Effect of Preservative on the Shelf Life of Yoghurt Produced from Soya Beans Milk

Leonardo Electronic Journal of Practices and Technologies
Issue 11, July-December 2007

ISSN 1583-1078
p. 131-142



Effect of Preservative on the Shelf Life of Yoghurt Produced from Soya
Beans Milk

Uduak G. AKPAN1, Abdullahi D. MOHAMMED2 and Ibrahim AMINU3

Department of Chemical Engineering, Federal University of Technology, Minna, Nigeria
ugaekon@yahoo.co.uk, abduldyamoh@yahoo.com, ibrodya@yahoo.com


Abstract
This study concentrated on the effects of preservatives on shelf life of yoghurt
produced from Soya beans milk. The yoghurt was produced by heating Soya
beans milk slurry, cooled and incubated with starter culture. After the required
yoghurt has been formed, sugar, flavour and preservatives were added. Study
of the effect of preservatives revealed that Sodium benzoate preservative used
at 20mg/ml give the best (optimum) preservation on both shelf and
refrigeration storage for 15 and 21 days respectively. This is because the
inhibitive ability of Sodium benzoate at lower temperature is higher than that
of Potassium metabisulphate preservative. The study also revealed that
40mg/ml concentration of the combined preservatives gives the best
(optimum) concentration level for both shelf and refrigeration storage with pH
values of 3.92 and 4.01 respectively after 14 days fermentation. The
preservatives concentration added are within the threshold values specified by
Standard Organization of Nigeria (SON) and National Agency for Food
Administration and Control (NAFDAC).
Keywords
Soya Beans; Yoghurt; Preservatives; Concentration; Fermentation; Culture.





131
http://lejpt.academicdirect.org

---

Effect of Preservative on the Shelf Life of Yoghurt Produced from Soya Beans Milk
Uduak G. AKPAN, Abdullahi D. MOHAMMED, Ibrahim AMINU

Introduction

Yoghurt is a semi-fluid milk product, prepared from fresh whole or skimmed milk,
boiled and concentrated by evaporation. The fermentation is caused by the addition of culture
bacteria. The thickness of the product is the result of the acidification by the Lactic Acid
Bacteria (LAB) [1].

Yoghurt appears as whitish liquid substance but in some cases, flavour is added to
bring about alteration in the colour and taste. The transformation of milk into yoghurt with a
starter culture composed of streptococcus salivarus ssp. Thermophilus and Lactobacillus
delbiueckil ssp) [2]. Bulgarius can easily extend the shelf life of the milk from few days to
weeks. Example of such post fermentation process includes heating, concentration, freezing
and drying, but it is evident that such treatment will alter the characteristics of the end product
[2].

Soya bean (Glycine Maxima), the primary material for Soya milk production has been
identified to be one of the most important legumes of the tropics with high protein content. It
is a potential food material that contains all essential amino acids that are very important for
the proper development of the body, indeed Soya beans, has a higher content of Lysine in
comparison to other plant proteins [3]. Soya bean when processed they give Soya beans milk
which can be converted to yoghurt which is valuable protein supplement or substitute for
adult and infant feeding [4]. Soya milk is lactose-free and can be consumed by the lactose-
intolerant people as a substitute to milk [5].

Preservatives are added to improve yoghurt consistency. These are generally additives,
which prolong the life span of foods and drinks by preventing micro-organisms attack.
Technically, preservatives are chemicals used to poison micro-organisms and to prevent the
food onto which it is added from fermentation and spoilage without causing any harmful
effect to the person who consumed the food. The uses of chemical preservatives enhance food
quality, reduce waste and enhance consumer acceptability [6].

