Membrane filtration sterilisation

Instead, membrane filtration may be used to sterilise the nutrient in this experiment. This can be accomplished by drawing the nutrient from a mixing jar and forcing it through an in-line filter (0.2 p,m pore size) either by gravity or with a peristaltic pump. The sterilised medium is fed into an autoclaved nutrient jar with a rubber stopper fitted with a filtered vent and a hooded sampling port. 

Material which is thermally unstable cannot be sterilised by autoclaving, and solutions should be sterilised by membrane filtration through a membrane with a pore size of less than 0.45 pm. Membrane filtration is dealt with in more detail in the chapter on water testing. It should be noted that membrane filters will only remove bacteria and do not necessarily remove viruses. 

Antibiotic solutions placed in reservoirs need sterilising. Autoclaving can only be used for a small number of antibiotics, and many need to be sterilised by membrane filtration. The use of cups to hold the antibiotic has the advantage that non-sterile solutions may be used. 

Heat is frequently used to sterilise or pasteurise the tubes before addition of the inoculum, but causes problems if the vitamin is not heat stable. In this case, membrane filtration may be used, and the vitamin preparation added aseptically to tubes of sterile medium. These procedures again imply a considerable level of microbiological expertise and competence. 

Steam sterilisation 15 min 121 °C Membrane filtration <1.2 pm Terminal sterilisation Terminal strailisation -... 

When heating at 100 °C (over boiling water) during 30 min in combination with membrane filtration sterile containers (dropper bottle or Redipac) and sterile solutions of excipients are required. The preparation will be performed in a Class A laminar flow workbench. After sterilisation the containers must be stored in the freezer. 

An ointment base is prepared by melting the ingredients together. Sterilisation can be performed by dry heating (see Sect. 30.5.2) or membrane filtration (see Sect. 30.6.1). Heat sterilisation requires a validated heat steriliser, which may be expensive. In addition, a disadvantage of dry heat sterilisation is the partial decomposition of the fat components. The degradation products could negatively influence the stability of the active substance and probably cause irritation of the eye. 

In an aseptic process, membrane filtration is the final step before filling. During the production of a terminally sterilised product membrane filtration is applied to reduce the bioburden. 

Classical sterilisation techniques using an autoclave and saturated steam under pressure, hot water or dry heat are practical and reliable. Other reliable sterilisation methods include membrane filtration, ionising radiatirm sterilisation (gamma and electron-beam radiation) and gas sterilisation (ethylene oxide, formaldehyde). Sterilisation equipment (autoclaves, membrane filters, and other sterilisers) is often used in industrial manufacturing, in preparation in pharmacies, and in other healthcare establishments. Standard sterilisation processes are described in the Ph. Eur., in other current Pharmacopoeias, in ISO standards and National guidelines. 

The efficacy of any sterilisation process depends on the nature of the product and container, the extent and type of any contamination before sterilisation, the production and sterilisation conditions. Pre-cleaning of materials and pre-filtration by membrane filtration result in a low bioburden. Process validation, quality assurance and quality control are necessary to secure sterility. 

Sterilisation Autoclave Sterile products Saturated steam Hot air Dry heat Radiation Ethylene oxide Hydrogen peroxide Plasma Membrane filtration Gas sterilisation... 

Because products that require sterilisation are often aqueous solutions, the initial microbiological contamination can easily be determined in the quality control laboratory by the membrane filtration method (see Sect. 19.6.3). 

Membrane filtration is the usual alternative sterilisation method in those cases. However membrane filtration as a sterilisation method is less effective and needs aseptic circumstances. If the product withstands a heat treatment at lower steam temperature (100 °C) for 30 min and no pressure, this treatment contributes to the effectiveness and safety of the membrane filtration process. If the solution contains a preservative, as is often the case with eye drops, the effectiveness of the 100 °C treatment may increase considerably. Heat treatment at 100 °C for 30 min over boiling water as such (without additional measures) is not a reliable sterilisation method and hence not recommended by the Ph. Eur. 

As shown in Table 30.1, non-spore forming bacteria do not survive 30 min at 100 °C, and neither do yeasts, fungi, and viruses. Combination of this process with sterilising membrane filtration and aseptic circumstances, gives a higher degree of certainty than aseptic filtration alone as is applied in preparation of eye drops, see Sect. 10.7, notably Tables 10.18 and 10.19. 

For non-aqueous liquids, semisolids and dry powders 2 h sterilisation at 160 °C in dry heat is preferred. Where it is not possible to carry out terminal sterilisation by heat due to formulation instability, a decision should be taken to utilise an alternative method of terminal sterilisation, filtration and/or aseptic processing. It is recognised that new terminal sterilisation processes other than those described in the pharmacopoeia may be developed to provide sterility assurance levels equivalent to present official methods and such processes, when properly validated, may offer alternative approaches. If necessary, a different time-temperature combination may be applied to obtain an SAL of 10 . If too much degradation occurs in dry heat, ionising radiation or gas sterilisation can be applied. If these methods are not suitable either, sterilising membrane filtration and validated aseptic processing, sometimes robotised or with barrier system technology are considered as a last resort. 

