EFFECT OF ADDITION OF SPELT AND BUCKWHEAT HULL ON SELECTED PROPERTIES OF YOGHURT

Buckwheat and spelled hulls are little tested in their use in the production of functional foods. The aim of this study was to evaluate the use of hull as a functional additive in yoghurt and to determine the effect of various doses of buckwheat and spelt hull on the physicochemical, organoleptic and microbiological properties of yoghurt. The enrichment of yoghurt with buckwheat and spelled hull resulted in a decrease in total acidity and a decrease in syneresis. Addition of spelt and buckwheat hulls to yoghurts reduced the colour brightness and increased the intensity of the red and yellow colours. The highest number of S. thermophilus was found in yoghurt containing 3% buckwheat hull and the beneficial effect of the addition of hull on L. bulgaricus growth was demonstrated. The type of hull determined the flavour and odour of yoghurts. Spelt hull gave the yoghurts a more intense floury and grainy flavour than buckwheat hull.


INTRODUCTION
Food industry companies have high expectations in terms of food products that meet the consumers' demand for a healthy lifestyle. In this context, functional food plays a special role, as apart from its fundamental goal, which is nutrition, it also has the psychological or physiological impact on the human body (Menrad, 1990). In Europe, the main categories of functional food are dairy products (50% of the market) and cereal products (30%).Yoghurt is considered as a healthy food because it contains viable bacterial cultures. However, milk and dairy products do not contain fibre, while fibre is found in the cell wall of fruits, vegetables and cereals (Brückner-Gühmann et al., 2019). Addition of fibre of various origins as a functional component to milk-based products could contribute to better water holding capacity, improving texture and structure properties, modifying the viscosity, swelling capacity or reducing the calorific value of the finished product by acting as a bulking agent (Elleuch et al., 2011). Consumption of food containing dietary fibre may prevent cardiovascular and gastrointestinal disorders, hypertension, hypercholesterolemia, obesity and diabetes (Hashim et al., 2009;Górecka et al., 2009;Esposito et al., 2005;Krkošková et al., 2005). The average daily demand for dietary fibre is 25g for women˂50 years of age, 21g for women> 50 years, 38g for men ˂50 years and 30g for men> 50 years. Most nutritionists suggest that 20-30% of the daily fibre intake should come from soluble fibre Elleuch et al. (2011). At present, the average global consumption of fibre is still lower than the recommended daily intake level. Thus fibre-rich by-products may be incorporated into food products as inexpensive, low-calorie bulking agents for partial replacement of flour, fat or sugar, as enhancers water and oil retention and to improve emulsion or oxidative stability. However, the percentage of fibre that may be added is finite, because it can cause undesirable changes to the colour and texture of the finished product (Elleuch et al., 2011;Hashim et al., 2009). One of solutions in preventing fibre deficiency in the human diet may be the addition of fibre to milk-based food products (Robertson et al., 2000). Yoghurt is considered a healthy food and fortifying it with dietary fibre will make it even healthier. The latest research results indicate that buckwheat deserves more interest as a valuable raw material of functional food due to the content of biologically active compounds. In buckwheat grains, dietary fibre accounts for 5 to 11%, the soluble fraction content is 3-7%, and the insoluble fraction is about 2-4% Krkošková et al. (2005). Buckwheat grain is characterized by a high content of starch, protein with a favourable amino acid composition, a low content of αgliadin fraction and a high content of dietary fibre (Krkošková et  Spelt wheat is also a rich source of dietary fibre. Whole grain spelt flour contains from 8.8% to 10.1% insoluble fibre and from 1.8% to 2.0% soluble fibre (Escarnot et al., 2007). The protein found in spelt is of high value, with high nutritional qualities, since compared with common wheat, it has 20-40% higher content of amino acids, including lysine, threonine, leucine and isoleucine (Rożnowski et al., 2015). Spelt in the lipid fraction is dominated by unsaturated fatty acids, of which about 50% is linoleic acid, and slightly more than 20% oleic acid. Moreover, fats found in spelt wheat contain phytosterols and fat-soluble vitamins (A, D and E). Spelt contains many valuable macro-and micronutrients, however, different authors give different contents of individual minerals in spelt (Christa et al., 2008;Kohajdová et al., 2008). The aim of this study was to evaluate the use of hull as a functional additive in yoghurt and to determine the effect of using various doses of buckwheat and spelt hull on the physicochemical, organoleptic and microbiological properties of yoghurt. Buckwheat and spelled hulls are little tested in their use in the production of functional foods. The aim of this study was to evaluate the use of hull as a functional additive in yoghurt and to determine the effect of various doses of buckwheat and spelt hull on the physicochemical, organoleptic and microbiological properties of yoghurt. The enrichment of yoghurt with buckwheat and spelled hull resulted in a decrease in total acidity and a decrease in syneresis. Addition of spelt and buckwheat hulls to yoghurts reduced the colour brightness and increased the intensity of the red and yellow colours. The highest number of S. thermophilus was found in yoghurt containing 3% buckwheat hull and the beneficial effect of the addition of hull on L. bulgaricus growth was demonstrated. The type of hull determined the flavour and odour of yoghurts. Spelt hull gave the yoghurts a more intense floury and grainy flavour than buckwheat hull. The milk mixtures were homogenised (20 MPa, 60 °C) in a homogeniser (Nuoni GJJ-0.06/40, Zhejiang, China) and pasteurized in a water bath at 95 °C for 15 min and then rapidly cooled in chilled water to 45 °C. After that, all the mixes were each inoculated with 1 g L -1 of yoghurt culture YC-X11. All experimental milk distributed in 100 ml sterile plastic containers and incubated at 42 °C until a pH was reached to 4.7 and stored at 5 °C (Cooled Incubator ILW 115, POL-EKO Aparatura, Poland). The yoghurts were evaluated after 7 days of cold storage.

