BITTERNESS IN CITRUS JUICES (part 1) – ‘NARINGIN’

Bitterness in Citrus juices, other than Grapefruit, is perceived as having a negative influence on taste.

In general, there are three naturally occurring components in citrus that contribute to bitterness:-

1. The flavonoid Naringin that occurs predominantly in Grapefruit and provides the expected bitter note.
2. The limonoid Limonin that exists in a tasteless pre-cursor form in all citrus and develops during or after processing. The bitter taste is largely unpleasant and is sometimes described as ‘dirty’ or ‘earthy’ with a lingering aftertaste.
3. Citrus Oil, which if present in excess amount produces a harsh, bitter like taste sensation.

This article describes the occurrence, properties and control of Naringin.

Occurrence

Citrus fruits are a rich source of flavonoids, of which Naringin is one.

Flavonoids are aromatic secondary plant metabolites based on a C6-C3-C6 carbon skeleton of two aromatic rings, A and B, connected through a pyrone ring, C. they are of interest in human nutrition having been associated with a diverse range of effects such as:-

• reduced risk for certain chronic diseases
• prevention of some cardiovascular disorders
• prevention of certain kinds of cancerous processes
• antiviral properties
• antimicrobial properties
• anti-inflammatory activity
• beneficial for capillary fragility
• inhibition of human platelet aggregation
• anti-ulcer effect
• anti-allergenic properties

Although it has to be said that most of the effects have been found from in vitro studies.
Naringin is metabolised to naringenin in humans and inhibits some drug-metabolising enzymes. Flavonoids generally can in increase or decrease intestinal absorption of some drugs. For example, consumption of grapefruit is frequently not advised when taking medication for hypertension.

In plants there are 6 classes flavonoids:-

• Flavanones*
• Flavones*
• Flavonols*
• Isoflavonoids
• Anthocyanins*
• Flavans

The four flavonoids marked * are found in Citrus, of which Flavanones are the most abundant, accounting for some 98% of the total in grapefruit. Anthocyanins are very minor for citrus, occurring mainly in blood orange, providing the red colouration.

Outline structures for citrus flavonoids are shown in Appendix 1.

In citrus, flavonoids usually occur as glycosides i.e with a sugar molecule attachment; the sugar free aglycones occur less frequently in juice as they are liphophilic. The nature and point of attachment of the sugar determines in particular the taste characteristic of the flavonoid. For example, all citrus flavanone 7 neohesperidoses are bitter, while the 7 rutinosides are tasteless (neohesperidoside and rutinoside differ only in the linkage of the rhamnose and glucose that they both comprise). Neohesperidose attached at the 7 position in other flavonoids does not result in bitterness, because of minor differences elsewhere in the molecule. Naringin comprises neohesperidose attached at the 7 position to the flavanone skeleton, making it intensely bitter. (The full chemical names for naringin are complex). The structure of Naringin is shown in

Appendix 2.
Naringin is found predominately in Grapefruit, but also occurs in

• Pummelo or Shaddock (Citrus grandis [Linn] Osbeck)
• Sour orange (Citrus aurantium)
• Hassaku orange (Citrus hassaku)
• Natsudaidai Orange (Citrus natsudaidai)
• Trifoliate orange (Poncirus trifoliata)
• Bergamot (Citrus bergamia)

In grapefruit, naringin occurs mainly in the flavedo, albedo and segment membranes, with far less in the juice vesicles. Some early figures for Texas grapefruit showed ppm levels of

• Flavedo 4100 – 54500
• Segment membrane 14900 – 53500
• Albedo 6800 – 30700
• Core 7400 – 54200
• Juice 130 – 1300

The non-juice parts of the fruit account for ~90% of the total naringin content.

Flavonoids increase in the early stages of fruit development, but little thereafter in absolute amount; the percentage decreases as the fruit matures.

Naringin is the dominant flavonoid in grapefruit primarily responsible for the immediate bitter taste, with the similarly bitter poncirin being present in far lower amounts

Properties

Both naringin and poncirin are the most bitter of the flavanone neohesperidoses having bitterness equivalent to about 20% that of quinine hydrochloride.

The perceived bitterness threshold for naringin is around 20 mg/L with a wide range of sensitivities from as low as 1.5mg/L to above 50mg/L. Grapefruit juice is expected to have a bitter character with the preferred level being around 300mg/L of naringin. Juice with 700 – 1100mg/L naringin has been found acceptable but too bitter at 1500 – 1900mg/L.

Naringin interacts with Limonin at sub-taste threshold levels in an additive manner for bitterness.

The solubility of naringin in water is 500ppm at 20°C and 1000ppm at 40°C. Early experience showed that naringin could crystallize from canned grapefruit juice during storage. Similarly naringin can crystallise from concentrated grapefruit juice, but tends to be inhibited by the viscosity of the material. Crystallisation is of more concern in grapefruit comminutes because of the far higher levels in such products. Naringin can be redissolved by warming, unlike hesperidin which often forms a coating on process equipment and has to be removed by caustic cleaning.

Analysis

The Davis test from 1947 has long been the method of analysis generally used in the citrus industry. The method relies upon the production of a yellow coloured chalcone when flavanones react with alkali. The colour intensity is determined at 427nm and is non-specific, giving a measure of the total flavanones present. However, the method is rapid, cheap and easy to perform and since naringin is the major flavanone in grapefruit only slightly overestimates naringin content. More recently, HPLC techniques are employed to give true results for naringin. (Other methods are available!)

Control

There has been much work to reduce the bitterness from naringin, particularly in Japan for natsudaidai orange. Enzyme treatment can be effective where naringinase converts naringin to the tasteless naringenin, but dependent on the enzyme source this may also result in cloud loss (immobilising the enzyme in hollow fibre membranes can assist as high molecular weight pectins are unable to pass through. Enzymes can also be immobilised in dextran and alginic acid gels, cellulose acetate in a bioreactor and in packaging film prior to casting). Other options are the use of adsorbant resins with or without ion exchange, formation of inclusion complexes with β-cyclodextrin or treatment with magnesium silicate. As in the case of limonin, such treatments are expensive and for grapefruit, by far the best option is choice of fruit and processing conditions.

Fruit should be fully mature and not frost damaged as naringin is consequently released from the structural components into the juice before processing.

Juice extraction should be under ‘soft squeeze’ conditions to limit the amount of naringin expressed into the juice. Then passing the juice quickly though finishers will reduce the contact time with pulp and minimise naringin leaching.

Other

Naringin can be converted to the intensely sweet naringin dihydrochalcone by catalytic hydrogenation in an alkaline solution. Naringin DHC is between 300 and 1800 times the sweetness of sucrose at threshold levels, but has a delayed onset.