which compositions are applied topically to the surfaces to be disinfected employing solution spray systems or the like. Somewhat similarly, U.S. Pat. No. 6,117,457 and European Patent Application 720814 both disclose the use of citric acid in aqueous PAA generation systems; either as a pH regulator in fish ponds or as a means of providing extended PAA generation, respectively.

[0007] However, it has now been unexpectedly found that when PAA stabilized with an organic acid such as citric acid is employed in vapor phase sterilization, the buildup of residue on heating surfaces is considerably less than when a phosphonic acid stabilizer is employed. This result is particularly unexpected in view of the showing that aqueous solutions of citric acid (in the absence of PAA) result in the undesirable buildup of residue on heating surfaces.

[0008] The present invention is directed to a method of sterilizing a material, said method comprising the steps of: a) providing a sterilizing composition comprising (i) peracetic acid and (ii) a stabilizer selected from the group consisting of citric acid, isocitric acid, aconitic acid and propane-1,2,3-tricarboxylic acid; b) introducing such sterilizing composition into a hot gaseous stream to produce a peracetic acid vapor; and c) contacting such peracetic acid vapor with the material to be sterilized.

[0009] This method permits the effective vapor sterilization of a material without the need for frequently disassembling the apparatus employed in order to remove the buildup of residue which occurs upon the heating surfaces of such apparatus. Accordingly, a wide variety of materials may be rapidly and economically sterilized employing the method of this invention.

[0010] The present invention is directed to a method of sterilizing a material, said method comprising the steps of: a) providing a sterilizing composition comprising (i) peracetic acid and (ii) a stabilizer selected from the group consisting of citric acid, isocitric acid, aconitic acid and propane-1,2,3-tricarboxylic acid; b) introducing such sterilizing composition into a hot gaseous stream to produce a peracetic acid vapor; and c) contacting such peracetic acid vapor with the material to be sterilized.

[0011] As is employed herein, the term vapor means a state in which the peracetic acid is substantially entirely in the gaseous form. This is in contrast to mist or fog, both of which contain a significant proportion of liquid droplets suspended in the air. Unlike the use of a mist or fog, it has been found that the use of peracetic acid in vapor form provides excellent sterilization of materials without the concomitant formation of water droplets on the material surface.

[0012] Peracetic acid is typically employed in the form of an aqueous equilibrium mixture of acetic acid, hydrogen peroxide and peracetic acid. The weight ratios of these components may vary greatly, depending upon the particular grade of PAA employed. Among the grades of PAA which may be employed are those having weight ratios of PAA:hydrogen peroxide:acetic acid of from 12-18:21-24:5-20; 15:10:36; 5:23:10; and 35:10:15.

[0013] The stabilizer employed in the PAA sterilizing composition is selected from the group consisting of citric acid, isocitric acid, aconitic acid and propane-1,2,3-tricarboxylic acid. Preferably, such stabilizer is citric acid.

[0014] Typically, the stabilizer is employed in an amount sufficient to stabilize the PAA for at least three months. Preferably, the stabilizer is present at a concentration sufficient to stabilize the PAA for a period of at least six months.

[0015] In general, such stabilizer will typically be present in an amount of between about 0.75% and about 1.5%; preferably of between about 0.9% and 1.25%; and more preferably of between about 1.0% and about 1.2% ; all such percentages being by weight, based upon the total weight of the PAA composition.

[0016] The composition employed in the process of this invention may further comprise sequestriants such as dipicolinic acid, as well as other ingredients such as mineral acid catalysts (sulfuric, nitric, or phosphoric acids); surfactants such as anionic laurylates, sorbitans and their respective esters, i.e. polyethylene sorbitan monolaurylates; and short chain fatty esters (C6-C12) forming mixed peracids in solution.

