Effect of Hydrocolloids and Drying Conditions on Stability, Rheological and Sensory Properties of Re

www.fmfi‐journal.org Focusing on Modern Food Industry (FMFI), Volume 5 2016 doi: 10.14355/fmfi.2016.05.002

Effect of Hydrocolloids and Drying Conditions on Stability, Rheological and Sensory Properties of Reconstituted Yoghurt (Instant Yoghurt) Marjan Esmailzadeh Nasiri 1, Soleiman Abbasi2* Department of Agriculture, Payam Noor University, P O BOX 19395‐3697 Tehran‐Iran

1

Food Colloids and Rheology Lab., Department of Food Science & Technology, Faculty of Agriculture, Tarbiat Modares University, P. O. Box 14115‐336, Tehran‐Iran 2

Tel No.0098‐21‐48292321 Fax No. 0098‐21‐48292200 *To whom correspondence should be addressed: sabbasifood@modares.ac.ir Short running title: Effect of gums and drying on instant yoghurt

Abstract In the present study, fresh skimmed yoghurt (total solids 10, 20, 28 %wt) was dried using microwave–vacuum drier (35, 135, and 260 Watt, absolute pressure 125 mbar). Then, the effect of this process on the survival of starters as well as yogurt powder color was investigated. Moreover, various concentrations of different hydrocolloids (tragacanth, guar, Persian, locust bean), as well as their mixtures were added, either to fresh yogurt or to yoghurt powder, in order to compare their influences on rheological and sensory properties of the reconstituted yogurt. Our findings showed that microwave power and total solids negatively affected color indices and number of survivals. In addition, adding the hydrocolloids to fresh yogurt produced more desirable sensory and rheological properties in comparison to yogurt powder. Furthermore, G’ and G”, complex viscosity and loss tangent of majority of hydrocolloid‐containing reconstituted yogurts (fresh yogurt method) were similar to the fresh one, indicating dominant elastic behavior.

Keywords Microwave; Drying; Instant yogurt; Hydrocolloids; Rheology; Rehydration

Introduction Yogurt is the product of the activity of two bacteria, namely Streptococcus thermophilus and Lactobacillus bulgaricus.

During fermentation, Streptococcus thermophilus first grows rapidly, consumes all amino acids produced by Lactobacillus bulgaricus, and generates lactic acid. The generated lactic acid and formic acid will, in turn, stimulate the growth of Lactobacillus bulgaricus as lactic acid regulates pH to an extent which is desirable for the growth of Streptococcus thermophiles (Tamime & Robinson, 2007). After production, yogurt is kept at 2–7°C throughout its supply chain. These conditions not only prevent yogurt from being spoiled by yeasts and molds, but also restrain the further activities of starter bacteria. Using the cold chain will nonetheless add to the final retail costs. Moreover, the durability of yogurt in its natural form is short normally 1 to 5 days under ambient (25–30°C) and 3 to 4 weeks under refrigeration (4°C) conditions. This is the major setback for commercialization of this product (Kumar and Mishra, 2004; Ghaderi et al., 2010).

Therefore, over the past decades, a number of attempts had been made for extending its shelf‐life via various dehydration techniques (Ghaderi et al., 2010; Kumar and Mishra, 2004; Kim et al., 1997; Kim and Bhowmik, 1995;

6

Focusing on Modern Food Industry (FMFI), Volume 5 2016 www.fmfi‐journal.org

Sharma & Arora, 1995; Kim and Bhowmik, 1994; Sharma et al., 1992; Cajigas, 1992; 1981). The primary objective of yogurt powder production was always to store yogurt sustainably and enabling its quick use. However, it was soon found that the reconstituted yogurt failed to have high amounts of live starter culture, appealing taste, hard texture and firmness, and desirable appearance, compared with regular yogurts (Tamime & Robinson, 2007). That is why nowadays majority of yoghurt powder is used as an ingredient in foodstuffs not as an instant yogurt on itself. Moreover, in recent years, microwave–vacuum drying has been introduced as a potential method for producing high quality dried foods. This hybrid method enjoys the advantages of both drying methods (vacuum and microwave drying), and it can improve energy efficiency and product quality (Ghaderi et al., 2010; Abbasi and Azari, 2009; Kim and Bhowmik, 1995; 1994). Moreover, microwave–vacuum drying (MVD) has been suggested as an inexpensive alternative to freeze drying (FD) for producing high quality, heat‐labile products in reasonably shorter time.

