The Effects of Ultra-filtered Milk Proteins on Chemical and Sensory Properties of Yoghurt

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Author: Ainaz Alizadeh

The Effects of Ultra-filtered Milk Proteins on Chemical and Sensory Properties of Yoghurt A.Alizadeh, M.R. Ehsani, M.Safari Department of Food Science, Technology and Engineering, Faculty of Biosystem Engineering, University of Tehran, Iran

Abstract

The chemical composition, sensory attributes and acetaldehyde content of yoghurts produced from ultra-filtered milk retentates and normal yoghurt made with SMP were studied and compared. UF yoghurts had more content of acetaldehyde, protein, lactic acid and total solid than control but lactose content and syneresis were at lower level. The amount of produced acetaldehyde was dependent to the protein amount and by increasing the latter the acetaldehyde got higher. The quantity of acetaldehyde decreased during the storage of yoghurt. Scores for sensory analysis of UF yoghurts were higher than control. Yoghurt made from UF retentate with 13.5 % TS and 5.3% protein got the highest rating for all sensory properties and was comparable with one of the most acceptable yoghurts produced in Iran.

Key words: yoghurt, acetaldehyde, ultrafiltration

INTRODUCTION

The formulation of products such as yoghurt with optimum consistency and stability to syneresis is of primary concern to dairy industry. Factors influencing yoghurt texture and syneresis include total solid (TS), protein, salts, homogenization, and type of culture, acidity and heat pre-treatment. The solid content of milk can be increased by applying evaporation of treated milk, addition of Skim Milk Powder (SMP) or protein concentrates and concentrating the milk by ultra-filtration (UF) or reverse osmosis (RO). (C.G.Biliaderis et al., 1992) Addition of milk powder or whey is the method most widely used to produce yoghurt. However, the thermal degradation of proteins and vitamins through heat treatment of milk can be very important in these cases, diminishing the nutritional value of yoghurt. (F.Alvarez et al., 1997) Also milk powder itself is a product witch has undergone extremely harsh thermal processing that alters some of its constituents. An alternative to obtain yoghurt with higher nutritional value is to increase the total solids in milk by means of membrane technology. (A.Brazuelo et al., 1995) The industrial scale production of yoghurt from milk concentrated by UF or RO has been reported by Jepsen (1977, 1979) and according to the data compiled by Tamime and Deeth (1980) and Ferguson (1989), whole milk concentrated by UF to 18-20% TS produced a smooth, creamy yoghurt with a typical acid flavor that homogenization was not required and skimmed milk concentrated by UF to 13% TS was suitable for yoghurt making. (Tamime and Robinson, 1999) Use of membrane technologies for the concentration of milk for production of cultured dairy products has been documented in several reports but most of them were focused on physical properties of UF yoghurts that are improved by the application these technique. No doubt that it is important as well to understand the effect of membrane processing on flavor and sensory properties of UF yoghurts. The present study was carried out to compare the chemical and sensory attributes of experimental yoghurts made from UF retentates and normal yoghurts produced by adding SMP to initial milk as control. Also acetaldehyde content of samples was compared with each others as the one of the most important factors of yoghurt flavor.

MATERIAL AND METHODS

1-Ultafiltration

Pasteurized milk (fat content: 1.8%) was ultra-filtered with a poly ether sulphone spiral wound membrane (nominal molecular weight cut-off 20,000 D) at 50șC. Retentates were obtained form the system at three steps with attention to their TS contents. Samples were named A, B and C respectively.

2- Fermentation step

After the necessary concentration, the samples were heated at 78 șC for 1 min in stainless steel buckets with stirring continued during cooling to 42șC. These samples were inoculated with the starter culture (code YC-350 from Ch.Hansen) at a level of 0.4% v/v (Nu-trish, 2004, Hansen) and dispensed into yoghurt pots of 200 ml. The pots were then incubated at 45 șC until the desired PH value. The PH of the samples was recorded during incubation using a PH-meter. After incubation, the samples were transferred to a cool room at 4șC. A control sample was prepared with pasteurized milk (fat content 3.23%) and addition of SMP (2%w/v) and named as D sample. Another control sample was obtained for acetaldehyde content comparing and sensory analysis as one of the most popular yoghurts in the market (Fat content 3%) and named as E sample.

