The main reasons for the heat treatment of juice, nectar and still drinks – or JNSD – are to make them safe for consumption and prolong shelf life. Pasteurization kills microorganisms that can grow during storage and inactivates enzymes that cause unwanted clarification (cloud loss). Since fruit beverages, in general, are high-acid products (pH 4,6 or less), they do not require ultra-high temperature treatment (UHT). This is because their high acidity inhibits the growth of bacteria, fungi and yeast. Their heat treatment should be safe, but still be able to deliver quality in terms of vitamins, colour and taste.
When discussing the heat treatment of fruit drinks, we can divide them into three categories:
Juice – containing only 100% natural ingredients from fruits and vegetables, from concentrate (FC) or not from concentrate (NFC).
Nectar – Juice content ranges from 25-99% fruit juices/purees that are too thick or too acidic to drink, such as apricot, pear or peach. They are diluted and sugar is added for drinkability.
Still drinks – fruit drinks that contain less than 25% fruit.
For clarity, we will concentrate on the heat treatment of orange juice, though the basics can be applied and adjusted to most fruit drinks and high-acid beverages. Orange juice is normally pasteurized at least twice before it reaches the consumer. It is pasteurized immediately after extraction, before bulk storage, then again before packaging. Some NFC (not from concentrate) juice is pasteurized only once.
The first pasteurization is usually at 95-98°C for 10-30 seconds, killing microorganisms and deactivating pectin methyl esterase (PME), the enzyme that causes cloud loss or gelation. The second time is to destroy any microorganisms that may have contaminated the juice after the first pasteurization, during reconstitution from concentrate or that have survived storage. This is normally done at 95°C for 15 seconds for fruit juices with a pH below 4.2. When aseptically packaged, the juice can be stored under ambient conditions.
Another major reason for pasteurizing juices and juice-based drinks is to maintain the properties that make them attractive and enjoyable to consumers, such as taste, appearance and “mouthfeel”. The clarification or gelation of juice is commonly referred to as cloud loss, which is the breakdown of pectin by enzymes. In practical terms, the tangible results of cloud loss are reductions in opacity and viscosity, the latter impacting mouthfeel. Proper pasteurization prevents this undesirable process by deactivating PME.
Within juice-drink categories, we must consider the variety of fruit from which the juice is extracted, as well as growing conditions and location. For example, the PME in Valencia, Pineapple and Hamlin oranges grown in Florida has varying heat stabilities. Among those grown in Brazil (Pera Rio, Pera Coroa, Natal and Valencia), the PME in Valencia and Pera Rio has the highest heat resistance and needs 2 minutes at 90°C vs 1.5 minutes at 90°C for Pera Coroa and Natal.
The higher the pH, the more intense the heat treatment required to inactivate PME, though this varies per variety of orange, with Pineapple being the most sensitive to pH changes. Since PME is related to cellular walls, longer heating is needed to treat juices with a higher pulp content. A pulp increase of just 5-10% lengthens the necessary duration of pasteurization by 2.5 times at a given temperature.
Conversely, a misconception sometimes exists in the industry that juice for aseptic filling requires a consid¬erably greater heat load than chilled (non-aseptic) juice. Orange juice, along with other high-acid products, requires a heat treatment of 80-95°C for 15-30 seconds to become microbiologically stable for chilled storage or storage at ambient temperatures. Ultra-high temperature (UHT) treatment is not required. The most important criterion for selecting pasteurization conditions is the microbial load of the juice to be pasteurized.
Our latest research finds that a decrease in temperature during the second pasteurization process from 95°C for 15 seconds to 80°C for 15 seconds demonstrates no reduction in safety or product quality. Potential positive implications of this study include an estimated saving in energy consumption of 19% and increased flexibility for juice producers, in terms of heat-exchanger design.
Vitamin C content is a very important quality factor for orange juice. Consumers recognize orange juice as an important source of vitamin C and a highly natural and “true” product that closely resembles the fresh fruit. Vitamin C contents are often stated on the label of the package, and as vitamin C decreases over storage time – due to oxidation and non-oxidative reactions – this decrease often determines the shelf life of the product.
Heat treatment plays an essential role in securing product quality and shelf life. It has long been thought that much of the vitamin C content in orange juice is destroyed during the pasteurization process. In principle, vitamin C degradation occurs faster as temperature increases, as with many chemical reactions. One might therefore conclude that pasteurization would result in large vitamin C losses. However, the latest research shows that during standard juice pasteurization (80-105°C for 15-30 seconds) the time the juice spends at an elevated temperature is very short, and the impact on vitamin C is negligible. In fact, the study proves that dissolved oxygen has a clear impact on vitamin C retention, whereas pasteurization has very little adverse effect.
Our customers in the beverage sector need to be extremely agile to meet consumer demands for novelty, value-added ingredients and high-quality products. They need to go wherever consumer tastes take them. Most often, they produce fruit juices and nectars as well as flavoured still drinks – all on the same line. At Tetra Pak, we are constantly researching ways in which to improve pasteurization and make it as safe and gentle as possible. This includes developing the most energy-efficient equipment for heat treatment, while controlling and maximizing product quality. We strive to make food safe, everywhere and at all times.