Other, more recently developed, uses include microwave oven parts, transparent pipelines, chemical plant pumps and coffee machine hot water dispensers. One exceptional use has been to produce, by an extrusion moulding process, very large rollers for textile finishing for use where cast nylons cannot meet the specification. Also of growing interest are medical equipment applications that may be repeatedly steam-sterilised at 134°C, filtration membranes and cartridges for ink-jet printers. 

The solid-liquid separation of shinies containing particles below 10 pm is difficult by conventional filtration techniques. A conventional approach would be to use a slurry thickener in which the formation of a filter cake is restricted and the product is discharged continuously as concentrated slurry. Such filters use filter cloths as the filtration medium and are limited to concentrating particles above 5 xm in size. Dead end membrane microfiltration, in which the particle-containing fluid is pumped directly through a polymeric membrane, is used for the industrial clarification and sterilisation of liquids. Such process allows the removal of particles down to 0.1 xm or less, but is only suitable for feeds containing very low concentrations of particles as otherwise the membrane becomes too rapidly clogged.2,4,8... 

Verification of the microbial retention efficiency of the membrane filters may be undertaken using either liquid or aerosol challenge tests. A liquid challenge test is more stringent. Furthermore, this test can provide retention information for process conditions such as extreme moisture after sterilisation or air entrained with water drops. A liquid challenge is performed using a protocol similar to that described for liquid filtration. 

For small volumes, 13 or 25 mm diameter, filtration membranes may be fitted into plastic or stainless steel holders (e.g. the Swinnex filter holder made by the Millipore Corp. Appendix 3) which, after autoclaving, are fitted onto a syringe containing the liquid to be sterilised. Care must be taken in the assembling of the membrane in the holder as incorrect assembly leads to the escape of the membrane from its retaining gaskets with subsequent failure of the filtration process. The plastic holders have a limited life time as they distort on autoclaving. 

A stock of the Hoechst 33258 bisbenzamid fluorochrome solution is made by dissolving 5 mg in 100 ml PBS-A (Appendix 1) using a magnetic stirrer. It must be free of bacterial contamination and should be sterilised by filtration through a 0.22 (im membrane and should be stored in the dark at 4°C. It should be diluted 100-fold with filtered PBS-A for use. 

Sterilise by filtration using a 0.22 jum membrane filter and store as aliquots of 5 ml at room temperature. 

Like suspensions, ointments can be more difficult to manufacture in sterile form. They can be terminally sterilised alternatively, they must be manufactured from sterile ingredients in an aseptic environment. Filtration through a suitable membrane or dry heat sterilisation is often used. 

Filtration processes may be classified as either depth filtration or surface filtration. Depth filtration relies on a layer of porous media in which suspended particles in the beer are trapped within the media. Examples in brewing include filter aid filtration, sheet filtration and some forms of filter cartridge. Surface filtration normally refers to membrane technology. A thin layer of membrane has pores throughout the structure. This means that it is possible to achieve very exact filtration, perhaps enabling sterilisation, but typically the quantity of suspended beer particles that may be removed is less than for depth filtration. 

Foreign particles can be removed by (pre)filtration over a membrane filter (<1.2 pm pore size). The use of this filter reduces the initial viable contamination as well. When autoclaving or steam sterilisation is not suitable for the product in order to remove viable contaminaticai, i.e. bacteria, the solutimi is passed through 0.2 pm membrane which will retain all bacteria. In practice a one-step procedure is preferred using only one membrane filter with a nominal pore size of 0.2 pm. For use in pharmacies this type is readily available. 

Final filtration through a sterile 0.2 pm membrane filter is not oifly a sterilisation method, but also an effective way to reduce the initial level of contamination. A pre-filtration (often in-line) with a coarser pre-filter (for example 1.2 pm) is necessary to reduce coarser foreign particles and contamination and to prevent early clogging of the 0.2 pm membrane filter. 

Certain products that cannot be terminally sterilised may be subjected to an aseptic filtration procedure [11] using a satisfactory sterile membrane filter membrane, tightly fixed in a filter holder. The operator passes the liquid product through a sterile and bacteria retentive membrane, mostly with a nominal pore size of 0.2 pm or smaller. Such membrane filters can capture most bacteria, yeasts and fungi, but not all viruses and mycoplasms. The liquid should be asepti-cally collected in a sterilised dedicated clean container directly after sterile filtration. 

---