Therefore, the present study attempted to examine the effect of microwave power in a MVD system and the total solids of yogurt on the color of powders and survival of starter bacteria. Moreover, the effect of different concentrations of added hydrocolloids, before and after preparing yogurt powder by MVD, on stability, rheological and sensory properties of reconstituted yogurt was investigated.

Materials and Methods Materials Skimmed milk (fat content < 0.5 wt%, Varna Co., Varamin, Iran), and baking powder (Baharan Tehran Co., Iran) were purchased from local producers. Microbial culture medium (MRS agar and M17 agar) and other chemicals were purchased from Merck Chemicals Co. (Darmstadt, Germany). The guar gum (GG, Pars Gum, Shiraz, Iran), gum tragacanth (GT) Astragalus gummifer (Herbal store, Tehran, Iran), locust bean gum (LBG, CP Kelco, Denmark), and Persian gum (PG) Amygdalus scoparia (collected from Arasbaran Forests, Azarbayjan, Iran) were used after milling and passing through a laboratory sieve (mesh size ‐# 60).

Preparing Yogurts with Different Total Solids To prepare fat‐free yogurt, fresh skimmed milk was heated (30 min, 95°C), cooled down (42–45°C), starters (Christine Hansen Co., Denmark) were added, and incubated for 4 hours (Ghaderi et al., 2010; Walstra et al., 2006).

After incubation, they were transferred to refrigerator (5°C) and were kept for 24 h. Afterwards, in order to produce yogurt samples with different levels of total solids (TS = 20 or 28 %wt), the prepared samples (TS=10 %wt) were poured in fabric bags with fine porosity, and then they were kept in the fridge (10–15 h) to reach the desired total solids. Before drying, baking powder (1.5 %wt) was added, and the mixture was mixed well with an electric mixer (SANYO, Japan) for 1 min. The resulted yogurt was placed in a microwave–vacuum drier.

Microwave–Vacuum Drier (MVD) The microwave–vacuum drier which had been designed and developed at our Food Colloids and Rheology Laboratory (Abbasi & Azari, 2009; Ghaderi et al., 2010) was used in this study. The microwave was generated by a domestic microwave oven (AEG Micro Mat 725, 2450 MHz, 1200W, Germany). The required absolute pressure (125±5 mbar) was generated by a vacuum pump (Kavake Airvac, model JP‐120H, Taiwan).

Drying with MVD The yogurts (TS = 10, 20 or 28 %wt) containing baking powder (zero or 1.5 wt%) were placed (thickness=2.2 mm) on a polytetrafluoroethylene (PTFE) plate (Ghaderi et al., 2010) and three microwave power levels (35, 130 and 260 W) were used for drying. After a few minutes (when there was no water drops on the inner wall of the desiccator), the microwave oven was turned off at every 3 min under vacuum conditions to prevent the samples from overheating (Kim & Bhowmik, 1995). To determine the end point of drying process, the moisture content was calculated using the mass balance law. Once the yogurt samples were at the proper moisture content (6–6.5 %wt), the dried samples were removed (scraped with a spatula from the PTFE plate) from drying chamber. The dried

7

www.fmfi‐journal.org Focusing on Modern Food Industry (FMFI), Volume 5 2016

yogurt flakes were then milled to produce yogurt powder. Finally, the powders which passed through sieve nest (‐ # 80 and +# 230) were used in the next stages (Ghaderi et al., 2010).

Adding Thickening Agents and Preparing Reconstituted Yogurt After Drying method (AD): In this method, different hydrocolloids (GT, GG, PG, and LBG) and casein powder were added to the yogurt powders (0.25, 0.5, 0.75, and 1 %wt). Moreover, in order to study their combined effect, the GT:GG, GT:LBG and GG:LBG at a various ratios (80:20, 50:50, 20:80) and constant concentration (0.6 %wt) were added. Then, the mixtures were dispersed in lukewarm water (50 °C) at a powder: water ratio of 1:5 (Kumar & Mishra, 2004). After 10–15 min, the texture of reconstituted yogurts was checked and the most appropriate ones in terms of appearance, and texture were identified.

