Pasteurization is the process of heating foods to the extent that the heat inactivates a substantial number of the pathogens in the food so that the remaining ones do not disease, but not to the extent that the heat alters the taste and quality of the food significantly. It is akin to partial sterilization. Full sterilization kills all micro-organisms but also alters the chemistry of the food.
Pasteurization is commonly associated with milk but also widely applied to fruits as part of processing them for jams, jellies, syrups, juice and canning. Very often, it is the semi-processed pulp of the fruit, purees or juice extracts obtained by pressure or other means rather than the whole fruit that is subject to pasteurization.
Bacteria, molds and spores face destruction when exposed to a certain minimum temperature for a certain minimum time. Ultra-pasteurization, which is akin to the sterilization of fruits and fruit juices, may alter the taste of the fruit, and moreover, cause deterioration of the pectin substances inherent to fruits. As such, the best option to pasteurize fruit is a moderate course that keeps the food in-between its raw state and ultra-pasteurized state.
Food pasteurizing time and temperature depends on the pH of the food. Fruits have a low pH that is usually below 4.5. This means that pasteurization inactivates harmful enzymes such as pectinesterase and polygalacturonase by applying the required temperature for a relatively short time.
In contrast, milk, which has a higher pH, requires more severe heat treatment to inactive the pathogenic bacteria and requires refrigeration post pasteurization to ensure that the remaining bacteria do not multiply and grow to unacceptable levels.
One best practice is to de-aerate the fruit before pasteurization to remove oxygen and reduce the oxidative degeneration of the food.
The basic methods of fruit pasteurization are:
- Batch or Vat Pasteurization, which is simply heating the fruit products to 145 degrees Fahrenheit for about 30 minutes in a vat. This is the most common method for home users and small time manufacturers.
- UHT Pasteurization or bringing the juice over to the boiling point of about 250 degrees Fahrenheit for a fraction of a second. This is the most common method adopted by fruit juice manufacturers as it increases the shelf life without refrigeration. This method may, however, give a slightly cooked taste to the juice.
- High Temperature Short Time (HTST) Pasteurization, which is used mainly in commercial applications. The “High Temperature” is typically 171 degrees Fahrenheit for the fruit juice, which is held in a “hold tube” or in a plate type heat exchanger for under a minute, followed by rapid cooling down to 45 degree Fahrenheit. The alternative is to heat to 149 degrees Fahrenheit and hold for 30 minutes, or heat to 191 degrees Fahrenheit and hold for 15 seconds.
High Temperature Short Time (HTST) is a middle ground between Batch and UHT pasteurizing and unlike the latter two allows for a continuous process. Manufacturers may apply this method to pasteurize the juice on a commercial-scale without altering the sensory character or nutrient composition of the fruit and still ensure the product remains free of pathogens with an extended shelf life under refrigeration.
The major steps to pasteurize fruit products using this method are:
- Pumping the cold and raw product from a holding tank to a balance tank controlled by a float.
- Pumping or pulling the product from the balance tank to a heat exchanger through a hold tube using a “timing system” that controls the flow of the product through the system. Possible options for such timing systems are a centrifugal pump, homogenizer, or positive displacement pump.
- Heating the product as it passes through the heat exchanger. The most popular heating medium is recycled hot water with the temperature controlled by the direct injection of steam.
- A sensor at the end of the hold tube checks whether the product is heated to the desired temperature. If not, place a divert valve to return the product to the balance tank. If pasteurized, the product is sent to the other side of the regenerative heater to transfer the heat of the pasteurized product to the incoming product. This process, although optional, allows for faster processing.
- The pasteurized product passes through another heat exchanger to cool the product; usually to 40 degrees Fahrenheit or lower. The most popular cooling media is chilled water or food-grade glycol.
- Success depends on controlling the flow rate and designing the flow tube to ensure that the product passes through the heating mechanism for no more or no less than the desired time such as 15 seconds, one minute or more as applicable.
This method is usually relevant for medium to large-scale commercial pasteurization, but small-scale and home users may also replicate this model to build up an efficient pasteurizing system.
Encyclopaedia Brittanica. “Food Preservation.” http://www.britannica.com/EBchecked/topic/212684/food-preservation/50563/Pasteurization
Author unknown, “Pasteurization of Apple (and other) Juice,” Dairy Engineering Company, http://www.dairyeng.com/applejuice.asp