[0017] In addition, the compositions employed in the process of this invention may further comprise one or more additional oxidants selected from the group consisting of chloroperbenzoic acid, perheptanoic acid, peroctanoic acid, perdecanoic acid, performic acid, percitric acid, perglycolic acid, perlactic acid and perbenzoic acid.

[0018] In the practice of the process of this invention, the peracetic acid sterilizing composition is preferably diluted, prior to its introduction into the heated gas stream, by the addition of high quality water (deionized water with ≧2 MOhm resistivity or ≦0.5 μSiemens conductivity), to a concentration of less than about 10,000 parts per million (ppm) of PAA, preferably to a concentration of less than about 4,000 ppm PAA.

[0019] The heated gas stream is typically sterile air, although other gases such as nitrogen, CO₂, or inert noble gas carriers may also be employed. Such gas stream is typically heated to a temperature of at least about 300° C., preferably to a minimal temperature of about 250° C. and can be in excess of 350° C. providing it can be cooled sufficiently for application. It then is typically cooled to between about 80° C. and about 120° C. prior to the introduction of the peracetic acid solution. The heated gas stream at the point of peracetic acid introduction should have a temperature of at least 5° C. higher than the dew point of peracetic acid (ca. 46.5°-49.9° C.); i.e., of at least about 55° C., in order to ensure that the peracetic acid is converted into a vapor rather than a fog or mist.

[0020] The peracetic acid may be introduced into the heated air stream by any means well known to one of skill in the art. One preferred method is by direct injection of a solution.

[0021] The peracetic acid vapor is then contacted with the material to be sterilized for a period sufficient to kill the contaminants of concern. This time period will vary according to variables such as the concentration of the peracetic acid vapor employed; the nature of the surface of the material to be sterilized; the particular contaminants to be sterilized; the concentration of the contaminants to be sterilized; and the like. Typically, such contact will maintained for a period of between about 15 and about 40 minutes.

[0022] A wide variety of materials may be sterilized employing the method of this invention, including hard surfaces of metals, plastics, polymers, and elastomers.

[0023] The present method may be used to sterilize materials contaminated with those bacteria typically controlled by peracetic acid in the liquid form. These include bacteria and spores of the genus Bacillus using B. thuringiensis and B. atrophaeus as surrogates for more pathogenic species (forms) such as C. botulinum as well as more typical genera of bacteria, fungi, and viruses and protozoans often controlled by PAA such as (but not limited to): Staphlococcus, Enterococcus, Salmonella, Campylobacter, Pseudomonas, Candida, Rhizopus, Mucor, Influenza, Bacilli, etc.

[0024] The following Examples are presented to offer further illustration of the present invention, but are not intended to limit the scope of the invention in any manner whatsoever.

[0025] In order to demonstrate the stabilizing effect of citric acid on PAA compositions, varying amounts of citric acid were added to concentrated PAA compositions, which compositions were stored at room temperature. Periodically, at the times indicated in Tables 1-4 below, samples of such compositions were analyzed for their peracetic acid (PAA); hydrogen peroxide (H₂O₂); and acetic acid (AA) contents (in percent by weight); and their Active Oxygen Recovery percentage (AO Rec) was calculated. The data presented in such Tables indicates that citric acid alone can stabilize PAA compositions for extended periods of more than 3 or 6 months.

