Preparation of liquid yogurt in the presence of pectin and its formation mechanism.

We had previously observed that adding pectin into milk before fermentation inhibited gelation of yogurt but did not affect the pH. Thus, this work aimed to prepare such liquid yogurt and clarify its formation mechanism. It was found that liquid yogurt was obtained in the presence of 0.10%-0.20% pectin. However, at lower or higher pectin concentrations, yogurt was gelled. Confocal laser scanning microscopy analysis demonstrated that 0.10%-0.20% pectin induced milk protein aggregating into separated particles rather than a continuous network, which explained why liquid yogurt was formed. Moreover, adding 0.10%-0.20% pectin into the casein micelle suspension induced aggregation of casein micelles at pH 6.8. After pH decreased to 4.3, casein micelles showed more aggregation but they were still separated particles, which was the same in the corresponding yogurt samples. These results suggested that pectin changed the aggregation mode of casein micelles and induced formation of liquid yogurt.

Enhanced hypoglycemic effects of konjac glucomannan combined with Polygonatum cyrtonema Hua polysaccharide in complete nutritional liquid diet fed type 2 diabetes mice.

In our aging society, dysphagia and malnutrition are growing concerns, necessitating intervention. Liquid nutrition support offers a practical solution for traditional dietary issues, but it raises a key issue: the potential for post-meal glucose spikes impacting efficacy. This study examined the effects of supplementation of Polygonatum cyrtonema Hua polysaccharide (PCP), konjac glucomannan (KGM) and their combination on acute phase postprandial glycemic response and long-term glucose metabolism in T2DM mice on a complete nutritional liquid diet. KGM was more effective in reducing postprandial glucose response, while PCP was more prominent in ameliorating long-term glucose metabolism. The KGM-PCP combination demonstrated superior outcomes in fasting blood glucose, insulin, and glucose homeostasis. PCP and KGM also influenced the composition and abundance of the gut microbiome, with the H-PCP group showing optimal performance. Moreover, the KGM-PCP combination improved body weight, lipid homeostasis, and liver health the most. PCP potentially regulates glycemia through metabolic pathways, while KGM improves glycemic metabolism by reducing postprandial glucose levels in response to viscous intestinal contents. This research identifies the structure, viscosity properties, and hypoglycemic effects of KGM and PCP in complete nutritional liquid diet fed T2DM mice, enabling their strategic utilization as hypoglycemic components in nutritional administration and glycemic regulation.

Yellow mustard gum: pilot-scale production and characterization.

BACKGROUND: Yellow mustard gum (YMG), which is extracted from the mucilaginous part of yellow mustard bran, has been considered an emerging natural hydrocolloid gum but lacks commercial development and production. To promote the commercial utilization of YMG, this study developed a pilot-scale YMG production protocol in an economic and environmentally friendly way to produce a clean-label YMG product. This YMG produced at pilot scale (YMW) was characterized in terms of chemical composition, rheological properties, and interaction with a commercial gum, κ-carrageenan, and was compared with purified YMG through ethanol precipitation (YME). RESULTS: The protocol processed up to 100 L of raw material with zero solvent and a minimal number of steps and showed strong quasi-industrial potential. The YMW showed a similar chemical composition as YME. However, the YMW contained a slightly lower amount of carbohydrate and a much larger amount of ash and potassium than the YME. The rheological results concluded that both the YMW and YME solutions exhibited shear-thinning flow behavior and a weak gel, with YME showing higher viscosity and stronger gel structure. Most interestingly, YMW could form unpourable gels when blended with native κ-carrageenan whereas YME barely achieved this despite the equivalent total gum concentration. CONCLUSION: This study demonstrated the feasibility of YMG production at a large scale with economic and green procedures and discovered its new functionality for commercial utilization. The gelling ability of YMG could provide it with wider applications as a result of a new potential synergistic combination. All this information should accelerate the process of full commercialization of YMG as a clean-label functional ingredient. © 2024 His Majesty the King in Right of Canada. Journal of The Science of Food and Agriculture © 2024 Society of Chemical Industry. Reproduced with the permission of the Minister of Agriculture and Agri-Food Canada.

The digestive fate of beef versus plant-based burgers from bolus to stool.

