MATERIALS AND METHODS

Study area: The study was carried out at the Department of Food and Nutrition Sciences, College of Agricultural and Food Sciences, King Faisal University, Al Hofuf, Kingdom of Saudi Arabia from June-August, 2019.

Lupine husks powder: Local Egyptian breeds of sweet lupin (Lupinus albus L.) variety Giza) were obtained from the Agricultural Research Centre, Giza, Egypt. The husks were removed from the seeds in a laboratory hammer mill, grounded husk fibers then re-milled and sieved to pass through sieve 500 μm. The samples were packed and kept under refrigeration for analysis. Obtained lupine husk have moisture 8.76±0.06, Crude protein 4.8±0.26, Total ash 2.57±0.18, Crude fat, Starch 0.20±0.04, soluble dietary fiber 43.7±2.19, Insoluble dietary fiber 41.5±1.12 and Total dietary fiber 85.20±4.38 according to Ahmed¹¹.

Starter culture: Direct Vat Set (DVS) containing Streptococcus thermophilus and Lactobacillus delbrueckii sub sp. bulgaricus (YCX31) was obtained from Chr. Hansen's laboratories, Denmark.

Preparation of lupine husks free fat stirred yogurt: Yogurt is produced from fat-free cow milk and lupine husk powder was added. The milk is heated at 90°C for 10 min in a stainless container and then cooled off at 42°C. The starter is added at 3%. The samples were incubated at 42°C until coagulation occurred, then stirred, cooled and stored in a refrigerator at 4°С for 15 days as depicted in Fig. 1. Samples were taken for sensory, rheological, chemical and microbiological analysis at zero time, 7 and 15 days, respectively.

Chemical and physical analysis: Milk and yogurt samples were analyzed for total solid, protein content and total acidity were determined by following the standard procedures of the Association of Official Analytical Chemists¹². The pH was determined by using a digital pH meter (Inolad model 720, Germany). Acetaldehyde contents were examined according to Lees and Jago¹³.

The syneresis of yogurt was calculated immediately after coagulation and during the storage time of 15 days at 4°C±1¹⁴. The viscosity of the samples was also measured using a rotational viscometer DV-II+Pro Brookfield Engineering Laboratories, Inc. (USA).

Fig. 1:

Steps of lupine husks free fat stirred yogurt preparing

The analysis was carried out at 20°C by using spindle (L2) at a speed of 100 rpm and the reading was recorded as cP.

Texture profile analysis: Texture Profile Analysis (TPA) was carried out by a compression test that generated a plot of force (g) versus time (s) and was evaluated instrumentally using a texture analyzer (Texture Pro CT V1.2 Build 9, Brookfield Engineering Labs, Inc. USA). In the first stage, it was carried out at a pre-test speed of 1 mm sec^–1 and a test speed of 1.7 mm sec^–1. The typical texture profile (force vs. time) curve was obtained with one complete run. Hardness, gumminess, cohesiveness, adhesiveness, and springiness of yogurt samples were calculated through the software program (Texture software, Brookfield Instruments).

Microbiological analysis: Regular microbiological methods (colony-forming unit [cfu]). Streptococci were counted on M-17 agar (Oxoid) and Lactobacilli were counted on Man Rogosa Sharpe agar (Oxoid). Both media were incubated under microaerophilic conditions at 37°C for 48 hrs.

Sensory evaluation: According to Peryam and Pilgrim¹⁵ A 9-point hedonic scale (1 = dislike extremely and 9 = like extremely).

Statistical analysis: All data is presented as mean±SD for three replicates for each sample. Multiple comparisons were performed through the application of Univariate Analysis of Variance (ANOVA) using Stat graphics 16.1.11 (Stat Point Technologies, Inc. Virginia, USA). The differences were considered significant at p<0.05.

RESULTS AND DISCUSSION

The effects of lupine husk supplementations on total solids, protein and ash were significant (p<0.05). Total solid contents ranged from all treatments between 8.56 and 13.53% and total protein and ash values also increased in correlations with the addition of lupine husk powder (Table 1).

