EL CONCEPTO DE 'ESTERILIDAD COMERCIAL'

El Código internacional recomendado de prácticas de higiene para alimentos poco ácidos y alimentos poco ácidos acidificados envasados (CAC/RCP 23-1979, Rev. 2 (1993)), establece en su Sección XI (especificaciones aplicables al producto terminado) y Anexo V (procedimientos básicos para determinar las causas microbiológicas del deterioro de alimentos poco ácidos y alimentos poco ácidos acidificados envasados), que los productos deberán ser comercialmente estériles y no deberán contener ninguna sustancia originada por microorganismos en cantidades que puedan representar un riesgo para la salud.

El término: "esterilidad comercial" para un alimento tratado térmicamente, aparece recogido en los documentos del CODEX (CAC/RCP 23-1979 y CAC/RCP 52-2003) como la condición que se logra por aplicación de calor suficiente, sólo o en combinación con otros tratamientos apropiados, con objeto de liberar a ese alimento de microorganismos capaces de reproducirse en él en unas condiciones normales no refrigeradas en las que se mantendrá probablemente el alimento durante su distribución y almacenamiento.

El tratamiento térmico necesario para hacer que los alimentos poco ácidos envasados sean comercialmente estériles dependerá de la carga microbiana, de la temperatura inicial, composición y pH del producto, de la presencia de sustancias conservadoras, de su actividad acuosa, del tamaño y tipo del envase, y de la temperatura de almacenamiento.

El procedimiento para establecer el tratamiento térmico necesario para un producto dado deberá:

1.    garantizar la esterilidad comercial, y

2.    determinarse teniendo en cuenta la calidad deseada del producto.

 

Los términos de “conserva” y “esterilidad comercial”, aparecen asociados en todas las referencias documentadas y el concepto de "comercialmente estériles" hace hincapié en las condiciones previstas de distribución y almacenamiento.

Para manejar una definición “cuantificable” del concepto de esterilidad comercial, podemos consultar el "Glossary of Fish Technology Terms" preparado por la "Fisheries Industries Division" de la FAO, en diciembre de 2000 (actualizado en febrero de 2002):

Commercial sterility: A compromise on the practical impossibility of achieving full sterility in the contents of a hermetically sealed container during commercial heat processing, whereby the initial bacterial load is destroyed through sufficient decimal reductions to reduce the possibility of a single organism surviving to an acceptably low level. This level depends on the organism, usually Clostridium botulinum, which the process is designed to destroy. The time required to reduce the number of spores of this organism (or any other micro-organism) by a factor of 10 at a specific reference temperature (121.1°C) is the decimal reduction time, or D value, denoted D₀. The D₀ value for Clostridium botulinum spores can be taken as 0.25 minutes. To achieve a reduction by a factor of 10¹², regarded as an acceptably low level, requires 3 minutes at 121.1°C, and is known as the process value, or F value, designated F₀ so, in this case, F₀ = 3, which is known as a botulinum cook.

Aquí ya tenemos una referencia sobre el valor de F₀ mínimo (teórico) necesario para alcanzar dicha condición.

si nos vamos a la definición de "enlatado":

 

Canning: A means of obtaining long-term microbiological stability for non-dried foods without the use of refrigeration, by prolonged heating in hermetically sealed containers, such as cans or retortable pouches, to render the contents of the container sterile. This entry does not deal with the detailed stages of the entire canning process, e.g. the pre-processing steps such as preparation of the fish, brining, salting, smoking, marinating, or the process steps of pre-cooking, exhausting, sealing, seal integrity, retorting, and post-process cooling. The process depends on a thermal process lethality, in terms of exposure time to a specific high temperature, sufficient to effectively eliminate the most dangerous and heat resistant pathogens, particularly Clostridium botulinum. Thermal processes are calibrated in terms of the equivalent time the thermal centre of the product, i.e. the point of the product in the container most distant from the heat source or cold spot, spends at 121.1°C, and this thermal process lethality time is termed the F₀ value. Low acid foods having a pH greater than 4.5, which includes most canned fish products, must receive a minimum thermal process of F₀ 3 (121.1 °C for 3 minutes), known as a botulinum cook, or an equivalent heat sterilisation process. The heat process F₀ 3 is designed to achieve a 12 log cycle reduction in spores of Clostridium botulinum, which means a less than one in 10¹² chance of survival. This is regarded as an acceptably low level possibility of survival of this dangerous pathogen and is the basis of so-called commercial sterility.