These chemical preservatives are classified into three main types: Anti microbial (such
as Benzoic acid, propionates, Di-methyl Pyrocirbonates), Antioxidant (such as Ascorbic acid,
Butylated hydroxyl anisole), and Antibiotic (such as Oxy tetracycline, Ninsin and Lacto
peroxidase). Other miscellaneous preservatives are Vinegar, Sorbet, Silica gel, Boric acid,
Boras, Formaldehyde, Thiabendazole et cetera [6].
132

---

Leonardo Electronic Journal of Practices and Technologies
Issue 11, July-December 2007

ISSN 1583-1078
p. 131-142


Ascorbic acid and its derivatives are added to milk and yoghurt to inhibit the rancidity
and spoilage rate of the yoghurt. Likewise Ninsin is used in preserving yoghurt, as it prevent
the growth of vegetable bacterial cells and end pores that are responsible for gas formation by
clostridium spp which may cause the yoghurt packages to expand or burst. A culture of
Lactoperioxidase is also used to preserve yoghurt because it damages the inner membrane of
bacteria. Most preservatives are readily available since they can be synthesized in the
laboratories [6].

This work is however aimed at studying the effect of preservatives on yoghurt
produced from Soya beans; and majorly focuses on the following specific objectives:
• Production of the Soya milk from the Soya beans;
• Preparation of the starter culture;
• Conversion of the milk produced to yoghurt;
• Application of the preservatives on the yoghurt;
• Estimation of the best (optimum) concentration level required to preserve the yoghurt.


Methodology

Materials
Soya beans (Glycine Maxima) yellow seeds were purchased at modern Market,
Minna, Niger State, Nigeria; a starter culture (developed from skimmed powdered milk);
distilled water; flavouring agent; sugar and chemical preservatives (Sodium benzoate and
Potassium metabisulphate).

Procedure for propagation of starter culture
0.6 kg skimmed powdered milk was weighed into a measuring cylinder. 3L clean
water at ambient temperature was added to the skimmed milk and thoroughly stirred to give a
homogeneous mixture. The mixture was heated to 90°C and held for 30 minutes at this
temperature, following the inoculation of the milk. With the culture, the milk was then
incubated at 42°C for a period of 12hrs.



133

---

Effect of Preservative on the Shelf Life of Yoghurt Produced from Soya Beans Milk
Uduak G. AKPAN, Abdullahi D. MOHAMMED, Ibrahim AMINU

Procedure for the production of yoghurt
7.0 kg of moth-free and mildew-free Soya beans grain was weighed and soaked in
warm water for a period of 24h. At interval of 8h, the warm water was drained off and
replaced with a new one to reduce the beanic flavour of the Soya beans grain. After soaking
overnight, the beans was de-husked, washed with water and ground into paste. Three liters of
clean water was added and thoroughly mixed to give slurry. The Soya beans milk was
extracted by transferring the slurry to a clean white sieve cloth where filtration by suction
took place. The beans dreg on the filter cloth was dried and used as livestock and poultry feed.
The extracted milk was transferred into a pot and pasteurized or rather heated to 85°C for an
hour and allowed to cool gradually to a temperature of about 42-45°C. The yellowish wad
appeared on the surface was continually packed off using a clean spoon. The cooled
homogenized milk was incubated with the already prepared starter culture. The mixture was
stirred properly and kept to stand at a temperature of about 30-35oC for a period of 24h. At the
expiration of the incubation period, the output was yoghurt. At this juncture, sugar, flavour
and preservatives are seldom added.

Procedure for addition of chemical preservatives
0.01g each of Sodium benzoate and Potassium metabisulphate was dissolved in 3mL
of distilled water and shaken thoroughly inside a beaker 3mL of the solution was then added
to 200mL of the prepared yoghurt. The yoghurt was stored in sample bottle with label on each
bottle for easy identification of each sample.

Determination of pH of the sample
The pH meter was switched on and allowed to warm up for 20min. The pH was
adjusted to neutral value by using distilled water at ambient temperature. The electrodes of the
meter where then cleaned, dried and dipped into the different samples and the reading was
noted. After the pH of the first sample was noted, the electrodes were re-washed with distilled
water before being dipped into the second and subsequent samples until all the samples were
tested.

Determination of viscosity of the sample
The viscosity of water at 27°C and 28°C were noted. The water was poured into a
134

---

Leonardo Electronic Journal of Practices and Technologies
Issue 11, July-December 2007

ISSN 1583-1078
p. 131-142

flow-cup viscometer to the brim, while the discharge outlet was blocked with the index finger.
This was so done to ascertain the time taken for the water to discharge on releasing. The
average time taken for each sample was recorded. By using the method of linear interpolation,
the viscosity of each sample was determined by comparing with standard viscosity of water at
27oC and 28oC respectively.