Physicochemical properties
The pH determination was performed with a pH-meter (FiveEasy Mettler Toledo, Switzerland). The titratable acidity (TA) of the milk was determined according to the Soxhlet-Henkel method (Normenausschuss, 1970). Syneresis (whey separation) was determined according to the method described by Santillan-Urquiza et al. (2017) with modifications (10 g sample of fermented milk was transferred into 50 mL plastic tube and centrifuged at LC-04 CENTRIFUGE, Zenithlab, China, 1790 g for 10 min at 5°C). The syneresis was estimated as the released whey over the original weight and was an average of five determinations. Colour was analysed by colourimeter Chroma Meter CR-400 (Konica Minolta Sensing, Inc., Osaka, Japan) using the CIELAB. Determined of L*, a* and b* parameters, where L* represents the brightness (from 0 -black to 100 -white); a* and -a* redness and greenness, respectively; and b* and -b* yellowness and blueness, respectively. Before testing the instrument was calibrated on a White Calibration Plate CR-A43.

Parameters of texture
Texture was determined with the CT3 Texture Analyzer with the Texture Pro CT software (Brookfield AMETEK, USA). A TPA (Cycle Count 2) test was performed on a 100 mL sample of solid state yoghurt, without mixing in the container, with the following: samplecylinder 66.00 mm x 33.86 mm, trigger Load 0.1 N, test Speed 1 mm s -1 , table TA-BTKIT, probe TA3/100 (acrylic cylinder -35 mm diameter). The following texture parameters were determined: hardness, adhesiveness, stringiness length, cohesiveness, springiness, gumminess.

Microbiological analysis
Determination of the number of Streptococcus thermophilus (S. thermophilus) and Lactobacillus delbrueckii ssp buglaricus (L. bulgaricus) was carried out by the plate method. Microbiological property of yoghurt was determined according to the method Gao [17] with some modifications. Viable counts of S. thermophilus were determined on an M17 medium (Biocorp, Poland), after aerobic incubation at 37°C for 48 h. Viable counts of L. bulgaricus were determined on Lactobacillus bulgaricus Agar (Base) used with acetate buffer isolation of L. bulgaricus (Sigma-Aldrich, India) and incubated anaerobically at 37°C for 72 h; ). The results were expressed as log cfu g -1 .

Organoleptic parameters
To assess organoleptic parameters a sensory pre-test was conducted. The organoleptic analysis was performed by a trained panel consisted of 10 women and 10 men at the age of 21-30. The panelists were served five samples at a time (in three-digit random number coded plastic cubs) and asked to rinse their mouths between samples with water. The panelists evaluated the presence of creamy milky flavour, sourness, floury flavour, grainy flavour, lactic acid odor, grainy odor (Tab 3) on a nine-point rating scale with edge markings (from 1 = not perceptible to 9 = extremely strong) (Baryłko-Pikielna et al., 2014).