TABLE 1
0.91% Citric Acid
Days After
Addition	PAA %	H2O2 %	AA %	AO Rec %
—	0.8438	15.0778	49.4288	88.94
1	2.8207	14.4139	47.1477	90.21
2	4.8579	13.7938	45.1340	91.89
3	6.8554	13.1946	43.4419	93.58
4	8.1230	12.7316	42.2202	94.18
8	12.1365	11.4882	38.1054	97.35
9	11.8892	11.2862	37.8752	95.55
10	12.8904	11.1100	37.6596	97.12
11	13.3851	11.0106	36.8579	97.82
14	15.3285	10.6744	35.9054	100.89
15	14.4656	10.6141	35.8921	98.32
16	14.7712	10.5483	36.6383	98.72
17	15.4082	10.5003	36.1812	100.09
18	15.0226	10.4909	36.6624	99.04
21	15.3530	10.3126	35.4124	98.87
28	16.1285	10.1703	34.9291	100.04
36	16.0191	10.0980	34.7712	99.35
43	16.6856	10.0932	34.0215	101.03
77	15.7208	9.8166	35.5012	96.96
95	15.8097	9.8772	35.7577	97.54
133	15.4084	9.7041	35.5974	95.51
162	14.2637	9.6438	36.2839	92.21
197	14.8616	9.5118	35.3907	92.99
228	14.5168	9.4222	35.9890	91.59
260	13.9939	9.2273	36.6908	89.12
285	13.8226	9.1102	36.1968	88.01
314	14.2674	9.0070	36.6037	88.56
344	13.3877	8.9848	36.4208	86.17
378	13.5115	8.8263	36.9739	85.57
403	13.0327	8.6849	37.7293	83.53

TABLE 2
1.00% Citric Acid
Days After
Addition	PAA %	H2O2 %	AA %	AO Rec %
—	0.8688	17.1802	47.8195	101.08
1	2.8990	15.7369	45.8501	98.00
2	5.1241	14.8095	43.8144	98.39
3	7.1400	13.9885	42.6610	98.86
4	8.4730	13.3357	41.1202	98.53
8	12.0610	11.7606	37.6254	98.70
9	12.8241	11.5194	37.3773	99.28
10	13.1537	11.3034	37.6218	98.88
11	13.3192	11.1213	37.4239	98.26
14	14.8345	10.7323	36.7059	99.92
15	14.8165	10.6578	36.3016	99.45
16	14.8555	10.5471	35.7440	98.91
17	14.9233	10.5083	36.4307	98.86
18	15.1771	10.4201	35.2700	99.01
21	15.5752	10.2907	35.7839	99.29
28	16.0195	10.1210	34.7292	99.45
36	15.4584	10.0619	35.0277	97.67
43	15.9848	10.0408	34.6613	98.90
77	15.6363	9.8988	35.8995	97.19
95	15.4839	9.8409	34.7765	96.46
133	15.0663	9.6691	36.3532	94.40
162	14.8545	9.6176	36.6276	93.56
197	14.8315	9.477	35.6838	92.69
228	14.4704	9.3548	36.0888	91.06
260	15.0582	9.2067	35.5108	91.72
285	14.3021	9.0621	36.2798	88.94
314	14.197	8.956	37.3694	88.06
344	13.9533	8.8917	36.8442	87.06
378	13.1943	8.7715	37.5735	84.42
403	12.9297	8.6296	37.1723	92.82

TABLE 3
1.15%, Citric Acid
Days After
Addition	PAA %	H2O2 %	AA %	AO Rec %
—	0.9493	14.9203	49.7722	88.31
1	2.7103	14.0927	47.8537	88.08
2	4.8674	13.5088	45.8163	90.27
3	6.7840	12.8510	44.2981	91.42
4	8.0746	12.4248	42.6000	92.29
8	11.8005	11.2592	38.8622	95.17
9	12.7506	11.1215	37.9775	96.82
10	13.0775	10.9839	37.7389	96.87
11	13.7122	10.8617	38.0374	97.80
14	14.4521	10.6744	36.8023	98.63
15	14.7832	10.5997	36.0188	99.05
16	14.9035	10.5517	35.5883	99.09
17	14.9890	10.5392	35.5743	99.23
18	15.1184	10.4180	35.4307	98.87
21	15.7941	10.3115	35.4520	100.00
28	16.1877	10.1272	34.7089	99.95
36	15.0727	10.0837	34.9070	96.83
43	15.8094	10.0461	35.0496	98.51
77	15.5986	9.8953	35.6593	97.10
95	15.5719	9.8333	35.9455	96.67
133	16.0577	9.6417	34.8538	96.82
162	15.1268	9.5816	36.0882	94.08
197	15.7167	9.5183	34.8321	95.23
228	13.9662	9.3003	36.2149	89.47
260	13.4297	9.1271	36.3155	87.09
285	14.3050	9.0125	36.4291	88.69
314	13.5222	8.9160	36.6740	86.12
344	13.8437	8.8414	37.2805	86.51
378	13.0769	8.7360	47.0423	83.93
403	12.9157	8.5879	37.2918	82.67