Before Drying method (BD): In this method, the hydrocolloids were added gradually to fresh yogurts. After 30 min, the samples were transferred into the MVD. After drying, dried flakes were pulverized and sieved and treated similar to the AD method.

Number of Survival Starters in Fresh and Reconstituted Yogurts The number of survived Lactobacillus bulgaricus and Streptococcus thermophilus was counted using MRS agar and M17 agar (anaerobic pour‐plate technique), at 37 °C for 72 and 48 h, respectively (Kim and Bhowmik, 1997).

Furthermore, the effect of rehydration (3–7 hours, 37 °C) on the number of survivals was examined by mixing 2.25 g of distilled water with 0.25 g of yogurt powder. Fresh yogurt (TS=10 %wt) was also used as a blank.

Measuring Color Indices of Powders A colorimeter (Hunter Lab Colorflex, Rexton, VA, USA) was used for comparing the color of powders. Apart from a*, b*, and L*, the Browning Index (BI) and total discoloration (ΔE) were also calculated. The BI indicates the purity of the brown color and is considered as an important factor in determining the enzymatic and non‐enzymatic browning processes (Abbasi and Azari, 2009).

  ( L0  L)2  (a0  a)2  (b0  b)2 (1)

BI  100( X  0.31) / 0.172 (2) Reconstitution of Yogurt Powder For reconstitution purposes, yogurt powders dried by AD method (sub‐section 2.5) were first mixed with the required amounts of gums in singular or combined forms, then the final mixtures were dispersed in lukewarm water (50 ˚C) in a ratio of 1:5 (powder : water). In case of ones prepared by BD method (sub‐section 2.5), powders were directly dispersed in lukewarm water (50 ˚C) at the abovementioned ratio (Kumar & Mishra, 2004). After cooling down (5 ˚C), they were used for sensory or rheological measurements.

Sensory Evaluation Sensory properties (taste, mouthfeel, appearance, uniformity, and overall acceptance) of reconstituted yogurts were evaluated using five‐level hedonic test (1, 2, 3, 4, and 5, corresponding to unacceptable, somewhat acceptable, good, very good, excellent, respectively) by 11 panelists aged between 25 and 43. The reconstituted yogurts were randomly coded and refrigerated (2 h, 5 °C) before evaluation (Abbasi & Mohammadi, 2013).

Rheological Properties To perform this test, fresh and reconstituted yogurts were placed between parallel plates geometry (d = 25 mm) using a rheometer (MCR300 Model, Anton Paar, Austria), and the changes in the storage modulus (Gʹ) and loss modulus (Gʹʹ) were measured as a function of frequency (Abbasi & Mohammadi, 2013) within 0.01 to 100 Hz at 7˚C. All experiments were performed at the linear viscoelastic region (strain = 0.1).

8

Focusing on Modern Food Industry (FMFI), Volume 5 2016 www.fmfi‐journal.org

Statistical Analysis All experiments and measurements were conducted at least 3 times unless stated. Results were analyzed in SPSS 16 by a completely randomized design, and means were compared using the Duncan test at the 95% level. Diagrams were also drawn in Excel.

Results and Discussion Effect of Total Solids and Microwave Power on Drying Time During different microwave–vacuum drying stages (Fig. 1), it was revealed that by increasing the microwave power (MP), the required drying time decreased. In addition, at higher total solids (TS) content, the drying time became longer. Moreover, the effect of their interaction on drying time was significant (p>0.05).

A major problem faced in this study was the insolubility of the produced powders likely due to intensive denaturation of proteins, occurrence of Maillard reaction, as well as the compactness of structure. Therefore, in order to increase porosity and solubility of yogurt powder, effect of baking powder, as a leavening agent, was examined.

250

Drying Time (min)

200

150

100

50

0 10

20

28

TS (%wt.)