Canners often use F₀ values ranging from 6 to 14 for low acid products (see below) to provide an additional safety margin to compensate for inaccuracies in temperature measurement. Canned foods destined for hot countries, or manufactured in hot countries, may require a much higher thermal process, such as F₀ greater than 10.

La situación obtenida tras la aplicación de valores de F₀ sobre 10-12 minutos, se conoce habitualmente como "esterilidad tropical"

y en la definición de "Heat- treated (heat-processing)":

«..Long term microbiological stability for non-dried products without the use of refrigeration is obtained by prolonged heating in hermetically sealed containers, such as cans or retortable pouches. The severity of the heat process is greater for low acid foods (pH greater than 4.5), which must receive a minimal thermal process, equivalent to a botulinum cook, sometimes called a full heat sterilisation process. Products manufactured in, or destined for, countries with hot climates may require a much more severe thermal process ».

El "Food Safety and Inspection Service" (FSIS) dependiente del Departamento de Agricultura de EEUU establece en los documentos "Principles of Termal Processing " y "Microbiology of Commercially Sterile and Shelf-Stable Products" que los productos en conserva son tratados con calor para hacerlos comercialmente estériles, o "shelf-stability":

As mentioned in the Introduction Section, "canned" products are treated with heat to make them commercially sterile. The condition of commercial sterility (or shelf-stability as it is characterized in the FSIS Canning Regulations -9 CFR 318.300 and 9 CFR 381.300) is recognized as follows:

Commercial Sterility – The condition achieved by application of heat, sufficient alone or in combination with other ingredients and/or treatments, to render the product free of microorganism capable of growing in the product at non refrigerated condition (over 50°F or 10°C) at which the product is intended to be held during distribution and storage.

A condition of commercial sterility will result in products that are safe to eat because the pathogens of concern are destroyed or inactivated. The product will remain shelf-stable as long as the container is intact because any spoilage organism that favors the environmental conditions within the container (i.e., anaerobic) and normal storage temperatures (i.e., mesophilic bacteria) are also destroyed with the thermal process.

En estos documentos también se establece un valor de F₀ mínimo de 3 para alcanzar la condición de "Minimum Health” and Commercial Sterility Processes.

Revisando los documentos del FSIS nos encontramos con un concepto nuevo, el de "shelf-stability", término que tradicionalmente siempre se ha asociado al concepto de “esterilidad comercial”. El FSIS admite que un producto “comercialmente estéril” pueda contener esporos termófilos viables (siempre que su destino sea el de distribuirse y almacenarse a temperaturas inferiores a los 40ºC y no representen un riesgo para la salud del consumidor):

«The term "shelf stability" traditionally has been used by the Agency and is synonymous with the terms "commercial sterility" or commercially sterile". Shelf stability is defined in CFR title 9, part 318, Subpart G, 318.300 (u) of the Food Safety and Inspection Service (meat and poultry) USDA regulations. Shelf stability (commercial sterility) means "the condition achieved by application of heat, sufficient, alone or in combination with other ingredients and/or treatments, to render the product free of microorganisms capable of growing in the product at non-refrigerated conditions (over 50°F, 10°C) at which the product is intended to be held during distribution and storage". Such a product may contain viable thermophilic spores, but no mesophilic spores or vegetative cells. These products usually are stable for years unless stored at temperatures of 115-130°F (46-55°C) which may allow swelling or flat sour spoilage to occur because of germination and growth of the thermophilic spores ».

