The rationale and a computer evaluation of a gamma irradiation sterilization dose determination method for medical devices using a substerilization incremental dose sterility test protocol

The experimental procedure described is designed to allow calculation of the radiation sterilization dose for medical devices to any desired standard of sterility assurance. The procedure makes use of the results of a series of sterility tests on device samples exposed to doses of radiation from 0.2 to 1.8 Mrad in 0.2 Mrad increments. From the sterility test data a 10^-2 sterility level dose is determined. A formula is described that allows a value called DS Mrad to be calculated. This is an estimate of the effective radiation resistance of the heterogeneous microbial population remaining in the tail portion of the inactivation curve at the 10^-2 dose and above. DS Mrad is used as a D ₁₀ value and is applied, in conjunction with the 10^-2 sterility level dose, to an extrapolation factor to estimate a sufficient radiation sterilization dose. A computer simulation of the substerilization process has been carried out. This has allowed an extensive evaluation of the procedure, and the sterilization dose obtained from calculation to be compared with the actual dose required. Good agreement was obtained with most microbial populations examined, but examples of both overdosing and underdosing were found with microbial populations containing a proportion of organisms displaying pronounced shoulder inactivation kinetics. The method allows the radiation sterilization dose to be derived from the natural resistance of the microbial population to gamma sterilization.     

Terminal sterilization of medical devices using vaporized hydrogen peroxide: a review of current methods and emerging opportunities

Medical devices are an important and growing aspect of healthcare provision and are increasing in complexity to meet established and emerging patient needs. Terminal sterilization plays a vital role in the provision of safe medical devices. While terminal sterilization technologies for medical devices include multiple radiation options, ethylene oxide remains the predominant nonthermal gaseous option, sterilizing c. 50% of all manufactured devices. Vaporized hydrogen peroxide (abbreviated VH2O2 by the International Organization for Standardization) is currently deployed for clinical sterilization applications, where its performance characteristics appear aligned to requirements, constituting a viable alternative low-temperature process for terminal processing of medical devices. However, VH2O2 has operational limitations that create technical challenges for industrial-scale adoption. This timely review provides a succinct overview of VH2O2 in gaseous sterilization and addresses its applicability for terminal sterilization of medical devices. It also describes underappreciated factors such as the occurrence of nonlinear microbial inactivation kinetic plots that may dictate a need to develop a new standard approach to validate VH2O2 for terminal sterilization of medical devices.     

Sterilization of Allograft Bone: is 25 kGy the Gold Standard for Gamma Irradiation?

For several decades, a dose of 25 kGy of gamma irradiation has been recommended for terminal sterilization of medical products, including bone allografts. Practically, the application of a given gamma dose varies from tissue bank to tissue bank. While many banks use 25 kGy, some have adopted a higher dose, while some choose lower doses, and others do not use irradiation for terminal sterilization. A revolution in quality control in the tissue banking industry has occurred in line with development of quality assurance standards. These have resulted in significant reductions in the risk of contamination by microorganisms of final graft products. In light of these developments, there is sufficient rationale to re-establish a new standard dose, sufficient enough to sterilize allograft bone, while minimizing the adverse effects of gamma radiation on tissue properties. Using valid modifications, several authors have applied ISO standards to establish a radiation dose for bone allografts that is specific to systems employed in bone banking. These standards, and their verification, suggest that the actual dose could be significantly reduced from 25 kGy, while maintaining a valid sterility assurance level (SAL) of 10⁻⁶. The current paper reviews the methods that have been used to develop radiation doses for terminal sterilization of medical products, and the current trend for selection of a specific dose for tissue banks.     

Radiation and Ethylene Oxide Terminal Sterilization Experiences with Drug Eluting Stent Products