FIG. 1: EFFECT OF TOTAL SOLIDS (10, 20, 28 %WT) AND MICROWAVE POWER (MP = 35 W □, 135 W ■, AND 260 W ■) ON YOGURT DRYING TIME USING MVD UNDER CONSTANT PRESSURE (AP = 125 MBAR)

Effect of Total Solids and Microwave Power on Yogurt Powder Color Our findings (Fig. 2) revealed that powders prepared at higher MP in MVD had darker color, in the way that all powders produced under powers higher than 260 W were completely burnt and were eliminated from the rest of the experiment procedure. This result confirmed the findings of previous studies (Ghaderi et al., 2010). It was also shown that under a fixed microwave power, increasing the dry matter of yogurt led to a darker color. According to colorimetric assays, (Fig. 2), higher yogurt concentrations (larger TS) adversely affected powder color, and the differences in both indices were statistically significant (P>0.05). Moreover, the presence of baking powder led to brighter colors. It seems that the occurrence of Maillard reaction was likely the main reason for increasing the color indices where at higher microwave powers, high temperatures accelerated this reaction. It is reported that the browning level rises 2 to 3 times per every 10°C increase (deMan, 2008). The relation between Maillard reaction and dry matter content is likely due to a raise in the amount of sugars and proteins in unit area, and simultaneously their closeness and availability. However, MP had a greater effect than TS content. Adding baking powder had a positive effect on the color of powder, more likely because it created highly porous structure as well as higher pH which, in turn, contribute in the prevention of Maillard reaction.

9

www.fmfi‐journal.org Focusing on Modern Food Industry (FMFI), Volume 5 2016

30

30

25

25

15

c b

5

15

a

10

b

20

c

a

∆E

BI

20

10 5 0

0 10

20

28

TS (% wt.)

FIG. 2: EFFECT OF TOTAL SOLIDS ON BI (♦, WITH BAKING POWDER; ◊, WITHOUT) AND ΔE (■, WITH BAKING POWDER; □, WITHOUT) INDICES OF YOGURT POWDER DRIED USING MVD (MP = 35 W, AP = 125 MBAR)

Effect of Hydrocolloids Based on our observations, the reconstituted yogurt had lower viscosity and consistency than the fresh one. It was reported that the weak rheological properties of reconstituted yogurt is likely due to the destruction of its gel‐like structure during drying (Kumar & Mishra., 2004). Therefore, after preparing the yogurt powder, different hydrocolloids at different concentrations (0.25, 0.5, 0.75 and 1 %wt) were used to improve reconstituted yogurt texture (AD method). Therefore, at first attempts, GT, GG, LBG, PG and also casein powder were added. As a result, the highest viscosity was achieved for GT, LBG, GG, and PG at 0.50, 1, 0.75 and 1 %wt, respectively. It needs to be noticed that two latter ones were still very fluid. Furthermore, casein powder (1 %wt) showed no satisfactory effect on viscosity. Therefore, in the next stage, we examined the capability of combined systems at various ratios.

As a result, adding 0.6 %wt of GT:GG, GG:LBG, and GT:LBG at distinct ratio (80:20) led to a desirable texture. It has already reported that 0.02 % LBG can significantly improve yogurt structure as well as hysteresis prevention (Ünal et al., 2003) which is significantly less than what we have reported, most likely due to destruction of original gel‐like structure of yogurt during drying process.

In order to reveal the effect of hydrocolloids addition stage (BD or AD methods), in this stage, the abovementioned ratios and concentrations of hydrocolloids were added to the fresh yogurt (BD method). As can be seen (Figure 3), the presence of hydrocolloids increased the drying time. This was likely due to the hydrophilic characteristic and their higher water holding capacity (WHC). Despite their longer drying period, they contained slightly higher moisture (6.5 %wt) in comparison to one without added hydrocolloids (5.0–5.5 %wt).