« samples of canned foods are not routinely incubated at 55°C because the results usually will be confusing and provide no sound information. Canned food lots that are to be held in hot vending machines or are destined for tropical countries are exceptions to this rule ».

El FSIS regula los productos cárnicos y aves de corral. Los productos de la pesca están bajo la competencia de la FDA.

En nuestro ámbito legislativo también se admite la presencia de un determinado tipo y número de termófilos para los productos en conserva:

• la Orden del 2 de agosto de 1991, por la que se aprueban las normas microbiológicas para los productos de la pesca y de la acuicultura (B.O.E. nº 195, 15.08.91), establece que los productos en conserva no deberán sobrepasar las limitaciones microbiológicas especificadas:

1.    Ausencia de microorganismos que crezcan y se multipliquen previas las pruebas de preincubación, durante 30 días a 31ºC ±1ºC y 10 días a 44ºC.

2.    Máximo de 10 esporos de Bacillaceae (por gramo) termoestables no patógenos, no toxigénicos e incapaces de alterar la conserva.

3.    Ausencia de toxina botulínica.

En el Real Decreto 1437/1992 por el que se fijaban las normas higiénico sanitarias aplicables a la producción y comercialización de los productos pesqueros y de la acuicultura, pedía por parte del fabricante, una incubación a 37ºC, 7 días ó 35ºC, 10 días y examen microbiológico. Actualmente, con la entrada en vigor del Reglamento 852/2004 relativo a la higiene de los productos alimenticios, los parámetros a controlar en alimentos envasados herméticamente tras el tratamiento térmico son la temperatura y la microbiología entre otros.

En el mismo ámbito, la  Food & Drug Administration (FDA), define el concepto de “esterilidad comercial” en su “Thermally processed low-acid foods packaged in hermetically sealed containers” (part 113, Title 21 del Code of Federal Regulations) como:

(e) Commercial sterility:

(1) ‘‘Commercial sterility’’ of thermally processed food means the condition achieved:

(i) By the application of heat which renders the food free of:

(a) Microorganisms capable of reproducing in the food under normal nonrefrigerated conditions of storage and distribution; and

(b) Viable microorganisms (including spores) of public health significance; or

(ii) By the control of water activity and the application of heat, which renders the food free of microorganisms capable of reproducing in the food under normal nonrefrigerated conditions of storage and distribution.

El "Compendium of Analytical Methods" del Departamento Federal de Salud de Canadá, podemos encontrar un documento (HPM Method MFHPB-01 y suplemento) donde se detalla un protocolo para la determinación de la esterilidad comercial y presencia de microorganismos viables en alimentos enlatados. En él tenemos una definición de "comercial Sterility" que expone lo que el resto de documentos no concretan:

Commercial Sterility: the condition achieved by the application of heat, alone or in combination with other treatments, to render a food free from viable forms of microorganisms, including spores, capable of growing in the food at temperatures at which the food is designed normally to be held during distribution and storage. If the normal temperature for storage or handling of the product is higher than 40°C then analyse for Thermophiles. Otherwise, analyse for Mesophiles only.

 Para mantener las cualidades de un producto en conserva, la integridad del envase deberá permanecer intacta y la temperatura de almacenamiento deberá ser inferior a los 35ºC (95ºF) (National Center for Home Food Preservation) - 40ºC (104ºF) (Código Internacional Recomendado de Prácticas para el Pescado en Conserva - CODEX CAC/RCP 10-1976), estando el intervalo óptimo de almacenamiento comprendido entre 10ºC y 21ºC según el National Center for Home Food Preservation. Una vez abierto el producto en conserva, su comportamiento se asemeja al de un alimenticio cocido, por lo que deberá almacenarse bajo condiciones de refrigeración durante no más de 3-4 días (CFSAN-Center for Food Safety & Applied Nutrition).