Results

The results obtained during the investigation on the effect of preservatives at different
concentration of yoghurt produced from Soya beans milk kept on both shelf and refrigeration
storage are tabulated in Table 1 to14.

Table 1. Results of sample preserved with Sodium benzoate at 10mg/ml on shelf storage
Days pH Viscosity
Kg/m2 Counts mg/ml Temp °C
Color
Odor
Taste
1 4.4
1.572
3
31
3 4.4
1.572
3
30
5 4.34
1.624
4
27
7 4.31
1.658
6
29
White Pleasant
Sweet
9 4.31
1.671
7
27
11 4.17
1.683
8
27
13 4.04
1.699
9
29
15 3.98
1.728
10
27
17 3.8
1.749
11
29
Brownish
19 3.67
1.756
12
26
Unpleasant Sour
white
21 3.59
1.774
12
26

Table 2. Results of sample preserved with Sodium benzoate at 10mg/ml on refrigeration
storage at 4oC
Days pH Viscosity
Kg/m2 Counts mg/ml
1 4.4
1.572
3
3 4.4
1.572
3
5 4.43
1.572
3
7 4.37
1.559
3
9 4.37
1.611
4
11 4.3
1.621
5
13 4.23
1.629
6
15 4.14
1.634
6
17 4.01
1.639
7
19 3.84
1.645
8
21 3.79
1.648
10

135

---

Effect of Preservative on the Shelf Life of Yoghurt Produced from Soya Beans Milk
Uduak G. AKPAN, Abdullahi D. MOHAMMED, Ibrahim AMINU

• Sweet taste, pleasant odor and white coloration were maintained by the sample during the
period under consideration.

Table 3. Results of sample preserved with Sodium benzoate at 20mg/ml on shelf storage
Days pH Viscosity
Kg/m2 Counts
mg/ml
Temp
oC
Color Odor
Taste
1 4.4
1.572
3
31
White
3 4.4
1.598
4
30
5 4.34
1.612
5
27
Sweet
7 4.31
1.626
6
29
Pleasant
9 4.31
1.635
6
27
11 4.17
1.647
7
27
13 4.04
1.652
8
29
15 3.98
1.66
10
28
17 3.8
1.678
11
29
Sour
Brownish white
Unpleasant
19 3.67
1.684
11
29
21 3.59
1.67
12
26

Table 4. Results of sample preserved with Sodium benzoate at 20mg/ml on refrigeration
storage at 4oC
Days pH Viscosity
Kg/m2 Counts mg/ml
1 4.4
1.572
3
3 4.4
1.572
3
5 4.37
1.57
4
7 4.36
1.593
4
9 4.33
1.597
5
11 4.38
1.623
6
13 4.31
1.617
7
15 4.26
1.693
8
17 4.2
1.701
9
19 4.13
1.743
10
21 4.07
1.764
10
• Sweet taste, pleasant odor and white coloration were maintained by the sample during the
period under consideration.

Table 5. Results of sample preserved with Sodium benzoate at 40mg/ml on shelf storage
Days pH Viscosity
Kg/m2 Counts mg/ml Temp oC
Color Odor
Taste
1 4.4
1.572
3
31
3 4.4
1.596
4
30
5 4.37
1.603
5
27
7 4.33
1.639
6
29
White Pleasant
Sweet
9 4.24
1.647
6
27
11 4.2
1.661
7
27
13 4.07
1.678
9
29
15 3.98
1.729
11
28
17 3.91
1.743
11
29 Brownish white Unpleasant Sour
19 3.84
1.787
12
29
21 3.65
1.853
12
26
136

---

Leonardo Electronic Journal of Practices and Technologies
Issue 11, July-December 2007

ISSN 1583-1078
p. 131-142

Table 6. Result of sample preserved with Sodium benzoate at 40mg/ml on refrigeration
storage at 4oC
Days pH Viscosity
Kg/m2 Counts mg/ml
1 4.4
1.572
3
3 4.4
1.572
3
5 4.37
1.59
3
7 4.35
1.596
3
9 4.31
1.601
4
11 4.29
1.619
5
13 4.1
1.624
5
15 4.01
1.629
6
17 3.97
1.638
7
19 3.98
1.643
7
21 3.83
1.657
8
• Sweet taste, pleasant odor and white coloration were maintained by the sample during the
period under consideration.

