Hot Filling is proven and recognized method for filling high -acid foods (pH-4.6) that will be shelf stable at ambient temperatures. This method is used extensively in the food industry for filling glass and plastic containers. It relies on heat treatment in tabular or plate-type heat exchangers to temperatures in the region of 90-95 degree C for at least 15 s (typically 15-30s). This process produces a "Commercially Sterile Product" by killing all microorganisms capable of growing in it. The product is then cooled and filled at temperatures ranging 82-85 °C into containers, sealed immediately with closures, and then held at this temperature for approximately 2-3 min. The hot filling will sterilize the inner surface of the container. The filled containers are usually placed on their sides so that the neck-finish and closure are also sterilized. The containers are then cooled in a cooling tunnel in order to minimize the thermal degradation of the product.
The main purpose of hot filling is to provide a product free from microorganisms capable of growing at ambient storage; that is, a "Commercially Sterile Product". Commercial sterility or thermally processed food refers to the absence of disease-causing microorganisms, absence of toxic substances and of spoilage - causing microorganisms capable of multiplication under a number of non-refrigerated storage and distribution conditions. Another benefit of this may be to reduce the viscosity of the food when filled at high temperature.
The properties of the food will largely determine the sensitivity to spoilage by microorganisms. Temperature and water activity are important factors that affect the microbiological spoilage of foods. Another factor of major importance for the growth and survival of microorganisms in foods is the pH or acidity. Low pH (high acidity) will restrict a number of microorganisms from growing and spoiling the food. It is, therefore, very important to know the acidity of the food when deciding the process conditions and the possible shelf life of the product. Foods are often divided into two main groups in terms of their acidity: high acid foods with pH 4.6 or lower and low-acid foods with pH higher than 4.6.
There are some important reasons for the acidity classifications:
- Most spores that may survive heat treatment and chemical will not grow or germinate at pH lower than 4.6.
- Pathogenic bacteria will not grow at pH lower than 4.6.
- The sensitivity to heat treatment is increased at lower pH.
Consequently, microbial spores generally do not have to be inactivated in high-acid foods and those products do not represents a public health risk since pathogenic bacteria will not grow and multiply in such an environment.
Generally, a hot-fill process can be safely performed with liquid foods with a pH of 4.0 or lower. Therefore, the food products that can be processed by a conventional hot fill process without any modification (acidification) are fruits, fruit drinks such as various nectars, ice-tea and isotonics.
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