Radiation and ethylene oxide terminal sterilization are the two most frequently used processes in the medical device industry to render product within the final sterile barrier package free from viable microorganisms. They are efficacious, safe, and efficient approaches to the manufacture of sterile product. Terminal sterilization is routinely applied to a wide variety of commodity healthcare products (drapes, gowns, etc.) and implantable medical devices (bare metal stents, heart valves, vessel closure devices, etc.) along with products used during implantation procedures (catheters, guidewires, etc.). Terminal sterilization is also routinely used for processing combination products where devices, drugs, and/or biologics are combined on a single product. High patient safety, robust standards, routine process controls, and low-cost manufacturing are appealing aspects of terminal sterilization. As the field of combination products continues to expand and evolve, opportunity exists to expand the application of terminal sterilization to new combination products. Material compatibility challenges must be overcome to realize these opportunities. This article introduces the reader to terminal sterilization concepts, technologies, and the related standards that span different industries (pharmaceutical, medical device, biopharmaceuticals, etc.) and provides guidance on the application of these technologies. Guidance and examples of the application of terminal sterilization are discussed using experiences with drug eluting stents and bioresorbable vascular restoration devices. The examples provide insight into selecting the sterilization method, developing the process around it, and finally qualifying/validating the product in preparation for regulatory approval and commercialization. Future activities, including new sterilization technologies, are briefly discussed.     

Insect Sterilization Activity of the 1-Methyl-1-nitroso-3-phenylurea Derivatives

The sterilization activity of the 3-substituted-phenyl-1-methyl-1-nitrosoureas was examined by a feeding test on house fly adults. This series of compounds exhibited toxicity as well as sterility, but, for some compounds, the toxicity was reduced to minimum still retaining the high sterility at lower dose levels in bait. It was suggested that the sterilization activity is dependent on physicochemical properties such as chemical reactivity and solubility in the insect body.     

Terminal sterilization of BisGMA-TEGDMA thermoset materials and their bioactive surfaces by supercritical CO2

The development of biomaterials endowed with bioactive features relies on a simultaneous insight into a proper terminal sterilization process. FDA recommendations on sterility of biomaterials are very strict: a sterility assurance level (SAL) of 10(-6) must be guaranteed for biomaterials to be used in human implants. In the present work, we have explored the potential of supercritical CO(2) (scCO(2)) in the presence of H(2)O(2) as a low-temperature sterilization process for thermoset materials and their bioactive surfaces. Different conditions allowing for terminal sterilization have been screened and a treatment time-amount of H(2)O(2) relationship proposed. The selected terminal sterilization conditions did not notably modify the mechanical properties of the thermoset nor of their fiber-reinforced composites. This was confirmed by μCT analyses performed prior to and after the treatment. On the contrary, terminal sterilization in the presence of H(2)O(2) induced a slight decrease in the surface hardness. The treatment of the thermoset material with scCO(2) led to a reduction in the residual unreacted monomers content, as determined by means of high performance liquid chromatography (HPLC) analyses. Finally, it was found that a thermoset coated with a polysaccharide layer containing silver nanoparticles maintained a very high antimicrobial efficacy even after the scCO(2)-based terminal sterilization.     

Opportunities for the application of real-time bacterial cell analysis using flow cytometry for the advancement of sterilization microbiology

Medical devices provide critical care and diagnostic applications through patient contact. Sterility assurance level (SAL) may be defined as the probability of a single viable micro-organism occurring on an item after a sterilization process. Sterilization microbiology often relies upon using an overkill validation method where a 12-log reduction in recalcitrant bacterial endospore population occurs during the process that exploits conventional laboratory-based culture media for enumeration. This timely review explores key assumptions underpinning use of conventional culture-based methods in sterilization microbiology. Consideration is given to how such methods may limit the ability to fully appreciate the inactivation kinetics of a sterilization process such as vaporized hydrogen peroxide (VH2O2) sterilization, and consequently design efficient sterilization processes. Specific use of the real-time flow cytometry (FCM) is described by way of elucidating the practical relevance of these limitation factors with implications and opportunities for the sterilization industry discussed. Application of FCM to address these culture-based limitation factors will inform real-time kinetic inactivation modelling and unlock potential to embrace emerging opportunities for pharma, medical device and sterilization industries including potentially disruptive applications that may involve reduced usage of sterilant.     

Blood tests for allergy in children. Allergy Section,Canadian Paediatric Society.

Skin tests and RASTs are important tools but do not by themselves make or exclude the diagnosis of allergy. Positive results require a confirmatory medical history, whereas negative results exclude IgE-mediated allergy with a high degree of reliability. Skins tests are preferred for routine use, because with appropriate procedural controls they are simpler, give rapid results and are cost-effective. RASTs are an excellent alternative when skin tests are precluded. Disorders such as asthma and allergic rhinitis may be provoked by irritants (e.g., cigarette smoke), physical factors (e.g., cold, dry air) and chemical substances (e.g., acetylsalicylic acid) and metabisulfite), none of which involve an IgE-antibody-mediated mechanism.     