Drying Time (min)

180 150 120 90 60 30 0 Control

GT:GG GT:LBG GG:LBG

GT

FIGURE 3: EFFECT OF DIFFERENT HYDROCOLLOIDS (0.6 %WT) ON DRYING TIME OF SKIMMED YOGURT (TS = 10 %WT), DRIED BY MVD (MP = 35 W, AP = 125 MBAR). THE RATIO OF MIXED HYDROCOLLOIDS WAS 80:20

Effect of Hydrocolloids on Rheological Properties Figure 4 presents storage (Gʹ) and loss (Gʹʹ) moduli as functions of angular frequency where the viscoelastic

10

Focusing on Modern Food Industry (FMFI), Volume 5 2016 www.fmfi‐journal.org

behavior or solid‐like properties was dominant (Gʹ>Gʹʹ) over the wide range of frequency (except one containing GG:LBG). Moreover, there was a direct relation between frequency and Gʹ, Gʹʹ possibly because at higher frequencies, the inter‐chain bonds had reasonably shorter time for recovery and relaxation.

As can be seen (Figure 4B), by increasing the frequency, Gʹʹ increased much faster than Gʹ and led to a liquid‐like property. This behavior was expected as both of these gums were non‐gel and non‐adsorbing and consecutively were unable to interact with colloidal systems namely caseins. Therefore, the effective mechanism for stabilizing was viscosity increase. This has been also reported in several studies (Abbasi & Mohammadi, 2013; Azarikia & Abbasi, 2010; Everett & McLeod, 2005).

As shown in Figure 5, the highest and lowest complex viscosity belonged to fresh yogurt and one containing GG: LBG, respectively. Moreover, very similar to fluids, all samples had a quasi‐plastic behavior, and their viscosity decreased as the frequency increased. It can also be seen that the highest and lowest loss tangent values belonged to one containing GG:LBG and fresh yogurt, respectively. In terms of phase angle, all samples had phase angles between 20–30 degrees, indicating dominant elastic and solid properties, except fresh yogurt and one containing GG:LBG with the lowest and highest values, respectively. Moreover, in all cases, except one with tragacanth, the phase angle decreased when the frequency increased. This behavior shows that the samples had a more elastic behavior at higher frequencies. It is interesting that one containing tragacanth, showed very different behavior at various levels of frequency more likely due to its composition which consists of soluble and insoluble fractions (Abbasi & Mohammadi, 2013; Azarikia & Abbasi, 2010).

FIGURE 4: EFFECT OF DIFFERENT HYDROCOLLOIDS (BD METHOD) ON STORAGE (♦) AND LOSS (×) MODULUS (STRAIN= 0.1, TEMP 7 °C) OF RECONSTITUTED YOGHURTS (TS = 16.6 %WT): A) GT:GG + BP, B) GG:LBG + BP, C) GT, D) GT:LBG + BP, E) GT + BP, F) FY (TS = 10 %WT). BP= BAKING POWDER (1.5 %WT.), FY=FRESH YOGURT. THE RATIO AND CONCENTRATION OF ADDED HYDROCOLLOIDS WERE 80:20 AND 0.60 %WT., RESPECTIVELY

11

Complex Viscosity (Pa.s)

www.fmfi‐journal.org Focusing on Modern Food Industry (FMFI), Volume 5 2016

10000

1000

100

10

Loss tan

1 10

1

Phase Angle (degree)

0.1 90

60

30

0 0.01

0.1

1 Frequency (Hz)

10

100

FIGURE 5: EFFECT OF DIFFERENT HYDROCOLLOIDS (BD METHOD) ON COMPLEX VISCOSITY, LOSS TANGENT AND PHASE ANGLE (STRAIN= 0.1, TEMP 7 °C) OF RECONSTITUTED YOGHURTS (TS = 16.6 %WT):♦ GT:GG + BP, ▲GG:LBG + BP, ■ GT, ∆ GT:LBG + BP, □ GT + BP, ● FY. BP= BAKING POWDER (1.5 %WT.), FY=FRESH YOGURT (TS =10 %WT). THE RATIO AND CONCENTRATION OF ADDED HYDROCOLLOIDS WERE 80:20 AND 0.60 WT. %, RESPECTIVELY