The pH values of all treatments were between 4.7 and 4.8 on day 1 and 4.36 and 4.46 on day 14, respectively. Lupine husk treatment pH values of the yogurt T₁ were non significant via control T₀ (p>0.05).

T₂ and T₃ were found to be significantly (p<0.05) different from control (T₀). This increase in pH may be attributed to the water holding capacity of lupine husk thus diluting the concentrations of lactic acid and other organic acids produced by cultures. This might have resulted in an increase in pH of fiber -fortified yogurt¹⁶. The more the lupine husk was added, the more water might have been absorbed, thus increasing the pH values of all yogurt treatments. It was reported that the addition of various dietary fiber sources did not influence the pH of the fortified yogurts¹⁷.

Titratable acidity increased in conjunction with the addition of lupine husk powder and storage. The highest titration acidity was detected on day 14 (0.6% of T₂ and T₃) and the lowest on day 1 (0.3% of T₀ and T₁). While the effects of titration acidity of the yogurts were significant at day 14, the lupine husk yogurt treatments were significant (p<0.05), with persimmon lupine supplemented yogurts exhibiting the highest titration acidity. Since the lupine husk content was increased from 1-5% (T₁-T₃), therefore, an increase in acidity is expected. But our results showed a low increase which may be attributed to the water holding capacity of the lupine husk, thus diluting the concentration of lactic acid and other organic acids produced by cultures in yogurt. However, the titratable acidity of yogurt samples was within an average of 0.9%, which recommended value for plain yogurts¹⁸.

Syneresis or whey separation is an important defect in yogurt. Lupine husk powder addition to yogurt milk decreased significantly whey separation values during storage. Although the addition of lupine husk powder reduced the separation of whey at a concentration of 1%, it increased with the addition of lupine husk powder by 3 and 5%. The effects of different fiber sources addition on yogurt were varied. Güzeler et al.¹⁹ described an increasing yogurt syneresis correlated with apricot fiber addition. While 1% orange fiber addition decreased syneresis and improved the textural properties of the yogurts as mentioned by Lario et al.²⁰. Whey separation in yogurt reduced by adding wheat bran fibers²¹, sweet potato²², Rice bran²³ and orange²⁴. The binding capacity of added dietary fibers to yogurt protein micelle makes the composition more capable to bind whey and avoid syneresis²⁵.

Acetaldehyde content as the main flavor compound in yogurt must be between 23 and 41 mg kg^–1 of yogurt²⁶. Yogurt free lupine husk had higher acetaldehyde values than lupine husk -added samples after both 7 and 14 days of storage (p<0.05). Acetaldehyde values of the yogurts in beginning in storage varied between 8.06 and 8.56 mg kg^–1, which were lower than the control treatment. That decrease is due to the alcohol dehydrogenase activity of yogurt starters on the relationship between acetaldehyde and ethanol^26,27.

The bacterial count of yogurt samples including S. thermophilus and L. bulgaricus is presented in Table 2. With an increase in the lupine husk, the total bacterial count for both strains increased. The mean values of the streptococci count of T₀ and T₁ differed significantly (p<0.05) on day 1. T₂ and T₃ were similar to the control one T₀. Lactobacilli count for T_{0 higher} than T₁, similar to T₂ and lower than T₀. During the storage period, increases in the total bacterial count with an increase in lupine husk level may be due to the increase in moisture content²⁸. The increase in the Lactobacilli count with the increase in lupine husk content may also be due to the prebiotic effect of the lupine husk, which agreed with the addition of grape fiber to yogurt as mentioned by Dibazar et al.²⁹.

Table 3 reveals the textural characteristics (Viscosity, Hardness, Adhesiveness, Cohesiveness, Gumminess and Springiness) of the lupine husk yogurts and their control. The viscosity is defined as the area under the force versus time curve. The maximum viscosity indices were respectively recorded on days 14 for T₃, T₂ and T₁. The effects of lupine husk addition on the viscosity index of the yogurts were a significant increase on days 1, 7 and 14 (p<0.05), free lupine husk yogurts displaying lower viscosity index values than the lupine husk yogurt. Espirito et al.³⁰ reported that yogurts supplemented with fruit fiber exhibited reduced fermentation times but increased firmness, consistency and cohesiveness.

