Cloudy or clear apple juice: the heat exchanger's role

Although the volume of apple juice consumed by Australians is fairly consistent, in line with market trends for other juice products, the value of the sector currently stands at $129.8 million, with each person consuming an average of $5.09 of juice a year.

While heat treatment is important in ensuring the safety of juice through pasteurisation or sterilisation, for some products heat plays an important role in the production process itself.

As with other juice products (and orange juice in particular), the apple juice market is increasingly fragmented, with new brands and high-end freshly pressed products increasing in popularity. The physical and chemical properties must be considered when they are juiced and processed. Sugar content is typically around 11%, while dry matters vary between 13 and 20% depending on variety and growing conditions.

Processing apples for industrial juice production typically involves the same technological processes, although, depending on the exact production method employed, they may not always occur in the same order. To produce a high-quality product, it is necessary that the treatment processes described below are non-aggressive and do not affect the product in a negative way. Such treatments also guarantee that there is no contamination from external sources.

Another factor to consider is that apples contain starches, which will foul surfaces during thermal treatment. This makes corrugated tube, or even scraped-surface, heat exchangers more appropriate than plate heat exchangers for such applications as they require less cleaning and are more energy efficient.

Because apples are a hard fruit, to obtain the most juice it is important to break them down prior to pressing — a process known as maceration. This mechanical process turns the whole fruits into pulp, and enzymes are sometimes used to increase the juice extraction. Some processes heat the pulp to a set temperature before juice is extracted; a scraped-surface heat exchanger, such as the HRS R Series, is suitable for such purposes. The juice is usually extracted using some form of mechanical pressing, and what happens to the raw (cloudy) juice which is obtained depends on whether a cloudy or clear product is required.

Separation of the various parts of the apple product and pulp is carried out using decanters and clarifiers during various stages of the process. Depending on end use and available heat sources (such as heat left over from pasteurisation), leftover pulp (pomace) may be dried or concentrated for other uses

Cloudy juice

Where a cloudy juice, which contains particles in suspension which will not precipitate out, is required, the pulp temperature is normally raised from around 10 to 25°C in order to efficiently extract the product, then further heated to 55°C to carry out the enzymatic treatment, which extracts more juice and makes the juice sweeter. The extracted juice is then sent for further treatment.

Clear juice

Producing clear apple juice follows a similar process, but the pulp temperature is raised to 55°C for the enzymatic depectinisation treatment. This removes pectins and other compounds which give the juice its cloudy appearance.

Once the juice has been obtained, it may be pasteurised (or sterilised, depending on market requirements) and, if being sold as an ingredient, it will also be concentrated to save on storage and shipping costs. Both processes may be carried out using heat exchanger-based systems, such as the HRS Thermblock Series of pasteurisers and sterilisers or the HRS Unicus Series of scraped-surface evaporators.

The HRS Thermblock MI pasteurisation system combines pasteurisation and aseptic packing in one solution.

From here the finished product then cooled to around 3°C for storage (either as part of the pasteurisation system or using another separate heat exchanger) before being aseptically packaged, either for the consumer (in bottles or cartons) or for industrial customers (in lined boxes or IBCs).

As can be seen from this brief overview, thermal treatments play an important role in apple juice production, and therefore the energy costs associated can be significant. It therefore makes sense to choose the most efficient equipment for each stage of the process.