TABLE 4
1.24% Citric Acid
Days After
Addition	PAA %	H2O2 %	AA %	AO Rec %
—	0.7259	12.9350	51.4255	76.31
1	2.6857	13.1213	48.8097	82.42
2	4.7080	12.8994	46.6388	86.35
3	6.6668	12.4803	45.0566	88.98
4	8.1618	12.0665	43.2028	90.45
8	12.3648	11.1363	39.9843	95.91
9	12.5204	10.9715	38.9763	95.37
10	13.1867	10.8514	38.4447	96.39
11	13.8380	10.7723	37.6754	97.61
14	14.3456	10.5460	36.8458	97.62
15	14.6369	10.5380	36.0910	98.32
16	14.7992	10.4648	35.8426	98.43
17	14.9864	10.4538	35.7862	98.74
18	15.3825	10.4189	36.6292	99.55
21	15.3706	10.2649	35.4215	98.64
28	15.8088	10.1306	35.0317	98.99
36	15.8843	10.0424	35.1196	98.68
43	15.9236	10.0629	35.2217	98.90
77	15.6937	9.8773	35.6800	97.24
95	15.7823	9.7861	36.1936	96.94
133	15.1511	9.6406	35.6101	94.48
162	15.0181	9.5280	36.4564	94.39
197	15.5437	9.3555	35.0428	93.85
228	14.1509	9.2284	36.2681	89.53
260	14.9844	9.0772	35.7010	90.81
285	14.3708	8.9362	36.5596	88.42
314	14.1109	8.7402	36.4426	86.62
344	13.9393	8.7677	36.9396	86.34
378	13.4533	8.6343	37.2074	84.32
403	13.3214	8.5447	38.2168	83.46

[0026] In order to determine the amount of residue buildup produced by vaporization, various solutions were prepared by blending the following ingredients:

Samples A1, A2 and A3: 250 ppm citric acid in deionized water

[0027] Samples C1 and C2: 4000 ppm PAA+250 ppm citric acid

[0028] The PAA employed had a PAA:hydrogen peroxide:acetic acid weight ratio of about 15:10:36.

[0029] An initial portion of the sample to be tested was placed into a 50 mL burette suspended over a 100 mL beaker placed on a Corning Stirrer/Hotplate. The hotplate was heated to 180° C. and the stopcock on the burette opened such that the solution was added drop wise for vaporization. The burette was refilled before it ran out of solution. The hotplate was shut off and the beaker allowed to return to room temperature. Once it had cooled to room temperature, the beaker was reweighed to determine the amount of residue present. The results observed and the calculated amount of residue (mg/mL) are summarized in Table 5 below.

	TABLE 5
		Residue	Volume of	Mg/mL of
	Sample	(mg)	Solution (mL)	Residue
	A1	20	92.5	0.22
	A2	31	109.9	0.28
	A3	27	117	0.23
	B1	131	94.3	1.39
	C1	0	121.6	0.00
	C2	3	92	0.03

[0030] The above results show that the method of the present invention (exemplified in Samples C1 and C2) produces considerably less residue of the heating surface than does a PAA composition of equal concentration which is stabilized using a phosphonate stabilizer. This result is completely unexpected in view of the showing that the vaporization of citric acid in deionized water results in the formation of a much more residue than does the vaporization of a composition comprising an equal amount of citric acid in a PAA solution.