Ultra-processed, plant-based burgers (PB) and traditional comminuted-beef burgers (BB) share similar organoleptic characteristics, yet a knowledge gap exists in understanding how consumption of these divergent physical structures alters the lipemic response and gut microbiota. PB, comprised of highly refined ingredients, is formulated with no intact whole food structure, while BB entraps lipids throughout the myofibrillar protein network. PB presented significantly higher free fatty acid (FFA) bioaccessibility (28.2 ± 4.80 %) compared to BB (8.73 ± 0.52 %), as obtained from their FFA release profiles over digestion time after characterizing them with a modified logistic model (SLM), using the simulated TIM Gastro-Intestinal Model (TIM-1). Additionally, the rate of lipolysis, k, obtained from the SLM for PB (90% CI [0.0175, 0.0277] min-1) was higher than for BB (90% CI [0.0113, 0.0171] min-1). Using the Simulated Human Intestinal Microbial Ecosystem (SHIME®), the Firmicutes to Bacteroidetes ratio (F/B ratio) was significantly higher for PB than BB; and linear discriminant analysis effect size (LEfSe) showed Clostridium and Citrobacter were more highly represented in the microbial community for the PB feed, whereas BB feed differentially enriched Megasphaera, Bacteroides, Alistipes, and Blautia at the genus level. Additionally, short-chain fatty acid (SCFA) production was altered (p < 0.05) site-specifically in each colon vessel, which could be attributed to the available substrates and changes in microbial composition. Total SCFAs were significantly higher for PB in the ascending colon (AC) and descending colon (DC) but higher for BB only in the transverse colon (TC). This research illustrates the crucial role of meat analog physical structure in modulating nutritional aspects beyond food composition alone.

Entrapment of cyanidin-3-O-glucoside in ß-conglycinin: From interaction to bioaccessibility and antioxidant activity under thermal treatment.

The thermal-induced interaction between ß-conglycinin (7S) and cyanidin-3-O-glucoside (C3G) on the bioaccessibility and antioxidant capacity of C3G was investigated. High ratio of 7S to C3G (1:100) led to a more ordered secondary structure of 7S. Thermal treatment promoted the formation of 7S-C3G complexes via hydrophobic and hydrogen bonds but did not induce the formation of 7S-C3G covalent products. Thermal treatment at 65 °C and 121 °C enhanced the binding affinity of 7S-C3G complexes by 46.19 % and 1203 % compared with 25 °C. The 7S-C3G interaction decreased C3G bioaccessibility by 4.37 %, 8.74 %, and 46.37 % at 25 °C, 65 °C, and 121 °C. Diphenylpicrylhydrazyl (DPPH) and ABTS antioxidant capacity assay indicated an antagonistic effect between 7S and C3G. The increased binding affinity of C3G to 7S limited the bioaccessibility of C3G and promoted the antagonism of antioxidant capacity between 7S and C3G. 7S addition was detrimental to the antioxidant capacity and bioaccessibility of C3G in vitro after thermal processing.

Therapeutic potential of non-starch polysaccharides on type 2 diabetes: from hypoglycemic mechanism to clinical trials.

Non-starch polysaccharides (NSPs) have been reported to exert therapeutic potential on managing type 2 diabetes mellitus (T2DM). Various mechanisms have been proposed; however, several studies have not considered the correlations between the anti-T2DM activity of NSPs and their molecular structure. Moreover, the current understanding of the role of NSPs in T2DM treatment is mainly based on in vitro and in vivo data, and more human clinical trials are required to verify the actual efficacy in treating T2DM. The related anti-T2DM mechanisms of NSPs, including regulating insulin action, promoting glucose metabolism and regulating postprandial blood glucose level, anti-inflammatory and regulating gut microbiota (GM), are reviewed. The structure-function relationships are summarized, and the relationships between NSPs structure and anti-T2DM activity from clinical trials are highlighted. The development of anti-T2DM medication or dietary supplements of NSPs could be promoted with an in-depth understanding of the multiple regulatory effects in the treatment/intervention of T2DM.

Conformational Properties of Flaxseed Rhamnogalacturonan-I and Correlation between Primary Structure and Conformation.

