An Overview of Cyanogens in Foods

Cyanogens are naturally occurring compounds that can release cyanide when hydrolyzed. These compounds are found in various foods and plants, especially those from the family Euphorbiaceae (which includes cassava), Leguminosae (beans), and Rosaceae (apples, almonds). Cyanogens exist mainly as cyanogenic glycosides, which are sugar-bound molecules. Upon enzymatic or acidic hydrolysis, they release hydrogen cyanide (HCN), a potent toxin.

Types of Cyanogens

Cyanogens primarily fall into three categories:

1. Cyanogenic Glycosides: These are the most common cyanogens and are found in various plants. When the plant is damaged (e.g., during chewing or crushing), these glycosides are broken down by plant enzymes, releasing hydrogen cyanide. The two main cyanogenic glycosides are linamarin and lotaustralin, which are especially prevalent in cassava.
2. Cyanohydrins: These are intermediates in the breakdown of cyanogenic glycosides. They form when cyanogenic glycosides are hydrolyzed by enzymes and can easily convert to hydrogen cyanide under mildly acidic conditions.
3. Hydrogen Cyanide: This is the final product of cyanogen breakdown and the most toxic form of cyanogens.

Cyanogens in Common Foods

1. Cassava (Manihot esculenta)
   • Cassava, also known as manioc, is one of the most significant sources of cyanogenic glycosides, primarily linamarin and lotaustralin. These compounds can release cyanide upon breakdown, especially when cassava is processed improperly.
   • There are two main types of cassava: sweet cassava and bitter cassava. Sweet cassava contains lower levels of cyanogenic glycosides, while bitter cassava has much higher levels, requiring thorough processing to remove the cyanide.
   • Cassava is a staple food in many tropical regions, particularly in Africa, South America, and parts of Asia. However, improper processing of cassava can lead to cyanide poisoning, a public health concern in areas where cassava is consumed in large quantities.
2. Bamboo Shoots
   • Bamboo shoots contain significant amounts of cyanogenic glycosides, particularly taxiphyllin. Unlike other cyanogenic glycosides, taxiphyllin breaks down rapidly at boiling temperatures, making it less toxic if the shoots are cooked properly.
   • Bamboo shoots are widely consumed in East Asian cuisines, and proper cooking is essential to reduce the cyanide content.
3. Stone Fruits (e.g., Apricots, Peaches, Cherries, Plums)
   • The seeds of stone fruits contain amygdalin, a cyanogenic glycoside that can release hydrogen cyanide. While the fruit flesh is safe, consuming large amounts of the seeds or pits can result in cyanide poisoning.
   • Apricot kernels, in particular, have gained attention due to claims of cancer-fighting properties, but consuming them in large amounts poses a serious risk of cyanide toxicity.
4. Lima Beans
   • Lima beans, especially wild varieties, contain cyanogenic glycosides such as linamarin. The level of cyanogens in commercially grown lima beans tends to be lower, but it is still necessary to cook them thoroughly to break down the cyanogenic compounds.
5. Almonds
   • There are two types of almonds: sweet almonds (which are widely consumed) and bitter almonds (which are not typically eaten raw due to their high cyanide content). Bitter almonds contain amygdalin, which can release hydrogen cyanide upon digestion.
   • Bitter almonds are used in the production of almond extract, and the cyanide is removed during processing. Eating raw bitter almonds, however, can lead to cyanide poisoning.
6. Sorghum
   • Sorghum, a cereal grain, contains cyanogenic glycosides in its young leaves and stems. While the grain itself is typically safe, consumption of improperly processed sorghum plants can lead to cyanide exposure, particularly in grazing animals.
7. Flaxseed
   • Flaxseed contains linamarin and lotraustralin, similar to cassava, but in lower concentrations. While flaxseed is considered healthy due to its omega-3 content and fiber, consuming large quantities of raw or unprocessed flaxseed can potentially lead to cyanide exposure.
   • Cooking and processing methods significantly reduce the cyanogen content in flaxseed, making it safe for consumption in typical dietary amounts.

Cyanogen Toxicity: Mechanisms and Symptoms

When cyanogens are hydrolyzed, they release hydrogen cyanide (HCN), a potent toxin that inhibits the enzyme cytochrome c oxidase, essential for cellular respiration. By blocking this enzyme, cyanide prevents cells from using oxygen, leading to cellular asphyxiation. The brain and heart, which require high amounts of oxygen, are particularly vulnerable to cyanide toxicity.

Acute Cyanide Poisoning

Acute exposure to cyanide can result in symptoms that manifest rapidly, often within minutes. These symptoms include:

• Headache
• Dizziness
• Nausea and vomiting
• Rapid breathing and heart rate
• Seizures
• Loss of consciousness
• In severe cases, respiratory failure, cardiac arrest, and death

Acute cyanide poisoning is often a result of consuming large amounts of improperly processed cyanogenic foods in a short period.