Medical devices manufactured from latex: European regulatory initiatives

In Europe the marketing of medical devices manufactured from latex is regulated by directives describing the essential (safety) requirements that products have to fulfill to obtain marketing approval. This paper describes the general requirements for marketing medical devices in Europe and, more specifically, the requirements for products manufactured from natural rubber latex. The requirements for marketing medical devices can be fulfilled by using the relevant harmonized European standards. These standards are regularly under revision to incorporate the latest scientific developments. For certain devices, for example, latex medical (examination and surgical) gloves, specific standards have been published. Medical devices manufactured from latex pose a serious problem because of the risk of induction of allergy both against the latex proteins inherently present (type I or immediate type allergy) and against chemicals added during processing (type IV or delayed type hypersensitivity) present as residues in the latex products. So, besides requirements for product quality in terms of barrier properties, strength, and sterility, the main focus consists of the allergy-inducing properties of the latex products. Recent developments have reopened the discussion on the value of total protein versus allergen determination in latex medical gloves. However, as long as minimal levels needed for both sensitization and elicitation have not been established, a safe maximum level for leachable proteins/allergens in latex products cannot be determined. A European Commission guidance document on the latex allergy problem is currently being drafted by experts from Competent Authorities.     

Improved Sterilization of Sensitive Biomaterials with Supercritical Carbon Dioxide at Low Temperature

The development of bio-resorbable implant materials is rapidly going on. Sterilization of those materials is inevitable to assure the hygienic requirements for critical medical devices according to the medical device directive (MDD, 93/42/EG). Biopolymer-containing biomaterials are often highly sensitive towards classical sterilization procedures like steam, ethylene oxide treatment or gamma irradiation. Supercritical CO₂ (scCO₂) treatment is a promising strategy for the terminal sterilization of sensitive biomaterials at low temperature. In combination with low amounts of additives scCO₂ treatment effectively inactivates microorganisms including bacterial spores. We established a scCO₂ sterilization procedure under addition of 0.25% water, 0.15% hydrogen peroxide and 0.5% acetic anhydride. The procedure was successfully tested for the inactivation of a wide panel of microorganisms including endospores of different bacterial species, vegetative cells of gram positive and negative bacteria including mycobacteria, fungi including yeast, and bacteriophages. For robust testing of the sterilization effect with regard to later application of implant materials sterilization all microorganisms were embedded in alginate/agarose cylinders that were used as Process Challenge Devices (PCD). These PCD served as surrogate models for bioresorbable 3D scaffolds. Furthermore, the impact of scCO₂ sterilization on mechanical properties of polysaccharide-based hydrogels and collagen-based scaffolds was analyzed. The procedure was shown to be less compromising on mechanical and rheological properties compared to established low-temperature sterilization methods like gamma irradiation and ethylene oxide exposure as well as conventional steam sterilization. Cytocompatibility of alginate gels and scaffolds from mineralized collagen was compared after sterilization with ethylene oxide, gamma irradiation, steam sterilization and scCO₂ treatment. Human mesenchymal stem cell viability and proliferation were not compromised by scCO₂ treatment of these materials and scaffolds. We conclude that scCO₂ sterilization under addition of water, hydrogen peroxide and acetic anhydride is a very effective, gentle, non-cytotoxic and thus a promising alternative sterilization method especially for biomaterials.     

Sterilization of enzyme glucose sensors: problems and concepts

A useful method of enzyme glucose sensor sterilization has not only to ensure the needs of sterility assurance but has also to guarantee the functional stability of the sensors. The action of 2 or 3% alkalinized glutaraldehyde solution, as well as gamma irradiation with a dose of 25 kGy caused changes of the in vitro functionality and polymer material irritations, respectively. After a combined treatment by 0.6% hydrogen peroxide solution acting over 4 days with 7 kGy gamma irradiation only a slight loss of sensitivity must be registered. The combination of a specially designed universal homogeneous ultraviolet irradiation over 300 s with a 3 days lasting treatment by an inclusion compound of hydrogen peroxide with tensides in urea (0.15% effective hydrogen peroxide concentration) did not cause any influence on the glucose sensor function in vitro. With all methods tested here, a Bacillus subtilis spore reduction over 8 log(10) cycles from 10(6) to 10(-2) spores per test object on an average could be proved experimentally. In general, if non-thermal methods must be used it seems to be impossible to guarantee a sterility assurance level of 10(-6) as it is demanded by the pharmacopoeias. Consequently, effective concepts to produce sterile glucose biosensors for medical and biological applications should be based not only on final product treatments but should include germ reducing measures in every manufacturing step.     