Hardness is represented as the highest force sensed and recorded by the load cell of the texture analysis device. Day 14 yogurts exhibited the maximum (T₃: 301 g) and minimum firmness (T₀: 173 g). Present results matched with that obtained by Perina et al.³¹ that the addition of passion fruit peel powder to yogurt significantly affected the hardness. Also, results are in agreement with those obtained by Crispin-Isidro et al.³² which reported that yogurt hardness increases at a level of 2-4% inulin addition. This may be because fibers are acting as ‘filler’ between yogurt components.

The decrease in adhesiveness with an increase in lupine husk level may be due to the decreased binding capacity of yogurt thus reducing adhesiveness and increasing cohesiveness³³. During the storage period T₁ lupine husk yogurt exhibited the lowest cohesiveness followed by T₂ and T₃. The effects of lupine husk supplementation on the cohesiveness of the yogurts were significant on days 1, 7 and 14 (p<0.05).

The organoleptic properties of experimental yogurt samples are shown in Table 4. Appearance scores were similar in free lupine husk yogurt treatment and 1% addition during the storage period. Increasing the level of the lupine husk there was a little bit decrease in the consistency, flavor and overall acceptance scores. Free lupine husk yogurt treatment showed the significantly (p<0.05) highest score, followed by T₁, T₂ and T₃ yogurt samples containing 1, 2 and 3% of the lupine husk, respectively. The decrease in the flavor of yogurt with every increment of lupine husk may be due to the prevention of the release of the flavoring from the network formed by fiber addition. Fernandez-Garcia et al.³⁴ have also reported a decrease in flavor quality of yogurt by the addition of fiber.

Consistency scores for T₁ were nearest to control T₀ followed by T₂ and T₃. The mean values with an increase in lupine husk may be due to the stabilizing effect of fiber on the texture of yogurt. Lupine husk (polysaccharides) acts as a stabilizer in yogurt, thus improving its consistency³⁵. However, a decrease in the texture of T₃ may be due to the undesirable grittiness produced by the addition of more lupine husk. Besides, more fiber content of lupine husk absorbs more moisture thereby making yogurt tougher³⁴ which may further support our results.

Hussein et al.³⁵ have reported that the addition of polysaccharides (fiber) improves body, texture, appearance and mouthfeel and retards whey separation of yogurt. The addition of polysaccharides (fiber) as a stabilizer in the manufacture of yogurt is a common practice. Texture enhancement and preventing syneresis is the main functions of hydrocolloids, which act as stabilizers³⁶ in yogurt. Tomic et al.³⁷ reported that the addition of different sources of fibers (soy, rice, oat, corn and sugar beet), at the level of 1.32%, led to lower overall flavor and texture scores a grainy flavor and a gritty texture were intense in all samples except in those made with oat fiber. However, fiber size is also an important factor in yogurt formulation because of its impact on general acceptance. Besides, Raju and Pal³⁸ reported that yogurt color changes with adding fiber, while no marked changes in yogurt are affected with bamboo or inulin fibers¹⁷.

CONCLUSION

Fortifying yogurt or dairy products with fiber is of great interest to improve the functionality and create foods with health benefits. The addition of lupine husk (as dietary fiber) would complement its healthy characteristics. The results of the present study showed that fortifying yogurt (T₁) with 1% lupine husk scored highest in all parameters including sensory characteristics.

SIGNIFICANCE STATEMENT

This study discovers the possibility of adding sweet lupine husk powder in yogurt preparation that can be beneficial for fat replacement and texture enhancement. This study will help the researcher to uncover the novel by-products additive areas of fat replacers and texture enhancers in yogurt that many researchers were not able to explore. Thus a new additive in yogurt may be arrived at.