Table 7. Results of sample preserved with Potassium metabisulphate at 20mg/ml on shelf
storage
Days pH Viscosity
Kg/m2 Counts mg/ml Temp oC
Color Odor
Taste
1 4.4
1.572
3
31
White Pleasant
Sweet

3 4.35
1.572
4
30
5 4.31
1.642
5
29
7 4.27
1.629
6
27
9 4.19
1.641
7
27
11 4.13
1.678
8
29
13 4.09
1.689
10
27
Sour
15 3.97
1.698
11
29 Brownish white Unpleasant
17 3.89
1.702
12
26
19 3.79
1.719
13
29
21 3.65
1.732
13
27

Table 8. Results of sample preserved with Potassium metabisulphate at 20mg/ml on
refrigeration storage at 4oC
Days pH Viscosity
Kg/m2 Counts mg/ml
1 4.4
1.572
3
3 4.4
1.572
3
5 4.37
1.589
3
7 4.29
1.59
4
9 4.22
1.612
5
11 4.17
1.642
6
13 4.11
1.661
6
15 4.08
1.715
7
17 4.03
1.729
8
19 4.0
1.763
9
21 3.93
1.781
9

137

---

Effect of Preservative on the Shelf Life of Yoghurt Produced from Soya Beans Milk
Uduak G. AKPAN, Abdullahi D. MOHAMMED, Ibrahim AMINU

• Sweet taste, pleasant odor and white coloration were maintained by the sample for 19
days, but brownish white color, unpleasant odor and sour taste were dictated on the 21st
day.

Table 9. Results of sample preserved with Potassium metabisulphate at 40mg/ml on shelf
storage
Days pH Viscosity
Kg/m2 Counts mg/ml Temp °C
Color
Odor
Taste
1 4.4
1.572
3
31
3 4.37
1.572
4
30
5 4.34
1.61
5
29
White Pleasant
Sweet
7 4.28
1.642
6
27
9 4.2
1.661
7
27
11 4.13
1.663
8
29
13 3.96
1.697
10
27
15 3.92
1.718
11
2
17 3.86
1.772
12
26 Brownish white Unpleasant Sour
19 3.79
1.821
12
29
21 3.64
1.867
13
27

Table 10. Results of sample preserved with Potassium metabisulphate at 40mg/ml on
refrigeration storage
Days pH Viscosity
Kg/m2 Counts mg/ml Temp °C
Color
Odor
Taste
1 4.4
1.572
3
4
3 4.4
1.572
3
,,
5 4.37
1.572
4
,,
White Pleasant
Sweet
7 4.34
1.593
4
,,
9 4.33
1.607
5
,,
11 4.26
1.635
5
,,
13 4.13
1.647
6
,,
15 4.01
1.657
7
,,
17 3.99
1.687
8
,,
Brownish white Unpleasant Sour
19 3.8
1.63
9
,,
21 3.74
1.736
9
,,

Table 11. Sample preserved with combined preservatives at 10mg/ml on shelf storage
Days pH Viscosity
Kg/m2 Counts mg/ml Temp °C
Color
Odor
Taste
1 4.4
1.572
3
29
3 4.4
1.572
3
30
5 4.33
1.598
4
29
7 4.03
1.612
5
29
White Pleasant
Sweet
9 4.39
1.628
7
28
11 4.01
1.639
8
28
13 4.27
1.648
10
28
15 4.17
1.66
11
29
17 3.83
1.678
12
29 Brownish white Unpleasant Sour
19 3.78
1.684
13
29
21 3.69
1.692
14
29
138

