The Science Behind Juice Aromas And Their Meanings

The aroma of a juice is an important quality indicator. Aroma compounds in juice are influenced by factors such as temperature, processing methods, storage time, and heat treatment. For example, freshly squeezed orange juice stored at room temperature or 37°C had higher total aerobic plate counts than juice stored at 4°C, and most terpenes decreased when stored at 4°C after 15 days. The aroma of juice can also be enhanced by adding fruit essences, which are highly aromatic waters collected during the juice concentration process.

Juice aroma can be collected and bottled to be used in food and beverages

When juice is heated during the production of concentrates, the aroma vaporises. This vapour can be collected and condensed back into a liquid, which is known as a fruit essence or distillate water. Fruit essences are clear, water-soluble, and do not contain any sugar or calories. They can be added to food and beverages to deepen and round out flavours without adding any sweetness.

Aroma recovery systems are used to collect the fruit aroma. These systems are embedded in the existing pipeline system and consist of a bell column and an aroma station. In the bell column, the initial condensation of the aroma vapours takes place, and in the aroma station, the aroma is condensed to an appropriate concentration level and then cooled. Cooling is performed in two stages: first using cooling water, and then using a refrigeration unit.

The principle of aroma recovery is to evaporate part of the juice and collect the vapours to recover the aroma. The vapour/aroma mixture is separated in a distillation column, and the aroma is then concentrated in a rectification column. The recovered and concentrated aroma must be stored in a refrigerator if it is being stored for any period before being added back to the juice for bottling.

The aroma of citrus fruit is a speciality, consisting of a water phase and an oil phase. Citrus juices contain small amounts of volatile oils, and during processing, a small amount of peel oil gets into the juice. The oil has a very special aroma and is different from the peel oil in terms of analysis and flavour. The oil and water phases are kept separate and added back to the juice according to individual requirements.

Heating juice releases its aroma, which can be collected and condensed back into a liquid

When juice is heated, the liquid evaporates, and the vapour can be collected and condensed back into a liquid. This liquid is known as fruit essence or distillate water. Fruit essence is a highly aromatic water solution that captures the essence of the fruit, including its aroma and flavour.

The process of heating juice to release its aroma involves using heat to separate the liquid from the solid matter in the juice. The liquid vapour is then collected and cooled to condense it back into a liquid form. This process is known as distillation and is commonly used to create fruit essences or distillate water.

Fruit essences have a delicate flavour and can be used to enhance the taste of juices and other beverages. They are clear and do not affect the appearance or calorie content of the final product. Additionally, fruit essences do not contain any sugar, making them a suitable option for those who are conscious of their sugar intake.

The heating of juice can also affect its quality. For example, a study on the effect of heat treatment on the quality of citrus juices found that as the duration and temperature of the treatment increased, there was a significant decrease in total phenolic content, reducing sugars, ascorbic acidity, and antioxidant activity. Therefore, it is essential to carefully control the temperature and duration of heat treatment to maintain the desired quality of the juice.

Aroma compounds in orange juice change during storage, with terpenes decreasing at 4 °C and increasing at room temperature and 37 °C

The aroma of freshly squeezed orange juice is due to a complex mixture of volatile compounds, including aldehydes, esters, alcohols, ketones, and terpenes. The aroma of orange juice is profoundly affected by processing, packaging, and storage conditions.

Freshly squeezed orange juice is unstable, and thermal processing is required to reduce enzyme and microbial activity. Heating protocols range from lightly heated not-from-concentrate (NFC) juices to twice-heated reconstituted from concentrate (RFC) juices. Heating reduces levels of reactive aroma impact compounds and creates off-flavours or their precursors from Maillard, Strecker, and acid-catalysed hydration reactions.

Aroma volatiles are further altered by subsequent time-temperature storage conditions. Storage temperature is the most important factor in altering the aroma of orange juice. The overall aroma of orange juice does not change significantly if stored at refrigerated temperatures (4-6°C) for up to 16 weeks, but changes in aroma compounds have been observed in orange juices stored at higher temperatures.

In a study of freshly squeezed orange juice stored at different temperatures, it was found that most terpenes decreased at 4 °C after 15 days of storage, while 10 and 8 terpenes increased during storage at room temperature and 37 °C, respectively. α-Terpineol was the only off-flavour compound found in juice stored at 4 °C, while terpinen-4-ol, 4-ethylguaiacol, and p-vinylguaiacol were found in juice stored at 37 °C.

The aroma of freshly-squeezed orange juice is due to a complex combination of volatile compounds, including esters, aldehydes, alcohols, ketones, and terpenes

The concentration of these volatile compounds can vary depending on factors such as temperature, processing methods, storage time, and heat treatment. For example, higher storage temperatures can lead to an increase in certain terpenes and sulphur compounds. Additionally, the addition of flavor packs, which contain ethyl butyrate, can enhance the perception of freshness in commercial orange juice.

The aroma of orange juice is not only important for its sensory appeal but also plays a role in determining the quality of the juice. Changes in aroma compounds during storage can indicate spoilage, with off-flavor compounds like α-terpineol, terpinen-4-ol, 4-ethylguaiacol, and p-vinylguaiacol being indicators of degradation.

The complex interplay of these volatile compounds in freshly-squeezed orange juice creates a unique and desirable flavor profile that contributes to its popularity as a beverage.

Aroma compounds in orange juice can be analysed using solid-phase microextraction–gas chromatography–mass spectrometry (SPME–GC–MS)

SPME–GC–MS is a sensitive and selective method that can detect and separate a wide range of volatile and semi-volatile compounds, making it ideal for analysing complex mixtures such as orange juice. The technique involves three main steps:

• Solid-Phase Microextraction (SPME): A fibre coated with a sorbent material is exposed to the headspace above the orange juice sample, allowing aromatic compounds to be absorbed onto the fibre. Different types of fibres, such as Carboxen/polydimethylsiloxane (CAR/PDMS) or divinylbenzene/Carboxen/polydimethylsiloxane (DVB/CAR/PDMS), can be used depending on the specific compounds of interest. The exposure time and sample equilibration conditions may also be optimised to improve the representation of the original orange juice aroma.
• Gas Chromatography (GC): The SPME fibre, now loaded with aroma compounds, is desorbed in the injector of a gas chromatograph. The compounds are separated based on their interaction with a stationary phase and their differential partitioning between the stationary and mobile phases. This separation step allows for the identification and quantification of individual compounds within the complex mixture.
• Mass Spectrometry (MS): After separation by GC, the compounds are ionised and their mass-to-charge ratios are measured. This provides additional information for definitive identification of the compounds, including their structural characteristics.

By applying SPME–GC–MS to orange juice, researchers can identify and quantify the specific aroma compounds present. This information can be used to characterise the overall aroma profile, understand the impact of different compounds on the perceived aroma, and potentially optimise juice processing methods to enhance desirable aroma characteristics.

Additionally, the SPME extracts can be evaluated using direct gas chromatography-olfactometry (D-GC-O), where a trained panel assesses the odor impressions to determine how well they resemble the original orange juice aroma. This approach provides a more direct measure of the sensory quality of the extracts.

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