Almonds, Apricots, and Peaches
Seeds from sweet almonds are a valuable food source. They contain very low levels of carbohydrates, and may therefore be made into flour for low-carbohydrate diets or for patients suffering from diabetes mellitus or any other form of glycosuria. Almond flour is gluten-free, and is therefore a popular ingredient in cookery in place of wheat flour for gluten-sensitive people, and people with wheat allergies and celiac disease. In addition, almonds are a rich source of riboflavin, magnesium, manganese, and, especially, vitamin E (alpha tocopherol), containing 24 mg/100 g (USDA, 2008). They are also rich in monounsaturated fatty acids, and almonds in the daily diet reduced LDL cholesterol by as much as 9.4%, reduced the LDL: HDL ratio by 12.0%, and increased HDL cholesterol by 4.6% (Jenkins et al., 2002). Claimed health benefits of almonds furthermore include improved complexion, improved transition of food through the colon, and even the prevention of cancer. Recent research associates the inclusion of almonds in the diet with elevating the blood levels of beneficial high density lipoproteins, and lowering the levels of low density lipoproteins. High concentrations of phenolics and flavonoids in the testa provide for antioxidative efficacy (Wijeratne et al., 2006). Bitter almonds (Prunus amygdalus var. amara) in particular accumulate substantial amounts of the cyanogenic di-glycoside amygdalin (D-mandelonitrile-b-D-gentiobioside; Figure 14.1) in their seeds. The seeds can contain up to 5% amygdalin (~ 1 mg hydrogen cyanide per seed), and 10—15 seeds are considered lethal for children while 50—60 seeds represent a critical amount for adults (for lethal dose of cyanide, see below). Seeds of sweet almonds (Prunus amygdalus var. dulcis) contain much lower levels of cyanide; however, up to 2% of sweet almond seeds are bitter — i.e., contain amounts of amygdalin comparable to P. amygdalus var. amara.
Like almonds, both Prunus armeniaca (apricot) and P. persica (peach) accumulate amygdalin in their seeds. The amount of cyanide in seeds of apricots ranges from 0.05 to about 4 mg/g, with an average amount of 0.5 mg of cyanide, but varies widely depending on a variety of poorly defined factors, including cultivation practices, variety, and moisture content. In general, the HCN content in peach kernels is lower than in bitter almond or apricot kernels, and ranges from 0.4 to 2.6 mg/g. In addition to amygdalin, apricot and peach seeds also contain prunasin (the corresponding monoglycoside) and several minor cyanogenic glycosides, including amygdalinic acid, mandelic acid b-D-glucopyranoside, benzyl b-gentiobioside, and benzyl b-D-glucopyranoside (Fukuda et al., 2003). Like bitter almonds, apricot kernels can sometimes be strongly bitter tasting. Consumed excessively, they can produce severe symptoms of cyanide poisoning or even death.
Flaxseed (or linseed) is a valuable oil seed that is very low in cholesterol and sodium (USDA, 2008). It is also a good source of magnesium, phosphorus, and copper, and an excellent source of dietary fiber, thiamin, and manganese. At the same time, flaxseed can contain considerable amounts of cyanogenic diglycosides, primarily linustatin and neolinustatin, together with the corresponding monoglycosides linamarin and, at lower concentrations, lotaustralin (Figure 14.1). The cyanide content of different cultivars of flaxseed can range from 124 to 196 mg/g (Chadha et al., 1995). The seeds contain about 35—45% oil, and are a desirable food product because of the high content of a-linolenic acid and lignans. Cyanogenic compounds are generally not detectable in processed flaxseed oil.
Dry, mature lima bean seeds are very low in saturated fat, cholesterol, and sodium (USDA, 2008). They are also a good source of protein, iron, magnesium, phosphorus, potassium, copper, dietary fiber, folate, and manganese (USDA, 2008). Depending on genotype, lima beans contain varying concentrations of cyanogenic glycosides, primarily linamarin (Figure 14.1), ranging from 2.1 (white seeds from Burma) to 3.1 mg HCN/g in seeds from Puerto Rico (black). The modern high-yielding and generally white varieties contain much less cyanide. Clinical toxicity in humans following the ingestion of lima beans is not well documented.
These seeds are a valuable food due to their very low cholesterol and sodium content. Furthermore, they are rich in thiamin, and an excellent source of manganese (USDA, 2008). All Macadamia spp. acumulate cyanogenic glycosides (proteacin and dhurrin, Figure 14.1) in their seeds. Cyanide concentrations in the commercially used seeds are low (~4.05 mg HCN/g
fresh weight), while Macadamia whelanii and M. ternifolia accumulate considerably higher amounts of cyanogenic glycosides in their seeds (260 mg/g in M. ternifolia) and are considered inedible (Dahler et al., 1995). Indigenous people in Australia, however, process these seeds to reduce the cyanide content so they can use these species as well.
The cycad plants have a long history of use as food and medicine. In traditional Chinese medicine, cycad seeds are used to treat hypertension, musculoskeletal disorders, gastrointestinal distress, cough, and amenorrhea. Although the seeds are also frequently consumed as a source of carbohydrates, detailed data on nutritional facts are not available. Cycad seeds contain the toxic compounds cycasin (0.2—0.3%; Figure 14.1) and neocycasin (methyl-azoxymethanol b-D-glycosides), which are unique toxins present in cycad species (DeLuca et al., 1980). These azoxyglucosides are glycosides of the same aglycone, methylazoxy-methanol. The glycosides do not contain cyano groups, and thus are considered "pseudo-cyanogens," but can decompose to yield HCN. However, the liberation of cyanide from this process is a minor pathway compared to the formation of nitrogen gas, formaldehyde, and methanol. While acute poisoning has been observed arising after consumption of cycad seeds, the role of HCN as the agent determining toxicity is not fully resolved. Patients suffering from acute poisoning after consumption of cycad seeds and tested for blood cyanide concentrations had elevated blood cyanide levels, but the blood cyanide concentrations were below the values (0.5—1 mg/l) causing serious toxicity. In addition to acute poisoning, neuro-degenerative diseases (WP ALS-PD; Western Pacific Amyotrophic Lateral Sclerosis/Parkinsonism-Dementia Complex) associated with consumption of cycad seeds have been reported (Cox & Sacks, 2002). However, the non-protein amino acid b-methyl-amino-L-alanine (BMAA) is the most likely neurotoxic compound, rather than a cyanogenic compound.
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