Thermal Disinfection Validation: The Relationship Between A0 and Microbial Reduction

Validating a thermal disinfection process for the processing of medical devices using moist heat via direct temperature monitoring is a conservative approach and has been established as the A₀ method. Traditional use of disinfection challenge microorganisms and testing techniques, although widely used and applicable for chemical disinfection studies, do not provide as robust a challenge for testing the efficacy of a thermal disinfection process. Considerable research has been established in the literature to demonstrate the relationship between the thermal resistance of microorganisms to inactivation and the A₀ method formula. The A₀ method, therefore, should be used as the preferred method for validating a thermal disinfection process using moist heat.

Disinfection, which is defined as reducing the number of viable microorganisms on a product to a level previously specified as appropriate for its intended further handling or use, can be achieved thermally by the action of moist heat.1 Thermal disinfection during the processing of medical devices, typically performed in a washer-disinfector, is widely used for two purposes. The first is for reducing product bioburden (disinfection) either as a terminal step (e.g., for noncritical or semicritical devices) or prior to packaging and sterilization (e.g., for critical devices) in preparation for patient use. The second is to render the devices safe for handling for central service professionals during inspection and packaging.2,3 Thermal disinfection requirements therefore should consider the potential levels of microbial contamination on reusable devices after use, the desired level of reduction to render those devices safe for handling and for their intended purpose, and the reliability of the disinfection process to consistently achieve that endpoint.The microbial load on device types after patient use has been established in the literature and can vary depending on the typical clinical use of the device. For example, critical (surgical) devices, on average, have demonstrated relatively low levels of viable microorganisms (bioburden level <10² colony-forming units [CFU]/cm²).4 However, these same studies have shown the concentration of other testing analytes (e.g., protein, total organic carbon, hemoglobin) to be more noteworthy. Although the data indicate that residual clinical soil (e.g., human secretions, blood, tissue) can harbor microorganisms, the incoming product bioburden levels are far below the microbial populations challenged during an overkill sterilization process (e.g., moist heat or gaseous processes).Conservative sterilization processes have been demonstrated to achieve at least a 12-log₁₀ reduction of microorganisms with a known higher resistance versus typical bioburden.3,5 Cleaning, which is defined as the removal of contamination from an item to the extent necessary for its further processing and its intended subsequent use, is an important step to render the device ready for sterilization and will further reduce the levels of microorganisms prior to sterilization. Therefore, with critical devices, adequate cleaning followed by sterilization is the minimum requirement to ensure the device is safe for patient use.It is not likely that, for the intended use of the device, a disinfection process is strictly necessary as an intermediate step prior to sterilization. A benefit may exist to having an interim disinfection step to render the device safe for handling during inspection and packaging for sterilization. For example, the expectation in the Occupational Safety and Health Administration''s Bloodborne Pathogens standard 29 CFR 1910.1030 is that an employer will minimize the occupational exposure to bloodborne pathogens.Thermal disinfection has been used by sterile processing departments as a universal precaution to reduce the risk of exposure to processing personnel postcleaning. Although routine thermal disinfection at less than 100°C (212°F) may not be effective in deactivating all types of microorganisms (e.g., certain types of bacteria spores), it is a reliable and consistent disinfection process. As the temperature increases above a certain point (typically ≥70°C or 158°F), so does the activity against microorganisms, with variable intrinsic and acquired resistance mechanisms to heat.3 Thermal disinfection therefore will provide processing personnel with a minimized risk of bloodborne pathogens exposure.In other situations, the microbiological load can be much higher (e.g., with flexible endoscopes used in the gastrointestinal system6) or more variable (e.g., with noncritical devices or surfaces depending on their use7). Where practical, thermal disinfection is still viewed as the preferred and more reliable method to render these devices safe for use due to its known efficacy against microbial pathogens.5 Chemical disinfection generally is only considered if thermal disinfection cannot be applied (e.g., due to thermo-sensitivity of device or surface materials).     