Chronic Cyanide Exposure

Prolonged exposure to low levels of cyanide, particularly from the habitual consumption of cyanogenic foods like cassava, can lead to chronic health conditions. The most well-documented condition is Konzo, a neurological disease associated with cyanide exposure from cassava. Konzo is characterized by:

• Sudden onset of spastic paralysis, particularly in the legs
• Difficulty walking
• Permanent motor impairments

Another condition linked to chronic cyanide exposure is Tropical Ataxic Neuropathy (TAN), which affects peripheral nerves and can lead to impaired movement and coordination.

Cyanogen Processing and Reduction in Foods

The toxicity of cyanogenic foods can be reduced through proper processing techniques that remove or degrade cyanogenic glycosides. These methods include:

1. Soaking: Soaking cassava and other cyanogenic plants in water for several hours or days can help leach out cyanogenic compounds. This method is commonly used in Africa, where cassava is a staple food. The soaking water is then discarded, reducing the cyanide content of the food.
2. Fermentation: Fermentation is another traditional method used to detoxify cassava. During fermentation, microbes break down cyanogenic glycosides, releasing cyanide gas that escapes from the fermenting material. Fermented cassava products, such as gari and lafun, have significantly lower cyanide levels.
3. Boiling: Boiling cyanogenic foods, such as bamboo shoots and lima beans, helps to degrade cyanogenic glycosides. The cyanide evaporates during cooking, reducing the risk of poisoning.
4. Drying and Sun Exposure: Drying cassava or other cyanogenic foods in the sun allows the cyanide to evaporate slowly, particularly when combined with fermentation or other treatments.
5. Grating and Roasting: In some regions, cassava is grated and roasted to make cassava flour. The heat from roasting helps to break down cyanogenic glycosides and remove cyanide.
6. Mechanical Processing: Modern methods of processing cassava include mechanical pressing or grinding, which physically removes cyanogens before further processing. These methods are used in industrial production to ensure safe cassava-based products.

Measurement of Cyanogens In Food

Measuring cyanide in foods like cassava and fava beans is critical because these foods contain cyanogenic glycosides (cyanogens) (such as linamarin and lotaustralin) that can release toxic hydrogen cyanide (HCN) when hydrolyzed. Various analytical methods are used to detect and quantify cyanide in food products. These methods are designed to ensure food safety by accurately measuring cyanide levels, and they range from simple colorimetric techniques to more sophisticated chromatographic and electrochemical methods.

Here are some of the main methods used to measure cyanide in food:

1. Colorimetric Methods

Colorimetric techniques are among the most commonly used methods for cyanide detection due to their simplicity and relatively low cost. These methods typically involve a reaction that produces a color change in the presence of cyanide, which can then be measured using a spectrophotometer.

a. Picric Acid Method

The picric acid method is one of the oldest and most widely used colorimetric techniques for cyanide detection. The method involves the reaction of cyanide with picric acid under alkaline conditions to produce isopurpuric acid, which has a yellow to red color. The intensity of the color is directly proportional to the concentration of cyanide and can be measured using a spectrophotometer at 520 nm (Bradbury, 2009).

• Advantages:
  • Simple and relatively inexpensive.
  • Suitable for routine testing in food safety labs.
• Disadvantages:
  • Lower sensitivity compared to more advanced methods.
  • Interference from other nitrogen-containing compounds.

Kits are available which use a paper disc impregnated with an enzyme linamarase to hydrolyse one of the main sources – linamarin.

b. König Reaction

The König reaction is another commonly used colorimetric method for cyanide determination. In this method, cyanide reacts with chloramine-T to produce cyanogen chloride, which subsequently reacts with pyridine and barbituric acid to form a red-blue complex. The intensity of the color can be measured at 578 nm (Bradbury et al., 1991).

• Advantages:
  • Reasonable sensitivity and specificity.
• Disadvantages:
  • The reagents involved can be hazardous.
  • Time-consuming compared to some modern methods.

2. Titration Methods

a. Volumetric Titration

Volumetric titration is another traditional method used to quantify cyanide, particularly in industrial settings associated with the metals recovery industry (Breuer et al., 2011). However, it could be equally applied to food analysis.  In this method, cyanide is usually titrated with silver nitrate (AgNO₃) in the presence of an indicator, such as p-dimethylaminobenzalrhodanine. Cyanide reacts with silver ions to form silver cyanide, and the endpoint of the titration is determined by the color change of the indicator.