Effect of Hydrocolloids on Sensory Properties Our findings regarding sensory properties showed (Table 1) a significant difference (P>0.05) between two methods of adding hydrocolloids to fresh yogurt (BD method) or yogurt powder (AD method) where the scores in almost all aspects for BD method were much better than AD. Moreover, the influence of baking powder on these parameters was noticeable as in case of first method, in the absence of baking powder the scores declined significantly in comparison to their identical which contained baking powder. All in all, the sample which contained GT was very similar to control (fresh yogurt) and even much better. The possible reason for these differences can be attributed to the occurrence of interactions between added hydrocolloids and proteins (i.e. whey proteins, caseins, and their complexes) particularly in the presence of baking powder which induces slightly higher pH (pH=4.60) in comparison to fresh yogurt (pH=4.00). Moreover, the drying temperature can also improve these

12

Focusing on Modern Food Industry (FMFI), Volume 5 2016 www.fmfi‐journal.org

interactions or even some chemical reactions. As a result, the hydrophilicity and water absorption capacity of the yogurt powders were improved and these are the possible reasons for which they behaved much better than their matching part which is produced by AD method but with exactly same formulation. Moreover, it is reported that the survival of acetaldehydes is increased after adding stabilizers (Kumar & Mishra., 2004).

TABLE1: EFFECT OF MIXING STAGE (BD OR AD METHOD), TYPE OF HYDROCOLLOIDS, AND PRESENCE OR ABSENCE OF BAKING POWDER ON SENSORY PROPERTIES OF RECONSTITUTED YOGHURT (TS = 16.6 %WT)

Mixing stage of Hydrocolloids

Formulation of reconstituted yogurt

Taste

Uniformity

Appearance

Mouthfeel

Overall acceptance

GT:GG + BP+ FY

2.91b

3.36b

2.82ab

3.00bc

2.91bcd

GG:LBG +BP+FY

3.36b

3.82b

3.27abc

3.36bcd

3.18cde

GT+FY

3.00b

3.45b

3.36abc

3.45bcd

3.18cde

GT:LBG +BP+FY

3.36b

3.91b

3.82bc

3.91cd

3.82de

GT+BP+FY

3.91b

4.27b

4.09c

4.09cd

4.00e

GT+BP+YP

2.00a

2.09a

2.64ab

1.91a

1.91a

GT:LBG +BP+YP

1.91a

1.91a

2.36a

1.82a

1.82a

GT:GG +BP+YP

1.91a

2.09a

2.73ab

1.82a

2.09ab

BD method

AD method

Control

GG:LBG + BP+YP

1.73a

2.18a

2.64abc

1.64a

1.91a

GT+YP

3.00a

1.91a

2.64ab

2.64ab

2.45abc

Fresh yogurt

3.91b

3.55b

3.27abc

3.82cd

3.64de

BP= Baking Powder (1.5 %wt.), GG= Guar Gum, GT= Gum Tragacanth, LBG=Locust Bean Gum, FY=Fresh Yogurt, YP=Yogurt Powder.

Different small letters in each column represent significant difference (P>0.05).

The ratio and concentration of added hydrocolloids were 80:20 and 0.60 %wt., respectively.

Effects of BP, Microwave Power and Hydrocolloids on Survival of Lactic Acid Bacteria The viability of lactic‐acid‐ bacteria (LAB or yogurt starters) in yogurt powder is an important qualitative factor and a proper index to assess the damage incurred during the drying process (Kim et al., 1997). As can be seen (Table 2), the presence of baking powder has increased the number of LAB more likely due to its effect on pH (rising pH about 4.60) as well as nutrients which come from BP. It is likely that the original pH of added hydrocolloids also had some influence on final pH of yogurt powders.

As shown in Figure 6, increasing the microwave power and total solids of yogurt dramatically dropped the number of LAB where this decrease was much more pronounced in terms of Lactobacillus bulgaricus in comparison to Streptococcus thermophilus which is more resistant to heating. It seems that by increasing total solids and accumulation of caseins, dehydration became harder and harder and as a result the internal temperature increased where it can harshly damage LABs. Similarly, trends had been reported by other researchers (Kumar & Mishra., 2004).

With regard to the effect of hydrocolloids on survival of starters in yogurt powder, it can be seen that their presence had positive and negative effects on the survival number of Streptococcus thermophilus and Lactobacillus bulgaricus. However, these differences were not significant (Figure 7).