Radiation biology of mosquitoes

There is currently renewed interest in assessing the feasibility of the sterile insect technique (SIT) to control African malaria vectors in designated areas. The SIT relies on the sterilization of males before mass release, with sterilization currently being achieved through the use of ionizing radiation. This paper reviews previous work on radiation sterilization of Anopheles mosquitoes. In general, the pupal stage was irradiated due to ease of handling compared to the adult stage. The dose-response curve between the induced sterility and log (dose) was shown to be sigmoid, and there was a marked species difference in radiation sensitivity. Mating competitiveness studies have generally been performed under laboratory conditions. The competitiveness of males irradiated at high doses was relatively poor, but with increasing ratios of sterile males, egg hatch could be lowered effectively. Males irradiated as pupae had a lower competitiveness compared to males irradiated as adults, but the use of partially-sterilizing doses has not been studied extensively. Methods to reduce somatic damage during the irradiation process as well as the use of other agents or techniques to induce sterility are discussed. It is concluded that the optimal radiation dose chosen for insects that are to be released during an SIT programme should ensure a balance between induced sterility of males and their field competitiveness, with competitiveness being determined under (semi-) field conditions. Self-contained ⁶⁰Co research irradiators remain the most practical irradiators but these are likely to be replaced in the future by a new generation of high output X ray irradiators.     

A fluorescence bioassay to detect residual formaldehyde from clinical materials sterilized with low-temperature steam and formaldehyde.

A microtiter plate toxicity test based on fluorescence was developed to determine the residual concentration of formaldehyde on medical items after LTSF sterilization. The residual formaldehyde on eight common materials, some of which are used in different clinical instruments and devices were analysed after sterilization with LTSF. Formaldehyde residues were detected on cotton, filter paper, natural rubber, PVC, and silicone-coated latex, but not on polyurethane, silicone or glass. Formaldehyde never exceeded the recommended maximum concentration on clinical devices of about 5 microg/cm2. The results were compared with those obtained by means of a chemical method, the correlation being good (R2=0.9396). The biological method proposed here is fast and can be automated, which means that it could be used as a screening method when there are doubts as to the accumulation of residues on clinical materials or instruments that are going to be sterilized with LTSF.     

Fecundity,fertility and reproductive recovery of irradiated Queensland fruit fly Bactrocera tryoni

Pupae of the Queensland fruit fly or Q‐fly Bactrocera tryoni (Froggatt) are irradiated routinely to induce reproductive sterility in adults for use in sterile insect technique programmes. Previous studies suggest that adult sexual performance and survival under nutritional and crowding stress are compromised by the current target dose of radiation for sterilization (70–75 Gy), and that improved mating propensity and survival under stress by irradiated males may be achieved by reducing the target sterilization dose without reducing the level of induced sterility. This raises the question of the amount by which the irradiation dose can be reduced before residual fertility becomes unacceptable. The present study measures the levels of residual fertility in male and female irradiated Q‐flies at different irradiation doses (20, 30, 40, 50, 60 and 70 Gy), and investigates the possibility that fecundity and fertility increase between 10–15 and 30–35 days post emergence. Male flies require a higher dose than females to induce sterility, with no residual fertility found in females irradiated at doses of 50 Gy or above, and no residual fertility found in males irradiated at doses of 60 Gy or above. Irradiated females are more fecund at 30–35 days post emergence than at 10–15 days. However, fertility does not increase between 10 and 15 days post emergence and 30–35 days, even at doses below 50 Gy. The present study shows that there is scope to reduce the target sterilization dose for Q‐flies below that of the current dose range (70–75 Gy) at the same time as retaining an adequate safety margin above radiation doses at which residual fertility can be expected.     

The radiation resistance of ascospores and sclerotia of Pyronema domesticum

The Food and Drug Administration has become aware of several instances where supposedly sterile medical surgical products made of Chinese cotton have been found to contain viable Pyronema domesticum. The aim of this research was to determine the gamma and electron beam radiation resistance values for the two dormant phases (ascospores and sclerotia) of P. domesticum. The resistance values were obtained by developing a standardized system to cultivate, purify, and harvest biological indicators containing sclerotia or ascospores. Ascospores were more resistant to radiation than sclerotia. The D ₁₀ values for sclerotia were 0.79 and 1.09 kGy for strains 32030 and 14881, respectively. The resistance value for wild type ascospores was 2.83 kGy. The current standard for assuring radiation sterilization of medical devices is ISO 11137. This standard was developed to address the propensity for highly radiation-resistant organisms such as P. domesticum. Prior to the standard, biological indicators such as Bacillus pumilus, having a nominal D ₁₀ value or 1.7 kGy, were used to determine the sterility of many medical devices. Journal of Industrial Microbiology & Biotechnology (2002) 29, 51–54 doi:10.1038/sj.jim.7000267 Received 09 October 2001/ Accepted in revised form 08 April 2002     

