Sterilization method

[0001] The present invention is directed to a method of sterilizing a surface employing a vapor comprising peroxyacetic acid. This vapor is created by introducing a solution comprising diluted peroxyacetic acid solution into a hot gaseous stream. The use of such a true vapor results in the desirable sterilization of a substrate without the deposition of condensate droplets onto its surface.

[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 known in the art which are useful for bacterial sterilization is peroxyacetic acid. Typically, peroxyacetic acid is employed in aqueous-based systems as an equilibrium mixture comprising peracetic acid, acetic acid, and hydrogen peroxide. While such systems have been shown to be effective, in many instances a separate rinsing and/or drying step is required. This can add considerable expense and time to the sterilization process, particularly when the substrate may be adversely affected by high temperatures needed to expedite the drying process.

[0004] It would therefore be highly desirable to possess a method for using peroxyacetic acid as a sterilizing agent which method did not require an energy intensive or time delaying drying step.

[0005] The present invention is directed to a method of sterilizing a material, said method comprising the steps of:

[0006] a) introducing a solution comprising peroxyacetic acid into a hot gaseous stream to produce a peroxyacetic acid vapor; and

[0007] b) contacting such peroxyacetic acid vapor with the material to be sterilized.

[0008] This method permits the effective sterilization of a material without the need for a subsequent drying step as solution droplets are not formed and not deposited upon the surface of the material so treated. Accordingly, a wide variety of materials may be rapidly and economically sterilized employing the method of this invention.

[0009] The present invention is directed to a method of sterilizing a material, said method comprising the steps of:

[0010] a) introducing a solution comprising peroxyacetic acid into a hot gaseous stream to produce a peroxyacetic acid vapor; and

[0011] b) contacting such peroxyacetic acid vapor with the material to be sterilized.

[0012] As is employed herein, the term vapor intended to mean a state in which the peroxyacetic 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 the use of peroxyacetic acid in vapor form provides excellent sterilization of materials without the concomitant formation of water droplets on the material surface.

[0013] Peroxyacetic acid is typically employed in the form of an aqueous equilibrium mixture of acetic acid, hydrogen peroxide and peroxyacetic acid. [ratios of 35:10:15]. Such composition may typically further comprise stabilizers such as phosphonic acids or phosphonates, i.e. Degquest 2010 or 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.

[0014] Prior to introduction into the heated gas stream, the peroxyacetic acid is preferably diluted, by the addition of water, to a concentration of less than about 10,000 ppm, preferably of less than about 4,000 ppm.

[0015] 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 peroxyacetic acid solution. The heated gas stream at the point of peroxyacetic acid should have a temperature of at least 5° C. higher than the dew point of peroxyacetic acid (ca. 46.5°-49.9° C.); i.e., of at least about 55° C., in order to ensure that the peroxyacetic acid is converted into a vapor rather than a fog or mist.

[0016] The peroxyacetic 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.

[0017] The peroxyacetic 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 peroxyacetic 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.

[0018] A wide variety of materials may be sterilized employing the method of this invention, including hard surfaces of metals, plastics, polymers, and elastomers.

[0019] The present method may be used to sterilize materials contaminated with those bacteria typically controlled by peroxyacetic 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, Capmylobacter, Pseudomonas, Candida, Rhizopus, Mucor, Influenza etc.

[0020] The following Example is presented to offer further illustration of the method of this invention.

[0021] Employing a DA 2000 aseptic pouch filler, 1.5 Liters of a diluted solution of peroxyacetic acid (having a concentration of 4000 PPM) were injected over a period of 17 minutes into a sterile air stream which had been heated to 300° C. and allowed to cool to 90° C. No misting, fog or condensate was observed, indicating that full vaporization had occurred. Coupons inoculated with the spore-forming organisms listed at the concentrations listed were placed within the machine “head” sterile areas and exposed to the peroxyacetic acid vapor. No condensate was observed on any of the coupons tested. Upon completion of the injection of the peroxyacetic acid solution, the coupons were immediately removed from the machine. Samples were taken from the coupons and transferred to appropriate growth media, cultured, and monitored for growth. The results of such testing are summarized in Table 1.

[0022]

TABLE 1
		Organism	Total	No
Test #	Organism	Concentration	Sites	Growth	Growth
1	B. thuringiensis	7.5 × 104	70	70	0
2	B. thuringiensis	7.5 × 104	70	70	0
3	B. thuringiensis	4.5 × 105	70	70	0
4	B. thuringiensis	4.5 × 105	70	69	1
5	B. thuringiensis	3.5 × 106	70	69	1
6	B. thuringiensis	3.5 × 106	70	68	2
7	B. atrophaeus	3.1 × 104	9	9	0
8	B. atrophaeus	3.1 × 104	9	9	0
9	B. atrophaeus	2.0 × 105	9	9	0
10	B. atrophaeus	2.0 × 105	9	9	0

[0023] The above results demonstrate the excellent activity demonstrated by the peroxyacetic acid vapor employed in the method of this invention.