• Advantages:
  • Simple and does not require sophisticated equipment.
  • Useful for large-scale cyanide detection in industrial processing.
• Disadvantages:
  • Lower precision and accuracy compared to modern chromatographic methods.
  • Interference from other ions (e.g., thiocyanate) that may be present in the food sample.

3. Ion-Selective Electrodes (ISE)

Ion-selective electrodes (ISEs) are used to measure free cyanide ions in food samples. The cyanide ISE is sensitive to the concentration of cyanide ions in solution and can directly measure cyanide levels by producing a potential difference that corresponds to the ion concentration.

• Advantages:
  • Fast and can provide real-time results.
  • Non-destructive and can be used for continuous monitoring.
• Disadvantages:
  • Only measures free cyanide ions, not bound cyanide in cyanogenic glycosides.
  • Calibration is required, and the electrodes can be prone to fouling.

4. Gas Chromatography (GC)

Gas chromatography (GC), often coupled with mass spectrometry (GC-MS) or flame ionization detection (FID), is a powerful technique for measuring cyanide in food. This method is based on the separation of volatile compounds, including hydrogen cyanide, which can be measured with high sensitivity and specificity.

a. GC-MS (Gas Chromatography-Mass Spectrometry)

GC-MS is used to detect cyanide after it has been released from cyanogenic glycosides. In this method, cyanide is first liberated from the food matrix through acid digestion or enzymatic hydrolysis. The released cyanide is then converted to cyanogen chloride using chloramine-T or a similar reagent. The volatile cyanogen chloride is separated by gas chromatography and detected by mass spectrometry.

• Advantages:
  • High sensitivity and specificity.
  • Can detect very low levels of cyanide (parts per billion).
• Disadvantages:
  • Requires expensive equipment and trained personnel.
  • Time-consuming sample preparation.

b. Headspace-GC

In headspace gas chromatography, the cyanide in a sample is converted to hydrogen cyanide gas, which is collected in the headspace (the gas phase above the liquid sample). The gaseous hydrogen cyanide is then injected into the GC for analysis.

• Advantages:
  • Sensitive and selective for volatile cyanide species.
  • Can analyze cyanide in complex matrices like food.
• Disadvantages:
  • Requires specialized headspace equipment.

5. High-Performance Liquid Chromatography (HPLC)

High-Performance Liquid Chromatography (HPLC) is another analytical method used to measure cyanide, especially cyanogenic glycosides like lotaustralin and linamarin in foods. In HPLC, the cyanogenic glycosides are separated based on their interactions with the column material and detected using various detectors, including UV, fluorescence, or electrochemical detectors.

• Advantages:
  • Can quantify cyanogenic glycosides directly.
  • High sensitivity and ability to analyze complex samples.
• Disadvantages:
  • Requires expensive equipment and expertise.
  • Time-consuming, especially for sample preparation.

6. Enzymatic Assays

Enzymatic assays involve the use of specific enzymes to hydrolyze cyanogenic glycosides, such as linamarin and lotaustralin, into hydrogen cyanide. The amount of cyanide released is then measured using spectrophotometry or other techniques.

a. Linamarase-Based Hydrolysis

In the case of cassava, enzymatic assays using the enzyme linamarase (β-glucosidase) can be used to hydrolyze linamarin and lotaustralin to release cyanide (Cooke, 1978). The cyanide is then measured using colorimetric or electrochemical methods. The detection limit is < 0.01 mg (0.1 parts 10⁻⁶) cyanide per 100 g fresh weight and peeled root. This is often used as a preliminary test for cyanide levels in cassava products.

• Advantages:
  • Specific to cyanogenic glycosides.
  • Can be used for screening purposes.
• Disadvantages:
  • Requires the isolation of specific enzymes.
  • May not be as precise as chromatographic methods.

7. Flow Injection Analysis (FIA)

Flow injection analysis (FIA) is an automated technique that involves injecting a liquid sample into a continuous flow of reagent solutions. For cyanide detection, the sample is mixed with reagents that react with cyanide to produce a detectable signal, usually colorimetric or electrochemical.

• Advantages:
  • Rapid and suitable for high-throughput testing.
  • Can be coupled with other detection methods (e.g., spectrophotometry, ion-selective electrodes).
• Disadvantages:
  • Moderate sensitivity compared to GC and HPLC.
  • Limited applicability for certain food matrices.

8. Electrochemical Detection

Electrochemical methods, such as amperometry or potentiometry, involve the direct detection of cyanide based on its electrochemical properties. In these methods, cyanide reacts at the electrode surface, producing a current that is proportional to its concentration.

• Advantages:
  • High sensitivity and fast response.
  • Can be miniaturized for portable field testing.
• Disadvantages:
  • Requires calibration and careful handling of electrodes.
  • May suffer from interference from other electroactive species.