---

Leonardo Electronic Journal of Practices and Technologies
Issue 11, July-December 2007

ISSN 1583-1078
p. 131-142

Table 12. Sample preserved with combined preservatives at 10mg/ml on Refrigeration storage
at 4oC
Days pH Viscosity
Kg/m2 Counts mg/ml
1 4.4
1.572
3
3 4.4
1.572
3
5 4.4
1.572
3
7 4.37
1.585
4
9 4.3
1.592
5
11 4.29
1.598
5
13 4.18
1.603
6
15 4.14
1.608
7
17 4.11
1.614
9
19 4.06
1.637
10
21 3.91
1.656
11
• Sweet taste, pleasant odor and white coloration were maintained by the sample for 19
days, but brownish white color, unpleasant odor and sour taste were dictated on the 21st
day.

Table 13. Sample preserved with combined preservatives at 40mg/ml on shelf storage
Days pH Viscosity
Kg/m2 Counts mg/ml Temp °C
Color
Odor
Taste
1 4.4
1.572
3
31
3 4.4
1.572
3
30
5 4.37
1.643
5
29
7 4.32
1.664
6
27
White Pleasant
Sweet
9 4.17
1.678
7
27
11 4.09
1.685
8
29
13 3.97
1.709
9
27
15 3.92
1.721
10
29
17 3.87
1.756
11
26
19 3.83
1.781
11
27 Brownish white Unpleasant Sour
21 3.71
1.821
12
29

Table 14. Sample preserved with combined preservatives at 40 mg/ml on Refrigeration
storage at 4oC
Days pH Viscosity
Kg/m2 Counts mg/ml
1 4.4
1.572
3
3 4.4
1.572
3
5 4.37
1.598
3
7 4.32
1.602
4
9 4.3
1.611
4
11 4.29
1.618
5
13 4.21
1.623
6
15 4.16
1.627
7
17 4.11
1.631
7
19 4.06
1.635
8
21 4.01
1.643
9

139

---

Effect of Preservative on the Shelf Life of Yoghurt Produced from Soya Beans Milk
Uduak G. AKPAN, Abdullahi D. MOHAMMED, Ibrahim AMINU

• Sweet taste, pleasant odor and white coloration were maintained by the sample during the
period under consideration.


Discussion

It was observed that all the samples kept on refrigeration storage with preservatives at
different concentrations maintained their color; odor and taste within the first seventeen days,
and thereafter, some samples immediately changed their qualities. These may be as a result of
rapid growth of bacteria as room temperature changes.

Parameters results obtained on the refrigeration storage of the sample preserved with
10,20,30,40 and 50mg/mL concentration of Sodium benzoate, appear almost similar with pH
value ranging from 3.97-4.4, viscosity from 1.572-1.764Kg/m2 and microbial count between 3
and 10 respectively. Perhaps, the sample color, odor and taste maintained their quality within
the preservative period of twenty one days.

The samples preserved with Sodium benzoate at different concentration and kept on
shelf storage maintained their quality within the first-fifteen days. After which a sour taste
was observed. This, of course, is as a result of acid formation as the room temperature
increases to 29oC. In addition, the microbial count in the sample increased, indicating further
deterioration of the yoghurt quality.

On refrigeration storage, the samples preserved with Potassium metabisulphate at
different concentrations, maintained their properties within the first-nineteen days with
decrease in the pH values. But at 20mg/mL the yoghourt maintained its properties for 21
days on refrigeration storage. This shows that other samples become more acidic as a result of
fermentation which subsequently results into clotting or curdling (coagulation) of the samples.
On shelf storage, the sample preserved with Potassium metabisulphate maintained their
quality within fifteen days. Only sample preserved with 40mg/ml concentration of the
preservative was noticed to change quality immediately after 13 days of storage, as the
microbial count activity increases from 3-13mg/ml and the pH value decreases from 4.4-3.64
respectively.

The results obtained on refrigeration storage of the samples preserved with a mixture
of Sodium benzoate and Potassium metabisulphate preservatives, revealed a slight change in
the pH values from 4.4 to 4.08 within the same time. Only sample preserved with 40mg/mL
140

---