The pectic polysaccharides extracted from flaxseed (Linum usitatissiumum L.) mucilage and kernel were characterized as rhamnogalacturonan-I (RG-I). In this study, the conformational characteristics of RG-I fractions from flaxseed mucilage and kernel were investigated, using a Brookhaven multi-angle light scattering instrument (batch mode) and a high-performance size exclusion chromatography (HPSEC) system coupled with Viscotek tetra-detectors (flow mode). The Mw of flaxseed mucilage RG-I (FM-R) was 285 kDa, and the structure-sensitive parameter (ρ) value of FM-R was calculated as 1.3, suggesting that the FM-R molecule had a star-like conformation. The Mw of flaxseed kernel RG-I (FK-R) was 550 kDa, and the structure-sensitive parameter (ρ) values ranged from 0.90 to 1.21, suggesting a sphere to star-like conformation with relatively higher segment density. The correlation between the primary structure and conformation of RG-I was further discussed to better understand the structure-function relationship, which helps the scale-up applications of pectins in food, pharmaceutical, or cosmetic industries.

The effects of ß-lactoglobulin on cyanidin-3-O-glucoside antioxidant activity and bioaccessibility after heat treatment.

The impact of heat treatment at different temperatures on the interaction of ß-lactoglobulin (ß-Lg) and anthocyanin-3-O-glucoside (C3G) was studied. Heat treatment and the addition of C3G changed the secondary structure of ß-Lg with decreasing ß-sheets and increasing random coils. Interactions between C3G and ß-Lg were mainly via hydrogen bonds and van der Waals forces at 25 °C. The elevated temperature promoted hydrophobic interactions between C3G and ß-Lg due to an increase in the hydrophobic groups and amino groups on the surface of ß-Lg molecules. The addition of ß-Lg to the C3G eliminated heat-induced thermal degradation of C3G. The ß-Lg-C3G interactions accompanied with increased particle size and constant zeta potential could increase the antioxidant capacity of C3G approximately by 4% to 10% and protect the colour of C3G from degradation under heat treatment. The C3G bioaccessibility with ß-Lg addition increased by 26.08%, 33.45%, 83.09%, 72.27%, and 354.62% compared with C-25, C-60, C-85, C-100, and C-121, respectively. The protective effect of the non-covalent interactions on C3G at high temperatures (85 °C to 121 °C) was significantly stronger than at 25 °C and 60 °C. The application of ß-Lg in foodstuffs could enhance the antioxidant activity and bioaccessibility of C3G.

Polyphenol-Polysaccharide Complex: Preparation, Characterization, and Potential Utilization in Food and Health.

Polysaccharides and polyphenols coexist in many plant-based food products. Polyphenol-polysaccharide interactions may affect the physicochemical, functional, and physiological properties, such as digestibility, bioavailability, and stability, of plant-based foods. In this review, the interactions (physically or covalently linked) between the selected polysaccharides and polyphenols are summarized. The preparation and structural characterization of the polyphenol-polysaccharide conjugates, their structural-interaction relationships, and the effects of the interactions on functional and physiological properties of the polyphenol and polysaccharide molecules are reviewed. Moreover, potential applications of polyphenol-polysaccharide conjugates are discussed. This review aids in a comprehensive understanding of the synthetic strategy, beneficial bioactivity, and potential application of polyphenol-polysaccharide complexes.

Fluorescent labeling affected the structural/conformational properties of arabinoxylans.

Three fluorescent-labeling methods, including DMSO/DBTD, reductive amination and fluoresceinamine (FA), were applied to label arabinoxylan (AX). The conjugates were named AF, ATF and ACF, respectively. Compared to AX, the water solubility of AF significantly decreased while that of ATF significantly increased (p < 0.05); both showed decreased solubility with increasing degree of substitution (DS). However, AX was not successfully labeled by the FA method due to poor solubility. The structural and conformational properties of the AF and ATF conjugates under different DS were compared. Results showed that fluorochrome (FITC) was successfully grafted onto AX molecule by using the DMSO/DBTD and reductive amination methods. A slight increased Mw was noticed for both AF and ATF. AF existed in a more compact conformation, while the chain conformation of ATF was more elongated at a lower DS (1.15 %) and more compact at a higher DS (1.34 %).

Catechin-grafted arabinoxylan conjugate: Preparation, structural characterization and property investigation.

