Name of experiment : Milk and Milk products, Yogurt Analysis
Purpose:
The purpose of this experiment was to analyze microbiological properties of yogurt and to investigate the number of lactic acid bacteria and formation of mold and yeast.
Theory:
The effect of yogurt as a dietary supplement was investigated with regard to the gut ecosystem and lipid metabolism of 12 healthy, elderly people (78.3 +/- 9.8 years, body mass index 23.6 +/- 5.3 kg m -2, and mean +/- SD). Commercial yogurt with homogenized fruit was prepared by fermenting milk with yogurt specific cultures Lactobacillus delbrueckii ssp. bulgaricus (strain AY/CSL) and Streptococcus thermophilus (strain 9Y/CSL). The subjects consumed their usual diet (equal to 6279-6698 kJ d -1) over a 2-week baseline period (baseline start to end) and then were supplemented for 4 weeks with 250 g d -1 of fruit yogurt. The yogurt was administered in 125 g portions twice per day: at breakfast in substitution of milk and in the afternoon in substitution of tea with milk (test). At the end of the 4-week period the volunteers returned to their usual diet for a further 4 weeks (follow-up). At the end of each trial period no changes were observed in faecal water content, pH, bile acid concentration or cytolytic activity of the faecal water. Throughout the study there was significant variation neither in dietary intake of macro- and micronutrients, nor in the plasma lipids and, during the experimental period, in the counts of the total anaerobic microorganisms, bifidobacteria, lactobacilli, coliforms or enterococci. The only significant difference was observed in the clostridia counts, which decreased (P < 0.05) after the consumption of yogurt. Moreover, this effect was still evident at the end of the follow-up period. Since this last result can be considered a positive modification of the colon ecosystem, as clostridia are involved in the production of putrefactive compounds, it is possible that a yogurt-supplemented diet can maintain and/or improve the intestinal microbiota of elderly subjects.
Procedure:
a) Total Count:
Firstly; from yogurt dilutions were prepared from 10-1 to 10-6 dilutions. For this, 25 g yogurt sample was weighed and added above 225 ml peptone water thus 10-1 yogurt dilution was prepared then; from 10-1 dilution 1ml was taken and it was added to 9 ml water thus 10-2 dilution was prepared. In the same manner, 10-3, 10-4, 10-5 and 10-6 dilutions were prepared. Finally; from each tube 0, 2 ml was taken with pipette and sample dilutions were put on the plate count agar (PCA) and then spread out with spread plate method and incubated at 30 oC for 1 to 3 days. After incubation number of microorganisms in gram was calculated in yogurt.
b) Lactic Acid Bacteria:
At previous experiment prepared dilutions were used for this experiment. Again sample dilutions were taken 0, 2 ml and put on the mean rogosa sharp agar (MRS). Then; plates were incubated at 35 oC for 24 to 48 hours. Finally; number of lactic acid bacteria was calculated in gram.
c) Mold and Yeast Count:
Again, at first experiment, preparaed dilutions were used for this analysis. In here; 0, 2 ml sample dilution was taken and was put on potato dextrose agar (PDA) and spread out with spread plate method. Then PDA was incubated at 25 oC for 2 to 5 days. Finally; number of microorganisms was calculated in gram.
MATERIALS:
Yogurt
Plate count agar
Mean rogosa sharp agar
Potato dextrose agar
Peptone water pipette
Spreader
Bunsen burner
Etuv
Test tube rack
Alcohol
RESULT and CALCULATIONS:
PCA
Total count
10-1
10-2
10-3
10-4
10-5
10-6
Open yogurt
25
5
1
0
2
0
Pasteurized yogurt
46
13
11
4
3
1
Open yogurt
19
4
1
0
4
7
PDA
Mold & Yeast
10-1
10-2
10-3
10-4
10-5
10-6
Open yogurt
TNTC
TNTC
TNTC
198
21
6
Pasteurized yogurt
1
0
0
0
0
0
Open yogurt
TNTC
TNTC
TNTC
189
16
8
MRS
LAB
10-1
10-2
10-3
10-4
10-5
10-6
Open yogurt
TNTC
TNTC
TNTC
TNTC
2
2
Pasteurized yogurt
TNTC
TNTC
TNTC
152
4
1
Open yogurt
TNTC
TNTC
TNTC
207
4
8
For PCA
Number of microorganisms = (46 * 101) / 0, 2 ml = 2300
For PDA
Number of microorganisms = (198 * 104) / 0, 2 ml = 9, 9*106
Number of microorganisms = (189 * 104) / 0, 2 ml = 9, 45*106
Average number of m / o’s = (9, 9*106 + 9, 45*106) / 2 = 9, 675106
For MRS
Number of microorganisms = (152 * 104) / 0, 2 ml = 7, 6*106 (pasteurized)
Number of microorganisms = (207 * 104) / 0, 2 ml = 1, 035*107 (raw)
Discussion:
In this experiment, we examined properties of microorganisms and calculated the number of microorganisms in yogurt. For this, we made three analyses. For the number of microorganisms on plate count agar, for lactic acid bacteria on mean rogosa sharp agar and for mold and yeast on potato dextrose agar were used for yogurt microbiologic analyses. In here; MRS was used to determine total number of lactic acid bacteria. PDA was used to determine the number of mold and yeast. When we diluted the yogurt sample, we used the peptone water has a great protective effect. For this, 1g peptone was dissolved in 1L of distilled water and pH was adjusted to 7, 0 and it dispenses in sufficient quantity to allow for loss during sterilization.
The end of experiment, at pasteurized yogurt 2300 microorganisms was calculated on PCA. Actually; in open yogurt more microorganisms should have been observed. On the other hand; also in here, these microorganisms were useful microorganisms like Lactobacillus bulgaricus and Streptococcus thermophilus.
On MRS, lactic acid bacteria were calculated as 7, 6*106 for pasteurized yogurt and 1, 035*107 for open yogurt.
