2.1. Soya-Based Products
Soybean (
Glycine max) provides high-quality protein, fats, and carbohydrates and contains no cholesterol or lactose. It is a good source of nutrients for lactose-intolerant individuals, vegetarians, and those with milk allergy [
82]. The production of soy products has been emerging as an interesting alternative to dairy products and their incorporation into human diets is increasing due to their nutritional and functional properties [
83].
Several studies have shown that soy products, especially soy yoghurt, is a good vehicle for probiotic delivery [
84,
85,
86,
87]. Due to the presence of raffinose and stachyose, soymilk is a good medium for
Bifidobacterium spp., as most of the strains belong to this genus can ferment these sugars. Strains of
Lb. acidophilus has also been reported to metabolize oligosaccharides present in soymilk during fermentation [
83,
85,
86]. Many probiotic strains possess α-galactosidase activity that allows their growth in soymilk [
83,
88]. These probiotic strains seem to have no negative effects when they incorporated with traditional yoghurt starter cultures. Farnworth et al. (2007) reported that the presence of probiotic bacteria [
Lb. johnsonii NCC533 (La-1),
Lb. rhamnosus ATCC 53103 (GG), and human-derived bifidobacteria] did not affect the growth of the yoghurt strains [
86]. Approximately 2 log increases in both
Lb. rhamnosus GG and
Lb. johnsonii La-1 were observed when each was added with yoghurt strains in the soy beverage.
Mounting evidence suggests that soymilk matrix may also provide adequate protection to the probiotics during gastrointestinal transit. For instance,
Lb. acidophilus and
Bifidobacterium spp. showed resistance to simulated gastrointestinal conditions when delivered through a fermented drink made of a mixed extract of soy and rice by-products with added waxy cornstarch [
89]. In another study, fermented soy matrix protected
Lb. acidophilus La-5 and
Bifidobacterium animalis Bb-12 against gastrointestinal juices, where the Bb-12 showed higher resistance to artificial gastrointestinal juices compared to La-5 [
83]. Moreover,
Lb. casei Zhang has also shown good tolerance to simulated gastric transit and intestinal juice in the fermented soymilk and maintained high viability (>10
8 cfu/g) during cold storage (at 4 °C for 28 days) [
87].
Soy oligosaccharides, mainly α-galactosides, are prevalently present in soy protein products and can result in unfavorable digestive effects when consumed. Certain probiotic strains are capable of decreasing α-galactoside content due to their high level of α-galactosidase activity while maintaining acceptable viability counts.
Lb. acidophilus LA-2 showed greater α-galactosidase activity when induced by raffinose and was able to retain viability over 14 weeks of cold storage (4 °C) when microencapsulated and freeze-dried [
90].
Soymilk is an excellent source of bioactive peptides and fermentation is an effective way of generating bioactive peptides. The β-glucosidase-producing probiotic
Lb. rhamnosus CRL981 allowed for obtaining a soy beverage with enhanced antioxidant capacity. The higher antioxidant activity was due to increased isoflavone aglycone contents during fermentation because of β-glucosidase activity towards isoflavone glucosides [
91].
Lb. plantarum C2 was excellent in terms of growth and peptide generation in soymilk, which showed excellent log count increases, protein hydrolysis, and α-galactosidase activities. Seventeen biofunctional soy peptides showing both antioxidant and ACE-inhibitory activities have been identified from the fermented soymilk produced using
Lb. plantarum C2 [
92]. ACE inhibitory activity in vitro has also been reported in fermented soy whey produced by using
Lb. acidophilus FTCC 0291 in the optimized soy-whey medium [
88]. Accordingly, soymilk consumption could improve some oxidative stress factors among patients with diabetic kidney disease [
93]. In addition, a regular intake of a soy-based probiotic drink (
Enterococcus faecium CRL 183 and
Bifidobacterium longum ATCC 15707) was reported to modulate the microbiota and reduce body weight gain in diet-induced obesity in mice [
94]. These pieces of evidence suggest that certain probiotics strains can be used for the preparation of soy-based functional fermented foods and bioactive food supplements.
Probiotics in the soy matrix could alter the sensory attributes of the products as well. A synbiotic soy yoghurt prepared using optimized FOS concentration (8.1%
w/
v) resulted in well-set products with very less whey separation (1.14%). The developed product showed good nutritional, textual, and sensory characteristics [
95]. Norouzi et al. (2019) compared the survival rate of
Lb. paracasei in fermented and non-fermented frozen soy dessert and their sensory properties over 180 days of storage at −24 °C. Results showed that the colour, mouthfeel and overall acceptability were significantly improved in probiotic products compared to frozen dessert without probiotics. Further, both fermented and non-fermented products reported maintaining viable
Lb. paracasei counts well over 10
6 cfu/mL [
96].
Albuquerque et al. (2019) evaluated the effect of passion fruit by-product (PFBP) and fructooligosaccharides (FOS) on the viability of
Streptococcus thermophilus TH-4 and
Lactobacillus rhamnosus LGG in folate bio-enriched fermented soy products, and on probiotic survival and folate bio-accessibility under in vitro simulated gastrointestinal conditions during storage (at 4 °C for 28 days) [
97]. Only
Lb. rhamnosus LGG retained the desired viability (>8 log cfu/mL) during storage, whereas
St. thermophilus TH-4 populations decreased to 5.5 log cfu/mL by day 28. Therefore, the bio-enriched probiotic fermented soy products present great potential as innovative functional food by delivering probiotic microorganisms and providing 14% of the recommended daily folate intake.
2.3. Miscellaneous Legume-Based Products
Recently, there is an increasing trend of utilizing legume sprouts as probiotic carrier foods. A probiotic drink produced from sprouted green gram showed viable
Lb. acidophilus NCDC14 counts of 10
10–10
11 cfu/mL after 8 h of fermentation [
101]. Swieca et al. (2019) investigated the effectiveness of lentil and adzuki bean sprouts as carriers for the probiotic yeast
S. cerevisiae var.
boulardii and found that the sprouts obtained from seeds soaked in the inoculum and further cultivated at 30 °C for 4 days gave the highest probiotic counts (>10
7 cfu/g). More importantly, the two matrixes effectively protected the probiotic yeasts during digestion in vitro. Further, the sprouts enriched with
S. boulardii were characterized by lower mold counts and coliform counts [
81]. Chick-pea sprouts fermented with
Lactobacillus casei 0979 hinder the growth of pathogenic microorganisms and result in products with safety.
Lb. plantarum 299v reported increasing starch digestibility in the lentil and mung bean sprouts [
102]. These results suggest that legume sprout-based food matrices are effective probiotic carrier foods and probiotics can be utilized to improve the microbial safety, nutritional composition, and nutrient digestibility of these products.
Lb. plantarum B1-6 has been successfully utilized to produce a probiotic food using the mung bean (
Vigna radiata) as a probiotic carrier. Probiotic fermentation resulted in viable counts of >10
8 cfu/mL and significantly higher ACE inhibitory activity at the end of fermentation [
103]. In another study, Romero-Espinoza et al. (2020) successfully used the combination of yeasts
S. cerevisiae and
S. boulardii and a mix of commercial probiotic bacteria composed of
Lb. acidophilus,
Lb. casei,
Lb. rhamnosus,
Lb. plantarum, and
Bifidobacterium infantis to ferment whole meal lupin (
Lupinus mutabilis var. bola L.). The probiotic fermentation resulted in significant degradation of oligosaccharides (27.3–82.3%), phytic acid (61.9–67%), and alkaloids (25.5–36.7%) which are the antinutritional factors that limit the consumption of lupin [
104].