Substantiation of 25 kGy radiation sterilization dose for banked air dried amniotic membrane and evaluation of personnel skill in influencing finished product bioburden

Preparation of amniotic membrane (AM) by air drying method followed by radiation sterilization is simple and valuable approach; sterility and quality of the final AM product are depending on the quality management system at the tissue bank. Validation and substantiation of radiation sterilization dose (RSD) for tissue allografts is an essential step for the development and validation of the standard operating procedures (SOP). Application of SOP is perfectly relying on trained staff. Skills differences among personnel involved in AM preparation could have an effect on microbiological quality of the finished product and subsequently on the RSD required. AM were processed by four different couples of the tissue bank technicians. The AM grafts were randomly selected and subjected to bioburden test to validate and substantiate the 25 kGy RSD. Bioburden test for AM grafts were also useful to evaluate the skill of the tissue bank technicians and thus, to validate the current SOP for air dried AM. Moreover, the effect of placental source on bioburden counts on AM grafts was assessed. Substantiation of the 25 kGy RSD at a sterility assurance level of 10^?1, and sample item portion = 1, was carried out using Method VD _max ²⁵ of the International Organization for Standardization, document no. 11137-2 (ISO in Sterilization of healthcare products—radiation—part 2: establishing the sterilization dose, Method VDmax—substantiation of 25 kGy or 15 kGy as the sterilization dose, International Standard Organization, 2006). The results showed that there were no significant differences in the bioburdens of the four batches (α = 1 %), this means no significant differences in the skill of the four couples of the tissue bank technicians in terms of their ability to process AM according to the air dried AM SOP. The 25 kGy RSD was validated and substantiated as a valid sterilization dose for the AM prepared with the current established SOP at the Biotechnology Research Center experimental tissue bank. The donor’s type of delivery, normal or caesarean, showed no significant effect on the levels of microbial counts on the tested AMs (α = 1 %).     

Sterilization of sealed PVDF pouches containing decellularized scaffold by electrical stimulation

A terminal sterilization process for tissue engineering products, such as allografts and biomaterials is necessary to ensure complete removal of pathogenic microorganisms such as the bacteria, fungi, and viruses. However, it can be difficult to sterilize allografts and artificial tissue models packaged in wet conditions without deformation. In this study, we investigated the sterilization effects of electrical stimulation (ES) and assessed its suitability by evaluating sterility assurance levels in pouches at a constant current. Stability of polyvinylidene fluoride pouches was determined by a sterility test performed after exposure to five microorganisms (Staphylococcus aureus, Bacillus subtilis, Pseudomonas aeruginosa, Escherichia coli, and Candida albicans) for 5 days; the sterility test was also performed with decellularized human dermal tissues inoculated with the five microorganisms. Sterilization using ES inactivated microorganisms both inside and outside of sealed pouches and caused no damage to the packaged tissue. Our results support the development of a novel system that involves ES sterilization for packaging of implantable biomaterials and human derived materials.     

Complete design analysis of a continuous sterilizer for fermentation media containing suspended solids

An improved mathematical model was developed to design a continuous sterilizer for liquid fermentation media containing suspended solids. Unsteady-state energy balances were used to determine the temperature distribution in the liquid medium and in the solid particles as a function of time and position within the particle, as the medium flows through the sterilizer. Such temperature profiles were used to determine the level of microbial reduction achieved by each component of the sterilizer and by the entire sterilization process. A considerable difference exists between the temperature in the particle core and in the surrounding liquid. This has a significant impact on the degree of sterility achieved by the process. The level of microbial reduction in the particles was found to be tens and or even hundreds of order of magnitude lower than the corresponding level achieved in the liquid. The role of the particle material on the degree of sterilization was also investigated. Solid materials typically found in fermentation media, such as wood or flour clumps were found to offer considerable resistance to the sterilization of the organisms lodged inside them. (c) 1993 Wiley & Sons, Inc.