In this study, a high molecular weight arabinoxylan (AX, Mw: 694 kDa) from wheat bran was alkaline extracted and covalently linked with Catechin (CA) by free radical catalytic reaction. Comparing to AX, arabinoxylan-catechin (AX-CA) conjugates demonstrated an extra UV-vis absorption peak at 274 nm, a new FT-IR absorption band at 1516 cm-1 and new proton signals at 6.5-7.5 ppm, which all confirmed the covalently linked structure. Grafting CA onto AX not only decreased the molecular weight, thermal stability and apparent viscosity of AX, but also enhanced its inhibition effects on starch digestibility in vitro. The in vitro fermentation test with pig feces showed that the degradation & utilization rate of AX, the total short-chain fatty acid (SCFA) and acetic acid levels produced all were significantly delayed after grafting. This study provided a novel approach to synthesize AX-CA conjugates that could be a novel dietary fiber of enhanced functional/bioactive properties using in the fields of functional foods and medicine.

Structural characterization and immunomodulatory activity of mycelium polysaccharide from liquid fermentation of Monascus purpureus (Hong Qu).

Alkaline extracted endopolysaccharides (MPS) from Monascus purpureus (Hong Qu) mycelium were successfully separated into four sub-fractions, namely MPS-1 (18.0 %), MPS-2 (27.1 %), MPS-3 (12.6 %) and MPS-4 (14.7 %), by DEAE-Cellulose column chromatography. By combining monosaccharide composition analysis, methylation analysis and 1D & 2D NMR, the structure of sub-fractions was systematically characterized. Both MPS-1 and MPS-2 were comprised of mannose, glucose and galactose in the molar ratio of 1.5:1.6:1.0 and 10.6:1.0:13.8, respectively. The backbone of them both consisted of 2-α-Manp with several different branched chains. However, MPS-1 contained glucose based sugar residues such as 3-Glcp and 4-Glcp which were not shown on MPS-2. The proposed structures of MPS-3 and MPS-4 were not obtained due to the fairly complex molecular structure and relatively low yield. Moreover, based on the RAW 264.7 cells model, MPS-2 could significantly promote cytokines secretion including IL-6, TNF-α, and IL-10 and improve expression levels of the related mRNA.

Role of Amino Acids in Blood Glucose Changes in Young Adults Consuming Cereal with Milks Varying in Casein and Whey Concentrations and Their Ratio.

BACKGROUND: Increasing the total protein content and reducing the casein to whey ratio in milks consumed with breakfast cereal reduce postprandial blood glucose (BG). OBJECTIVES: We aimed to explore associations between plasma amino acids (AAs), BG, and glucoregulatory hormones. METHODS: In this repeated-measures design, 12 healthy adults consumed cereal (58 g) and milks (250 mL) with 3.1 wt% or high 9.3 wt% protein concentrations and with casein to whey ratios of either 80:20 or 40:60. Blood was collected at 0, 30, 60, 120, 140, 170, and 200 min for measurement of the primary outcome, BG, and for the exploratory outcomes such as plasma AA, gastric emptying, insulin (INS), and glucoregulatory hormones. Measures were made prior to and after an ad libitum lunch at 120 min. Exploratory correlations were conducted to determine associations between outcomes. RESULTS: Pre-lunch plasma AA groups [total (TAA), essential (EAA), BCAA, and nonessential (NEAA)] were higher after 9.3 wt% than 3.1 wt% milks by 12.7%, 21.4%, 20.9%, and 7.6%, respectively (P ≤ 0.05), while post-lunch AA groups were higher by 10.9%, 19.8%, 18.8%, and 6.0%, respectively (P ≤ 0.05). Except for NEAA, pre-lunch AAs were higher after 40:60 than 80:20 ratio milks by 4.5%, 8.3%, and 9.3% (P ≤ 0.05). When pooled by all treatments, pre-lunch AA groups associated negatively with BG (r/ρ ≥ -0.45, P ≤ 0.05), but post-lunch only TAA and NEAA correlated (r ≥ -0.37, P < 0.05). Pre-lunch BG was inversely associated with Leu, Ile, Lys, Met, Thr, Cys-Cys, Asn, and Gln (r/ρ ≥ -0.46, P ≤ 0.05), but post-lunch, only with Thr, Ala, and Gly (r ≥ -0.50, P ≤ 0.05). Pre-lunch associations between AA groups and INS were not found. CONCLUSIONS: Protein concentration and the ratio of casein to whey in milks consumed at breakfast with cereal affect plasma AA concentrations and their associations with decreased BG. The decrease in BG could be explained by INS-independent mechanisms. This trial was registered at www.clinicaltrials.gov as NCT02471092.

