Preliminary research on pretreatment methods and analysis of Jerusalem tuber dispersions made from untreated and treated raw materials
Pre-treatment of plant raw materials can contribute to the release of low molecular weight and water-soluble compounds through the cell membranes that were damaged [14–16]. Physico-chemical indicators and mineral composition are subject to changes depending on the type and duration of processing of raw materials (Table S1).
Pre-treatment had the greatest effect on the content of dry matter, protein, polyphenols, and carbohydrate contents in the Jerusalem artichoke dispersion. The highest content of dry matter, protein, and carbohydrates was in the control sample that was obtained from untreated raw materials. Any type of pretreatment led to a decrease in the content of protein, polyphenols, dry substances, and carbohydrates. Freezing at -80°C had the most negative effect on carbohydrate, and dry matter content decreasing the value from 5.99 ± 0.89 and 6.91 ± 0.11% to 4.08 ± 0.52% and 4.86 ± 0.11% and 16.58 ± 0.96%, respectively. During ultrasonic treatment, the hydrophobicity of proteins increases due to their partial denaturation in their secondary and tertiary structures. These changes can affect the dispersion of protein molecules and lead to a decrease in their distribution, which facilitates the movement of the protein phase into the solution [17–19]. The decrease in protein content was maximum when ultrasound treatment for 6 min was used or the raw material was pre-frozen before processing and reached the value from 0.66–0.42%.
It is known that pretreatment can modify plant material in such a way as to stimulate the release of bioactive components weakly bound to cell wall polymers [20]. Damaging the membrane can be sufficient to release the active compounds and can improve the extraction efficiency of bioactive compounds [21, 22, 14]. But at the same time, cell disruption methods can negatively affect the biological activity of the resulting product. It is known that pre-freezing, commonly used to preserve the quality of frozen foods, can degrade cell structure, resulting in discoloration, loss of moisture, softening, and loss of nutrients and biologically active compounds [23, 24]. Polyphenolic compounds constitute a diverse array of secondary metabolites characterized by multiple phenol functional groups. Found in a range of dietary sources like fruits, vegetables, and beverages such as tea and wine, polyphenols have been extensively studied for their bioactive properties. One of the primary biological activities ascribed to polyphenolic compounds is their capacity for free radical scavenging. Studies have demonstrated that polyphenols possess robust antioxidant properties, effective in neutralizing reactive oxygen species (ROS) and thereby ameliorating oxidative stress [25]. According to data obtained, it can be seen that the preliminary freezing of Jerusalem artichoke tubers decreases the polyphenol content, perhaps, due to the fact associated with the native composition of plant cells and the destruction of cells that occurs during freezing that led to the release of biologically active substances and their degradation due to chemical and enzymatic oxidation reactions. [26–28].
Different ultrasound and freezing treatments can affect the increase or decrease in the content of various mineral components [29, 30]. We have studied the effect of preliminary ultrasonic treatment and freezing methods of Jerusalem artichoke tubers on the mineral composition of plant dispersions (Table S2). There was a tendency to decrease the iron content after freezing and especially after sonication over time compared to the control sample. As the sonication time increased and decreased the freezing temperature, the iron content decreased by an average of 23–45% for the samples subjected to ultrasound treatment and 7–8% for the frozen samples. About the pre-freezing, there was no consistent pattern. When compared with the untreated Jerusalem artichoke tubers, frozen at -80°C tubers contained significantly more Ca, Si, Mg, Sr, and P. There was also an increase in Al, Ba, K, Cu, and Cr in the samples that were frozen at -17°C. An increase and decrease in the content of microelements may be due to damage to the cellular structures of vegetable raw materials during the preliminary freezing at a temperature regime of -17℃ and − 85℃ as cell membranes are disrupted by ice crystals during the freezing process [31].
Organoleptic evaluation of samples of Jerusalem artichoke tuber dispersions (Fig. S1) showed that pre-treatment of raw materials affects their sensory properties. None of the pre-treatment methods had significant changes in the viscosity of the samples, as well as bitter aftertaste, sour taste, homogeneity, putrid aroma, and fungus/earthy flavor of plant dispersions. Prolongation of ultrasound treatment led to the tendency of bitter taste. The 2-minute ultrasound treatment increased the fungus/earth aroma but then reduced this intensity with the expansion of ultrasound treatment time. The ultrasound pre-treatment also increased the Jerusalem artichoke tuber aftertaste but slightly decreased Jerusalem artichoke tuber flavor and aroma when 4 min ultrasound and longer were used. Ultrasonic treatment had a slight effect on wateriness, sweet taste, and pea flavor. As for freezing treatment, the − 17°C influenced a greater decrease of wateriness, Jerusalem artichoke tuber flavor and aroma, and pea flavor, but an increase in sweet taste descriptor. Due to the formation of small crystals of ice and the least extent of cell disruption the freezing at -80°C led to insignificant changes in sweet taste compared to the control sample and pre-frozen at -17°C.