Creamy -milky flavor
The taste stimulated by milk powder (noneintensive) Sourness The taste stimulated by acid (none -intensive)

Floury flavor
The intensity of flavor associated with floury (noneintensive) Grainy flavor The intensity of flavor associated with cereals, (none -intensive) Odour

Lactic acid odor
The intensity of odor associated with sour milk, i.e. lacid acid (none -intensive) Grainy odor The intensity of odor associated with cereals, (none 1intensive 9)

Statistical analysis
The obtained results were given as the mean and standard deviation was calculated statistically using the Statistica v. 13.1 software (StatSoft, USA). ANOVA was used to investigate the overall effect of the hull type and hull dose on properties of yoghurt. Significance of differences between the averages was estimated with Tukey's test (P˂0.05).

RESULTS AND DISCUSSION
After seven days of storage at low temperature, the highest pH was found in yoghurt with 3% of spell hull and the lowest in the control (Tab 4). The introduction of a lower dose of hull, i.e. 1.5%, did not significantly change the pH of yoghurt as compared with the control. Analysis of variance indicates that the type of hull did not significantly affect the pH value of yoghurts, but the dose was significant ( Tab   In yoghurts with buckwheat and spelt hull, the total acidity was lower than that in the control (Tab 4). Both the dose and the type of hull added significantly differentiated the total acidity (Tab 5). Introduction of a 3% addition of buckwheat hull reduced the acidity by about 6 0 SH, and of spelt hull by as much as about 7.5 0 SH compared with the control. Colour-space parameters (L*a*b*) of yoghurts were presented in Tab 4. The L* values of control yoghurts were significantly higher than those of yoghurts with the addition of buckwheat and spelt hull (P<0.05). The L* values decreased gradually along with increasing addition of hull dose. Similar reduction in L* value has also been reported by Costa et al. [21] when they fortified yoghurt with cupuassu (Theobroma grandiflora) pulp. Also, Demirci et al. (2017), adding rice bran, showed a decrease in the L * value of yoghurts and probiotic drinks. Addition of spelt hull to yoghurts increased the proportion of red colour and intensified saturation with yellow colour in comparison with the control. Spelt grain contains, among others, colour-forming carotenoids and riboflavin. Żuk-Gołaszewska et al. (2018) proved that spelt grain was also characterized by a significantly higher proportion of redness a* (6.64-8.11) and yellowness b* (17.2-19.0) than that of common wheat. According to Batifoulier et al. (2006), in spelt there is 0.77-0.88µg/g of riboflavin, which is characterized by intense yellow colour. The yellow-brown colour of yoghurts with the addition of triticale was also described by Tomic et al. (2017). Chromatic colour parameters in yoghurts with buckwheat hull assumed positive values for a *, indicating a significant proportion of the red colour, and positive values for b *, indicating the proportion of the yellow colour. The high proportion of the red colour, increasing with the dose of buckwheat hull, results from a high content of phenols in the buckwheat hull. According to Zhu et al. (2014), the total content of phenolic compounds in micronized buckwheat hull is 127µg/g. About six flavonoids have been isolated from buckwheat grains, of which rutin predominates. Rutin, quercetin, orientin, vitexin, isovitexin, and isoorientin were identified in buckwheat hulls (Christa et al., 2009;Dietrych-Szostak et al., 1999). Syneresis is an important defect in yoghurts caused by excessive disarray of curd stability. As it can be seen in Tab 4, yoghurts with buckwheat and spelt hulls had lower syneresis values as compared with the control. The reduction in syneresis was the most effective by the addition of spelt hull. The introduction of 1.5% spelt hull reduced syneresis by 4.0% and a higher addition of 3.0% reduced syneresis by about 6.0%. The use of buckwheat hull also caused a reduction in syneresis by 2.5 -3.0%, however, the dose of buckwheat hull introduced was not significant. A two-way ANOVA (Tab 5) indicates that the type of hull and interactions between the dose and the type of hull had a significant effect on the extent of syneresis. This can be explained by the fact that water holding capacity of the hull is related to the porous matrix structure formed by polysaccharide chains which can hold large amounts of water through hydrogen bonds (Kethireddipalli et al., 2002). According to Zhu et al. (2014), water holding capacity of micronized buckwheat hull is 2.24 g/g, and swelling capacity is 7.48mL/g.  , 2004). In another study, yoghurts with oat concentrate were analysed and it was shown that not the oat protein, but the oat starch improved the water holding capacity in the yoghurt samples and reduced syneresis (Brückner-Gühmann et al., 2019). After 7 days of storage, the count of S. thepmophilus was on average from 9.83 -9.99 log cfu g -1 (Fig. 1).  Hardness is considered as the resistance of a sample to deformation until an external force is applied (Lal-Dar et al. 2014). The hardness results in Tab 6 showed that fortification of yoghurt with spelt and buckwheat hull at a dose of 1.5% significantly reduced the hardness compared with the control. The use of pineapple peel fibre by Sah et al. (2016) also resulted in lower hardness of yoghurt. Specifically, pineapple peel powder incorporation resulted in lower hardness of yoghurt, resulting in a weak gel attributed to thermodynamic incompatibility between milk proteins and polysaccharides from pineapple peel powder (Corredig et al., 2011). In the study by Basiri et al. (2018) it was found that the presence of carboxy methyl cellulose (CMC) and linseed mucilage reduced the gel hardness compared with the control. Increasing the dose of hull up to 3.0% only in case of buckwheat hull resulted in significantly greater hardness. A two-way ANOVA showed that both the type and dose of hull and the interactions of these two factors significantly affected the hardness of yoghurts (Tab 5). In this study, the highest adhesiveness value was determined in the control sample. The addition of spelt hull significantly reduced the adhesiveness of yoghurts. The addition of spelt hull did not significantly differentiate the adhesiveness of yoghurts in comparison to the control. The addition of hull influenced the stringiness length of the yoghurt gel. Significantly lower stringiness length values were obtained in yoghurts with the addition of spelt hull. In yoghurts with the addition of 3.0% of spelt hull, stringiness length similar to the control was demonstrated. cohesiveness values were determined in yoghurts with the addition of spelt hull. There were no significant differences in the springiness of the control yoghurts and those with the addition of hull. Gumminess is a defect in which a sticky feeling is perceived in the mouth (Lal-Dar et al. 2014). The gumminess values shown in Tab 4 indicate that the addition of buckwheat or spelt hull in a dose of 1.5% decreased this defect in the yoghurt. Tab 7 presents the results of the analysis of the flavour and odour of yoghurts with hull and controls. The addition of buckwheat hull reduced the intensity of milk-cream and sour flavour and increased the sense of floury and buckwheat flavour. Yoghurts with buckwheat hull had the most intense sour and buckwheat odour.

