1. Introduction
Calcium, one of the important microelements, has been demonstrated to exhibit great advantages to our health
[1,2][1][2]. Calcium is not only essential for the growth of bones and teeth, but it also takes part in various physiological metabolisms of our body, such as the regulation of muscle contraction, blood coagulation, etc.
[3,4,5][3][4][5]. Recently, the literature has offered findings that show the occurrence of various diseases, mainly osteoporosis, cardiovascular, male infertility, etc., is highly correlated to calcium deficiency
[6,7,8][6][7][8]. So, the consumption of foods with high calcium levels is highly encouraged
[9,10][9][10]. The intake of calcium from dairy sources, such as milk, cheese, yogurt, etc., is a commonly recommended way to satisfy the body’s calcium requirement
[10,11][10][11]. Some non-dairy calcium sources from vegetables, including broccoli, kale, Chinese cabbage, etc., are also suggested
[10]. The majority of people realize that calcium is a vital mineral for our health; however, humans are not getting sufficient calcium in their diets, in accordance with the recommendations of many nations and agencies
[12,13,14,15][12][13][14][15]. To satisfy the need for calcium, the intake of calcium from calcium supplements is usually adopted. Many calcium supplements, including CaCl
2, CaCO
3, calcium gluconate, calcium amino acid chelate and peptide calcium, have been made available, which have played a great role in providing calcium to the human body
[16,17][16][17]. However, these calcium supplements have some drawbacks, for instance, the low solubility of inorganic calcium, poor absorption and utilization efficiency, etc.
[17,18][17][18]. Therefore, to meet the calcium requirements, the development of higher-bioavailability, safer and cheaper calcium supplements is rather necessary.
Many beneficial microorganisms such as probiotics, yeasts, edible mushrooms, etc., show great potential for the accumulation of minerals
[17,19,20,21][17][19][20][21]. Among these microorganisms, edible mushrooms can be regarded as an interesting object of supplementation. In numerous countries, edible mushrooms have been part of the daily diet for several thousand years
[22,23][22][23]. They are regarded as a nutritious food, referring to the many nutritious substances they contain, including polysaccharides, minerals, dietary fibers, proteins, vitamins, etc.
[23,24][23][24]. Furthermore, owing to their many bioactive characteristics, for example, anti-cancer, anti-bacterial, anti-oxidation, anti-inflammatory, etc., edible mushrooms can also be considered as functional foods with potential advantages for our health
[24,25,26,27,28,29][24][25][26][27][28][29]. There are about 100 species that are commercially available in the global mushroom market. Around 20 species have the potential to be cultivated at an industrial level
[30,31][30][31]. The world outputs of edible mushrooms have increased dynamically year by year, varying from 10.5 million tons in 2016 to 11.8 million tons in 2019, with an increase of around 57% over the last 10 years
[32]. It is important to highlight that the worldwide market for edible mushrooms in 2019 was USD 16.9 billion in 2019, whereas it is anticipated to reach USD 19.04 billion by 2026
[33]. China is the largest manufacturer of edible mushrooms in the world, and its production is still on the rise
[34,35,36,37][34][35][36][37].
Owing to edible mushrooms’ excellent capacity to accumulate minerals, numerous studies on minerals enriched in edible mushrooms have been carried out to help improve the nutritive value of edible mushrooms. Edible mushrooms fortified with calcium are extremely interesting, showing great potential as a calcium dietary supplement
[38,39,40][38][39][40]. In view of the increasing demand for natural dietary supplements, Ca-fortified edible mushrooms can be regarded as a type of marketable product with great commercial potential. Compared to studies of other enriched minerals such as selenium
[41[41][42][43],
42,43], the investigations for Ca accumulation in edible mushrooms are relatively limited. The most common calcium enrichment method involves the addition of exogenous calcium salts into a substrate or fermentation medium. Consequently, calcium-fortified edible mushrooms have the potential to be a safe and effective source of daily Ca supplementation, exhibiting the benefits of safety and effectively promoting organic Ca formation
[44,45,46][44][45][46].
2. Effects of Various Factors on Calcium Enrichment in Edible Mushrooms
Edible mushrooms are rich in many essential minerals
[47[47][48][49][50][51],
48,49,50,51], including potassium, calcium, phosphorus, and magnesium, which are often deficient in our daily diet
[52,53][52][53]. Accordingly, the investigation of Ca-enriched edible mushrooms has been a growing research area. Through the incorporation of Ca into active biomacromolecules during the metabolic process, the mycelium and the fruiting bodies of edible mushrooms are able to convert inorganic-state Ca to organic-state Ca, which has higher bioavailability and is safer compared to the inorganic form
[38,54][38][54]. Several studies have been conducted to investigate the capacity of edible mushrooms, including
Pleurotus eryngii,
Lentinula edodes,
Hypsizygus marmoreus,
Pholiota nameko and
Ganoderma lucidum, to accumulate calcium from a variety of Ca sources
[55,56,57,58,59,60][55][56][57][58][59][60]. Ca content (
Table 1) in edible mushrooms highly depends on several factors, for example, edible mushroom species, growing environments, etc.