According to the results obtained the pre-treatment decreased the overall acceptability attribute to a different extent and the preference was for the control sample without pre-treatment. Therefore, for the next studies, the control sample was chosen.
Impact of probiotic strain fermentation on quality of Jerusalem artichoke tuber dispersions
Acid accumulation
Based on the results presented in Fig. S2 we can conclude that changes in microorganism strain of starter cultures significantly affect the product fermentation time. The most rapid acid accumulation process among all selected L. acidophilus strains was performed by L. acidophilus H9, which attained a pH value of 4.72 and titratable acidity of 0.301% in 7 h. The most rapid fermentation till 4.75 was for S. thermophilus for 5 h. The use of B. longum B379M and B. bifidum led to a decrease in pH value which dropped between 8 h and 10 h to 4.68 and 4.73, respectively. The slowest acid accumulation was when using B. coagulans MTCC 5856 and P. shermani KM-186 for 13 h. Generally, all selected probiotic bacterial strains were able to adapt to plant dispersion from Jerusalem artichoke tubers.
The maximum acidification rate (Vmax, рН/min) was for L. acidophilus AT-41 and is equal to 0.007, for L. acidophilus 57S and L. acidophilus 8 reached 0.005. During fermentation by L. acidophilus H9 and L. bulgaricus, the maximum acidification rates were 0.006, and 0.009, respectively. For the samples fermented with B. longum B379M the maximum acidification rate was 0.009, with B. bifidum the rate was 0.006, with B. coagulans MTCC 5856 was 0.003, with S. thermophilus was 0.007, and with P. shermanii KM-186 was 0.003.
Antioxidant activity
Resultant values of antioxidant activities assayed (Fig. S3) during storage when using different starter cultures showed, that changes in DPPH radical scavenging activity depend on the probiotic microorganism used. Results showed that fermentation by B. bifidum and strains L. acidophilus AT-41, 57S, and H9 allowed to reach maximum values (51.34%, 51.54%, 54.85% and 55.39%, respectively) after the fermentation process was completed. While results indicated that there was no significant difference between fermented samples by B. coagulans MTCC 5856 and B. longum B379M compared to the same samples before the fermentation process. As for L. bulgaricus the chosen strain could inhibit 43.37% of DPPH radicals. The minimum value for the percentage of radical scavenging activity was for samples fermented with L. acidophilus 8 (38.23%), P. shermanii KM-186 (38.12%), and S. thermophilus (41.11%). Refrigerated storage increased DPPH inhibition in samples fermented with L. acidophilus AT-41, 57S, 8, H9, and B. coagulans MTCC 5856 on the 5 days and the maximum values for the samples fermented with B. longum B379M, B. bifidum, P. shermanii KM-186, S. thermophilus and L. bulgaricus were on 3 days of storage with the subsequent decrease till the 11th day. Some studies have shown that fermentation and post-acidification products can improve the antioxidative activity by increasing the release, for instance, organic acids and other phenolic compounds, produced during storage and hence could be possible sources of DPPH inhibitors [32, 33].
The greatest decrease in antioxidant activity on day 11 of storage occurred when L. acidophilus 57S was used for fermentation, and was 44.23%, which was 10.62% down from the antioxidant activity values after fermentation on day 0. The samples fermented with strains L. acidophilus H9 reduced DPPH scavenging activity on day 11 by 2.12% compared to day 0. The values of antioxidant activity in samples fermented by L. acidophilus AT-41 on day 11 of storage returned to the initial value, which was on day 0. The use of B. longum B379M, P. shermanii KM-186, L. bulgaricus, S. thermophilus, and L. acidophilus 8 as starter cultures showed an increase in antioxidant activity on day 11 compared to day 0 by 3.13%, 9.34%, 7.24%, 2.78% and 5.87%, respectively. As for the strains B. bifidum and B. coagulans MTCC 5856, the antioxidant activity on day 11 decreased by 3.35% and 4.62%, respectively, compared to day 0.