9. Microwave-Assisted Distillation and Spectrophotometry

This method combines microwave-assisted distillation with spectrophotometric detection to measure cyanide levels in foods like cassava. The microwave energy helps release cyanide more efficiently from the food matrix, while spectrophotometry measures the cyanide concentration based on the reaction with a colorimetric reagent.

• Advantages:
  • Efficient sample preparation.
  • Relatively simple and cost-effective.
• Disadvantages:
  • May not be as sensitive as GC or HPLC.

10. Mass Spectrometry (MS)

In addition to being coupled with gas chromatography (GC-MS), mass spectrometry can also be used independently for cyanide ion detection in complex food matrices. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) can be used to measure trace levels of cyanide and other related ions in foods.

• Advantages:
  • High sensitivity and precision.
  • Can measure cyanide and other ions simultaneously.
• Disadvantages:
  • Expensive equipment and complex operation.

Several analytical methods are available for the detection and quantification of cyanide in foods such as cassava and fava beans. These methods range from traditional techniques like colorimetry and titration to more sophisticated approaches like gas chromatography, HPLC, and mass spectrometry. The choice of method depends on factors such as sensitivity requirements, equipment availability, and the specific cyanide species being measured. Given the potential toxicity of cyanide, these methods play a crucial role in ensuring the safety of cyanogenic foods for human consumption.

Regulatory Guidelines and Safety Limits

To protect public health, various food safety organizations have established guidelines for cyanide levels in foods. These regulations are especially important for cassava and other high-cyanogen foods.

• World Health Organization (WHO): The WHO has set a safe upper limit for daily cyanide intake at 0.02 mg/kg body weight. This means that an average adult weighing 60 kg should not consume more than 1.2 mg of cyanide per day.
• Codex Alimentarius: Codex has established guidelines for the maximum allowable cyanide content in cassava flour and other products. For example, the maximum level of cyanide allowed in cassava flour is 10 mg/kg.
• Food and Agriculture Organization (FAO): The FAO has also provided recommendations on processing methods to reduce cyanogen content in cassava and other foods.

Public Health Implications and Control Measures

In regions where cassava and other cyanogenic foods are dietary staples, the risk of cyanide poisoning is a significant public health concern. In particular, areas facing food insecurity or drought may be more vulnerable, as people may rely on bitter cassava varieties with higher cyanogen levels or skip important processing steps in times of scarcity.

To address these concerns, several strategies have been implemented:

1. Education: Public health campaigns focused on educating communities about the importance of proper cassava processing have been effective in reducing the incidence of cyanide poisoning.
2. Improved Cassava Varieties: Agricultural research has led to the development of low-cyanogen cassava varieties that are safer to consume, even with less intensive processing.
3. Surveillance and Monitoring: Regular monitoring of cyanogen levels in cassava products is crucial for preventing cyanide poisoning outbreaks. This includes testing both locally produced and imported cassava products.

Cyanogens are naturally occurring compounds in many plants, including cassava, bamboo shoots, and stone fruits. While these foods are important dietary staples in many regions, the release of hydrogen cyanide from cyanogenic glycosides poses a significant risk if they are not processed properly. Cyanide poisoning can result in both acute and chronic health effects, including life-threatening conditions like Konzo.

By understanding the sources of cyanogens, their health risks, and the methods for reducing their content, it is possible to mitigate the dangers associated with cyanogenic foods. Proper food processing techniques, public health education, and regulatory measures are essential for ensuring that foods containing cyanogens can be safely consumed.

References

Bradbury, J.H., 2009. Development of a sensitive picrate method to determine total
cyanide and acetone cyanohydrin contents of gari from cassava. Food Chemistry
113, pp. 1329–1333

Breuer, P. L., Sutcliffe, C. A., & Meakin, R. L. (2011). Cyanide measurement by silver nitrate titration: Comparison of rhodanine and potentiometric end-points. Hydrometallurgy, 106(3-4), pp. 135-140.

Cooke, R. D. (1978). An enzymatic assay for the total cyanide content of cassava (Manihot esculenta Crantz). Journal of the Science of Food and Agriculture, 29(4), pp. 345-352.

Egan, S.V., Yeoh, H.H., Bradbury, J.H., 1998. Simple picrate paper kit for determination of the cyanogenic potential of cassava flour. Journal of the Science of Food and Agriculture 76, pp. 39–48 .

Gleadow, R., Pegg, A., & Blomstedt, C. K. (2016). Resilience of cassava (Manihot esculenta Crantz) to salinity: implications for food security in low-lying regions. Journal of Experimental Botany, 67(18), pp. 5403-5413 (Article). .