3- Analysis

Total solid, fat, protein and lactose content of retentates were determined by a Milko-Scan (133 B N.FOSS Electric, Denmark). PH and titrable acidity of yoghurt were measured according to AOAC (2002) methods. Acetaldehyde was extracted by distillation of yoghurts and detected by Gas Chromatography using a Philips PU 4410 (packed column, FID detector, Helium as carrier gas and temperature programming procedure) at 1 and 14 days of storage. Panel taste and rating scores analysis was used to assess the sensory properties of yoghurts by employing 50 untrained panelists at the day 14 of storage.

4- Statistical analysis

Samples for chemical analysis were taken in triple batches and the results were assessed by SAS software version 8. Sensory data were subjected to analysis of Friedman.

RESULT AND DISCUSSION

The UF experiments were aimed at providing membrane retentates with solid levels at aprox. 12, 14, 16 % for subsequent production of yoghurt. The apparent protein content of retentates increased from 3.07% in pasteurized milk to 6.80% in the most concentrated retentate having total solids of 16%. Lactose was slightly decreased, fat content increased from 1.80 to 3.74% and also total solid increased from 10.24% to 16.18% in the final received sample(C).

Fig.1: Milk constituents' changes during ultra filtration and comparing with the control.

Table1. Compositions of pasteurized milk, UF retentates, permeate and control sample sample fat protein lactose TS SNF Pas. milk 1.80 ±0.006 3.07 ±0.01 4.78 ±0.017 10.24 ±0.032 8.44 ±0.025 A(cf=1.5) 2.46 ±0.01 4.28 ±0.02 4.62 ±0.01 11.93 ±0.005 9.46 ±0.02 B(cf=1.8) 2.91 ±0.07 5.26 ±0 4.55 ±0.06 13.68 ±0.02 10.31 ±0.09 C(cf=2.2) 3.74 ±0 6.80 ±0.11 4.47 ±0 16.18 ±0.03 12.48 ±0.02 D(control) 3.23 ±0.03 3.06 ±0.005 4.92 ±0.05 11.80 ±0.03 8.57 ±0.07 permeate 0.90 ±0.005 0.13 ±0.01 4.07 ±0.005 4.83 ±0.02 3.77 ±0.02

Acidity was detected for all the samples during storage that results are shown in fig.2.

Fig.2: Acidity changes during storage period of different products.

As it can be seen the acidity has increased for all the samples at the day one of storage during fermentation and followed within storage period. Also acidity of UF yoghurts are always more than control (D) that is because of more protein content of these samples and accordingly more buffering capacity that permit more acid production.

Acetaldehyde has been detected and compared as one of the major flavor compounds in yoghurt for all the samples .The results are shown in fig. 3.

Fig. 3. Acetaldehyde content of different samples measured by GC at first and days 1 and 14 of storage

Higher content of protein causes increased amount of acetaldehyde for the UF yoghurts than control. Thus despite the lower lactose content of UF retentates, acetaldehyde production by starter bacteria was not impaired. Acetaldehyde and other carbonyl flavor compounds can be produced by more than one metabolic pathway and from various precursors including lactose, valine, pyruvate, threonin and acetyl phosphate that threonin is the most important way of its production. (Tamime and Robinson, 1999) The acetaldehyde content of yoghurts decreased with storage confirming the findings of C.G. Biliaderis et al. (1992) however at all total solid levels or protein levels the UF yoghurts showed more amounts of acetaldehyde compared with control.

The mean rating scores for sensory attributes of yoghurts with respect to protein content are shown in fig.3.

Fig.3. The mean rating scores for sensory properties of yoghurts

In general sensory analysis revealed differences between samples that UF yoghurts had higher rating scores comparing with control (D). Also B sample (Pr 5.3%, TS 13.5%) was comparable with one of the most popular yoghurts in Iran market. Although C sample had the most amount of protein, TS and acetaldehyde content but because of its so firm texture was not accepted by many panelists. All UF yoghurts were much resistant to syneresis because of the more protein and acid content comparing with control. Also syneresis decreased during storage period.

CONCLUSION

The results of chemical analysis showed that protein increase of the ultrafiltered samples causes more but slow acid production because of the more buffering capacity of these samples. Also more protein content showed more acetaldehyde production that decreases during storage period. Sensory analysis confirms the chemical analysis results and finally concentrated milk to 13.5% solids by UF process (B sample) can produce yoghurt with good acceptability.

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