Analysis of kinetic parameters and mechanisms of nanocrystalline cellulose inhibition of α-amylase and α-glucosidase in simulated digestion of starch.

This study evaluated the in vitro inhibitory influence of particle size of nanocrystalline cellulose (NCC) fractions against α-amylase and α-glucosidase using cooked potato starch-protein food model system. The kinetics of the resulting inhibitions in the presence of NCC of the two tested enzymes were examined and characterised. Both the size and dose of NCC significantly (p < 0.05) inhibited α-amylase and α-glucosidase by modulating the rate of hydrolysis of starch in the food model system lower than that of the control (no added fibre). At equal concentrations of each NCC fraction, the smallest particle size (≤125 nm) exhibited the highest potency as an inhibitor (median inhibitory concentration (IC50) = 2.98 mg mL-1 and 4.57 mg mL-1 for α-amylase and α-glucosidase, respectively). Increasing concentrations of each NCC fraction caused an apparent significant decrease in Vmax values (p < 0.05) with insignificant change in the Km values for both the tested enzymes. Furthermore, binding assays demonstrated that NCC particles may bind to the two tested enzymes in a non-specific manner. Analysis of the kinetics of the enzymes suggested that the mechanism of inhibition showed that the two tested enzymes mainly exhibited non-competitive mode of inhibition. The observed inhibition of the two tested enzymes suggests that reducing the cellulose size ≤125 nm may enhance its inhibition potency and potentially attenuate starch hydrolysis when added to diet.

Enhancing the prebiotic effect of cellulose biopolymer in the gut by physical structuring via particle size manipulation.

Cellulose is generally recognised as dietary fibre with no limit of permissible quantity in food, and its consumption may modulate digesta content and impact positively on the gastrointestinal physiology and gut microflora. However, cellulose in its native form possessed inherent undesirable physical properties, making it unattractive for food applications. Here, we postulate that by changing cellulose size to nanometric scale, its prebiotic effect would be altered and fermented differently in contrast with micro size cellulose by the gut microbiome and promote the yield of metabolites such as short chain fatty acids (SCFAs). Using faecal matter from three healthy human donors as microbial source, in vitro fermentation of variable size fractions of cellulose from the same were fermented under anaerobic conditions, and SCFAs as well Bifidobacterium selectively isolated and analysed. The increase in production of acetate (194%), butyrate (224%) and propionate (211%) after 24 h of fermentation was significantly promoted by the size reduction and revealed size-dependent relationship as exemplified R2 values >0.83. Consequently, gavaging rats with nanometric size cellulose (125 nm) significantly (p < 0.05) increased these SCFAs yields as well Bifidobacterium counts in contrast with both control and the micro scale size cellulose. Therefore, engineered nanocellulose might have beneficial physiological impact on the gut with improved prebiotic effect.

Versatile preparation of spherically and mechanically controllable liquid-core-shell alginate-based bead through interfacial gelation.

Developing alginate-based beads with liquid-core-shell structure is highly appealing for industrial applications as a promising delivery matrix material. Herein, based on the reaction-diffusion mechanism, a facile method that includes dissolving natural polymer in calcium ion core solution followed by dripping it to alginate shell bath is proposed through interfacial gelation. By facilely tuning the viscosity and surface tension, the boundary condition for forming spherical beads with applicable mechanical properties was obtained. The universal viscosity-boundary relationship was independent of the type or charge condition of polymers in liquid-core. However, chitosan in the core solution significantly affected mechanical properties due to polyelectrolyte interaction with alginate, based on FTIR and SEM analyses. Moreover, a larger spherical zone was obtained by adding a surfactant into the shell bath. By varying calcium ion concentration and reaction time, beads of superior mechanical properties were obtained with an increase in shell membrane compactness.

Improvement in physicochemical properties of collagen casings by glutaraldehyde cross-linking and drying temperature regulating.