Impact of fermentation on viability of probiotic bacteria
Data shown in Fig. S4(a) demonstrated the viability of probiotic bacterial cultures on fermented Jerusalem artichoke tubers dispersion. The viability microorganisms of fermented dispersions increased significantly from zero time before fermentation to the end of fermentation, with viable counts biomass increase 2.3, 2.27, and 2.12 log10CFU/ml during 9, 10, and 13 h for L. bulgaricus, L. acidophilus AT-41 and B. coagulans MTCC 5856, respectively and these were the highest viability among all selected probiotic microorganisms. L. acidophilus H9, S. thermophilus, and L. acidophilus 8 also showed significant increases in viability, but less than the former microorganisms and were 1.55, 1.46, and 1.58 log10CFU/ml to 7, 5, and 9 h, respectively. In contrast, the lowest viability showed B. longum B379M to 8h of fermentation, B. bifidum to 10 h, and P. shermanii KM-186 to 13 h (0.41, 0.21 and 0.37 log10CFU/ml increase, respectively).
L. acidophilus 57S showed no changes in biomass increase and had a value of 7.03 log10CFU/ml. So, it can be concluded that Jerusalem dispersion might be deficient in nutrients essential for their growth.
According to the data in Fig. S4(b, c) the specific growth rate (µG) among the L. acidophilus strains were 0.0226, 0.0281, 0.028, and 0.00039 h− 1, the doubling time of microbial cell concentration (td, h) was 30.65, 24.68, 24.26, 1782.89 h, multiplication rate (MR) was 0.033, 0.041, 0.041, 0.0005 and for Jerusalem artichoke dispersions fermented by L. acidophilus 8, L. acidophilus AT-41, L. acidophilus H9 and L. acidophilus 57S, respectively. Data also revealed that the specific growth rate (µG) was 0.0071, 0.0378, 0.0316, 0.0030, 0.0039, and 0.0204 h− 1, the doubling time (td) was 97.42, 18.29, 21.96, 234.50, 174.94, and 34.06 h, multiplication rate (MR) was 0.010, 0.055, 0.045, 0.004, 0.005, and 0.029 for the fermented samples of Jerusalem artichoke dispersions with B. longum B379M, S. thermophilus, L. bulgaricus, B. bifidum, P. shermanii KM-186, B. coagulans MTCC 5856, respectively.
Viscosity values
After fermentation, the presence of a structure (clot) that could be destroyed under the action of shear deformation was not visually detected in the studied samples. The same was confirmed when measuring rheological properties on a rotational rheometer. It can be seen from data in Fig. S5 that the viscosity of the studied samples does not depend on the shear rate and is on average only 2.16 times greater than the dynamic viscosity of water at the same temperature (4°C). The sample fermented by B. longum B379M sample has the highest viscosity. The increase in viscosity compared to the control sample before the fermentation was 8.56%. However, in general, the viscosity of Jerusalem artichoke dispersions after fermentation by various bacterial cultures does not differ significantly. Fermented vegetable dispersions of Jerusalem artichoke tubers can be consumed as a beverage itself, somewhat like resembling whey-based beverages. It is also possible to further increase the viscosity by adding various fillers and thickeners to obtain a product similar to yogurt-like beverage.
Impact of fermentation on the organoleptic liking
When making Jerusalem artichoke dispersion the dark color is appears due to the oxidation of polyphenolic compounds by peroxidase and polyphenol oxidase. However, during fermentation process, released acids suppress the action of enzymes. During the fermentation process, all samples change color from dark brown to light beige. According to the data presented in Fig. S6 using hybrid hedonic scale, depending on the starter culture added, the organoleptic properties of the fermented dispersions of Jerusalem artichoke tuber varied greatly, due to the fact that each strain performs a unique organoleptic profile of the product.
In terms of taste liking attribute, the least scores gained the sample fermented with L. acidophilus AT-41 due to the most pronounced sour taste in comparison to other samples. The samples L. bulgaricus gained the highest scores as they had the moderate sweetness and there is no bitterness in taste. The samples fermented by B. coagulans MTCC 5856, S. thermophilus, and B. longum B379M had the lowest scores from assessors for flavor, as the former had the flavor of rotten eggs, the two latter samples had the unusual off-flavors, despite that the sample fermented with B. longum B379M had the pleasant strawberry flavor. Sample with L. acidophilus 57S and L. bulgaricus achieved the highest scores due to the similarity of the former samples with brine of pickled cucumbers with honey, and the latter with diluted sauce BBQ. Assessors also noted the similarity of the samples fermented with L. acidophilus H9 to carrot and potato puree in flavor attribute. These samples as well as the samples fermented with L. bulgaricus received a commensurately high rating in terms of appearance liking. The lowest results for appearance liking parameter were demonstrated by B. coagulans MTCC 5856 and B. longum B379M, since flakes and fractionation were formed the fermentation process. In terms of texture liking the samples fermented with B. coagulans MTCC 5856 gained the least scores. The samples with S. thermophilus, L. bulgaricus and P. shermanii KM-186, on the contrary, received high marks from assessors. Ultimately, the sample fermented with L. bulgaricus received the highest overall liking scores and the lowest were for B. coagulans MTCC 5856.