Christa et al. (2008)
identified condensed catechins, phenolic acids, including hydrobenzoic acids, synigric, p-hydroxy-benzoic, vanillic and p-coumaric acids in the bran-aleurone layer of buckwheat grains. Most likely, soluble oligomeric condensed catechins along with phenolic acids provided characteristic astringent buckwheat flavour. Yoghurts with spelt hull were characterized by intensely floury flavour with a moderate proportion of grainy flavour. The intensity of these flavours effectively masked the taste of sour and milky-cream flavour. There was no sour odour in them, only a slightly perceptible grainy odour. Brückner-Gühmann et al. (2019) also indicated a significantly perceptible oat flavour in all yoghurts with oat protein concentrate and in samples with oat protein isolate as compared with the sample fortified with SMP (skim milk powder). Pyrroles and thiazoles have also been described as flavour compounds present in extruded flour (Parker et al., 2000). In the study by Tomic el al. (2017), grainy flavour, more intensive in triticale yoghurts compared with those fortified with wheat or oat fibre, was described as a flavour which is pleasant and typical of cereal-rich yoghurt.

CONCLUSION
The market for functional yoghurts is systematically increasing and the introduction of the addition of hull may be attractive to consumers due to the content of dietary fibre. Yoghurts with hull differ in terms of physicochemical and organoleptic properties. Fortification of yoghurts with buckwheat and spelt hull has resulted in lowering total acidity and reducing syneresis. Addition of spelt and buckwheat hulls to yoghurts reduced the colour brightness and increased the intensity of the red and yellow colours. The highest number of S. thermophilus was found in yoghurt containing 3% buckwheat hull and the beneficial effect of the addition of hull on L. bulgaricus growth was demonstrated. The type of hull determined the flavour and odour of yoghurts. Spelt hull gave the yoghurts a more intense floury and grainy flavour than buckwheat hull. However, the addition of hull may be attractive to consumers due to the content of dietary fibre. Most likely, the introduction of an intensely aromatic flavour additive, such as cinnamon, apple, blueberry -would effectively mask the flavour and odour of hull uncharacteristic of yoghurt.