The physicochemical properties of collagen casings were successfully improved by glutaraldehyde (GA) cross-linking, where the properties could be further regulated by drying temperature. Transverse direction (TD) showed a lower heat shrinkage rate than that in machine direction (MD). GA cross-linking significantly improved the mechanical properties of films under wet and boiled state. The mechanical properties of films in MD were more susceptible to wet and boiling water. The chemical composition was unchanged after GA cross-linking, but higher drying temperatures led to higher triple helix contents. The GA cross-linking mainly promoted the low temperature thermostability of collagen casings. All film samples had a rough fibrous morphology and a majority of collagen fibers was oriented under the lower drying temperature (55 â„ƒ). These results reported in this study can be used to better guide the preparation of collagen casings.

Facile preparation of collagen fiber-glycerol-carboxymethyl cellulose composite film by immersing method.

Carboxymethyl cellulose (CMC) and glycerol were added by immersing neutralized collagen fiber paste into CMC/glycerol aqueous solutions with different mass ratios, forming composite films. CMC decreased the glycerol adsorption by 49.5%. The presence of CMC formed rougher film surface and more compact internal structure. Glycerol increased the distances between collagen molecules and decreased the relative triple helix contents. The tensile strength and Young's modulus of film in dry condition maximum respectively increased by 125% and 277% with the immersion in CMC (1.5%)/glycerol (4%) solution, compared to the sample immersed into pure glycerol solution, but mechanical properties decreased at high glycerol content. Besides, lower wet mechanical strength was observed as the addition of CMC, corresponding to the higher water swelling ratio of film. CMC also improved the shrinkage stability under boiling and the thermostability of film at a fixed glycerol level, but higher glycerol content caused the decrease of denaturation temperature of film.

Lipid digestion of oil-in-water emulsions stabilized with low molecular weight surfactants.

Altering sn-fatty acid position of glycerol mono-oleate (GMO) from sn-1 to sn-2 decreases fatty acid bioaccessibility by 25.9% providing possible strategies to tailor lipemic responses of food emulsions. Lipid digestion kinetics and fatty acid bioaccessibility of monomodal O/W emulsions stabilized at their minimum surfactant concentration (0.5 < MSC > 0.7 (w/w)) were studied in the TNO Intestinal Model (TIM-1) gastrointestinal (GI) tract. No significant differences were observed between induction times nor rate constants when using 1-GMO and 1-GMS, Span 60, Tween 60 and Tween 80 as surfactants in O/W emulsions, as determined by fitting a three-parameter shifted logistic model to the cumulative bioaccessibility. Comparable trends were observed between area under the curve (AUC) of the absolute bioaccessibility and total overall bioaccessibility.

Correlating in vitro digestion viscosities and bioaccessible nutrients of milks containing enhanced protein concentration and normal or modified protein ratio to human trials.

This work compared in vitro and in vivo digestion of breakfast cereal with milks containing high protein concentration (9.3 wt%) and the normal protein ratio (80 casein : 20 whey) or a modified ratio (40 casein : 60 whey) and with a water-permeate control. The in vivo study indicated that high protein concentration and modified ratio in milks delays the postprandial appearance of blood glucose (BG) and amino acids due to delayed gastric emptying and hormonal responses. However, the role of viscosity and/or protein structure during digestion was not examined. Therefore, milks and the control were digested in vitro (oral (O, 2 min), gastric (G, 62 min) and duodenal (D, 92 min)) to determine viscosity, particle disintegration, protein solubility and hydrolysis, and the bioaccessibilities of sugars and total amino acids (TAA). The normal ratio (80 : 20) treatment demonstrated higher structural viscosity during digestion (P < 0.05) due to the formation of casein aggregates and interaction with cereal and greater TAA (mg per g protein of undigested breakfast) caused by gastric hydrolysis (DH%; P = 0.01). Overall, there were no treatment differences for disintegration, solubility and d-glucose. Protein-containing treatments inhibited amylolysis and lowered reducing sugars (mg g-1 available carbohydrates of undigested breakfast) compared to the control. Similar trends were observed between higher viscosity (Pa s) during gastric stage and slower in vivo gastric emptying (paracetamol, mmol L-1). Also, protein treatments inhibited amylolysis and lowered reducing sugar (mg g-1 of carbohydrates) and may have contributed to lowered BG (mmol L-1 g-1 of carbohydrates) after the duodenal phase. However, increased viscosity, elicited by higher proportions of casein, did not inhibit starch hydrolysis and appearance of BG. In vitro systems provide similar trends in biomarkers to in vivo studies and can be used to answer specific physiological questions.