[50]. Edible mushrooms, for instance,
Flammulina velutipes [60],
P. ostreatus,
H. marmoreus,
Auricularia auricula [61[61][62],
62], Coprinus comatus, are excellent calcium-enriched candidates (
Table 1). Generally, the total calcium is lower in edible mushrooms than in vegetables
[63,64][63][64]. In an effort to enrich edible mushrooms with calcium, Tabata and Ogura found that the Ca level in fruiting bodies of
H. marmoreus was improved as potato sucrose agar (PSA) and sawdust media were added with 1.0% Ca salts
[65]. Choi et al. determined the calcium-enriching effect of
P. eryngii in sawdust medium with a supplement of calcined starfish powder
[66]. These authors also expected that numerous environmental factors, such as pHs, moisture concentrations, climate conditions, etc., could have an additional influence on calcium accumulation within the fruiting bodies of edible mushrooms
[66]. In addition, the abilities of
P. ostreatus and
P. nameko to accumulate calcium in PSA and sawdust media have also been well characterized
[59,67][59][67].
Table 1.
Calcium level in some edible mushrooms.
As one of the typical edible mushrooms,
P. eryngii is acknowledged as an antioxidant resource, containing a large number of beneficial compounds and secondary metabolites, which may prevent oxidative damage
[68]. It is also regarded as a high-efficiency calcium accumulator and can change inorganic calcium into organic calcium
[69,70][69][70]. Akyuz et al. found that
P. eryngii tended to have higher mineral accumulations, because the Mg and Ca contents in fruiting bodies were higher than other minerals
[71]. Similarly, increased Ca content (14.94 mg/100 g) was observed in
P. eryngii cultured on rice straw
[72]. These differences in the calcium contents of
P. eryngii were also attributed to the different culture media used or different substrate components. Moreover, the wide variation in the Ca content of
P. eryngii grown on different media was similar to previous investigations
[73,74,75][73][74][75]. In 2023, He et al. investigated the influence of five kinds of exogenous calcium sources (calcium chloride, calcium amino acid chelate, calcium lactate, calcium nitrate and calcium carbonate) on
P. eryngii mycelia and fruiting bodies and found the optimum exogenous calcium (calcium lactate) could improve the yield of
P. eryngii fruiting bodies and shorten its growth cycle
[69]. However, in the investigation of Bu et al., the authors found different edible mushroom species (
Pleurotus nebrodensis,
P. eryngii and
Pleurotus citrinopileatus) showed a significant effect on calcium enrichment. In addition,
P. nebrodensis was a more suitable Ca-enriched edible mushroom candidate compared to other kinds of edible mushrooms
[18].
In general, the main Ca metabolic products present in edible mushrooms are in an organic state. The distribution of Ca metabolites in edible mushrooms differs according to the cultivated cultivar and growing conditions. Specifically, 62.4% of Ca was combined with protein in
Cordyceps sinensis, and the polysaccharide fraction contained 11.5% of Ca. A total of 80.5% of inorganic Ca was transferred into organic Ca
[20,59][20][59]. The calcium enrichment of
Laetiporus sulphureus showed similar findings. The degree of organic calcium reached 85.85% when the calcium content was 100 mg/L
[54]. However, for
Poria cocos, although 97.91% calcium was absorbed, only 24.57% organic calcium was detected
[76,77][76][77].
Although edible mushrooms are excellent at accumulating Ca and can be grown over a wide range of Ca levels, their abilities to accumulate Ca differ from cultivar to cultivar and with culturing conditions, Ca sources and dosages (
Table 2). Particularly, Ca sources and doses can highly affect Ca enrichment in edible mushrooms (
Table 2). Current studies on Ca accumulation in edible mushrooms principally use CaCO
3, CaCl
2 and Ca(NO
3)
2 as Ca sources
[78].
F. velutipes is one type of popular food in China due to its excellent anti-cancer and immunostimulating abilities
[60,79,80][60][79][80]. Fan et al. showed that with the addition of 1~2% light CaCO
3 and 1~2% shellac, the mycelia of
F. velutipes grew denser, and the output and the quality of fruiting bodies improved
[60,80][60][80]. In addition, in support of these results, it has been shown that adding 0.5% CaCO
3 into potato sucrose agar (PSA) medium slightly enhanced the mycelium growth of
H. marmoreus, while adding 5.0% CaCO
3 into the same medium resulted in total inhibition
[65]. However, it was observed that adding Ca phosphate and Ca carbonate into sawdust media did not affect the growth of
P. eryngii cultivated on both potato dextrose agar (PDA) and sawdust media with a supplement of Ca salts, while adding CaSO
4 inhibited the growth of mycelium
[81].
Table 2.
Calcium enrichment in some edible mushrooms.