TABLE 2: EFFECT OF DIFFERENT HYDROCOLLOIDS, AND BAKING POWDER ON SOME CHEMICAL AND MICROBIAL PROPERTIES OF RECONSTITUTED YOGHURT

(TS = 16.6 %WT) PREPARED USING BD METHOD. Sample

Lactobacillus bulgaricus (cfu/ml)

Streptococcus thermophilus (cfu/ml)

pH

Acidity (◦D)

FY

23×106

16×106

3.97

126.03

FY + BP

10 ×101

10 ×100

4.54

110.91

GT + BP +FY

-

-

4.47

111.39

GT:GG +BP + FY

-

-

4.66

101.03

GT:LBG + BP + FY

-

-

4.65

101.09

GG:LBG + BP + FY

-

-

4.49

111.39

6

6

BP= Baking Powder (1.5 %wt.), GG= Guar Gum, GT= Gum Tragacanth, LBG=Locust Bean Gum, FY=Fresh Yogurt. The ratio and concentration of added hydrocolloids were 80:20 and 0.60 %wt, respectively.

13

www.fmfi‐journal.org Focusing on Modern Food Industry (FMFI), Volume 5 2016

100

A L.bulgaricus (cfu/ml)

S. thermophilus (cfu/ml)

10000

1000

100

10

B

10

1

1 35

135 MP (Watt)

35

260

135

260

MP (Watt)

FIGURE 6: INFLUENCE OF DIFFERENT MICROWAVE POWERS (MP) AND TOTAL SOLIDS (□ 10, ░ 20, ■ 28 %WT) ON SURVIVAL OF A) S. THERMOPHILUS AND B) L. BULGARICUS IN YOGURT POWDER

1

100

L.bulgaricus (cfu/ml)

10

S. thermophilus (cfu/ml)

1000

0.1 Fresh Yogurt

GT

GT:LBG

GT:GG

GG:LBG

FIGURE7: EFFECT OF MVD DRYING (MP=35 W, AP= 125 MBAR) OF FRESH YOGURT (TS = 10 %WT) MIXED WITH DIFFERENT HYDROCOLLOIDS (BD METHOD) IN THE PRESENCE OF BAKING POWDER (1.5 %WT) ON THE VIABILITY OF LABS (■ S. THERMOPHILUS, ▲ L. BULGARICUS). THE RATIO AND CONCENTRATION OF ADDED HYDROCOLLOIDS WERE 80:20 AND 0.60 %WT, RESPECTIVELY

Conclusions Based on our findings, microwave–vacuum drying was recognized as a suitable method for yogurt drying where at lower microwave powers (35 Watt, abs pressure=125 mbar) one can get yogurt powders with higher solubility, better structure, less color changes as well as higher survival rate for starter bacteria particularly Streptococcus thermophilus. In addition, total solids of fresh yogurt showed a significant effect on quality and drying time, high total solids led to low quality and short drying time. Moreover, it was found that the type of hydrocolloid, its concentration and stage of its incorporation were really important for achieving instant yogurt powders. The most appropriate concentration was 0.6 wt% on the basis of fresh yogurt weight, in a single or combined forms of GT and its combination with GG, LBG at a constant ratio (80:20) using a BD method where these gums were added to fresh skimmed yogurt before drying by MVD. It is noteworthy that the presence of a leavening agent (baking powder, 1.5 %wt on the weight basis of fresh yogurt) was necessary due to its foaming properties as well as its effect on pH and occurrence of some possible interactions. All in all, the yogurt powders produced under the abovementioned conditions were instantly reconstituted and made firm yogurts where their rheological and sensory properties were significantly better than fresh yogurt.

REFERENCES

[1]

Abbasi, S., and Azari, S. (2009). Novel microwave‐freeze drying of onion slices. International Journal of Food Science & Technology, 44: 974–979.

14

Focusing on Modern Food Industry (FMFI), Volume 5 2016 www.fmfi‐journal.org

[2]

Abbasi S., and Mohammadi, S. (2013). Stabilization of milk–orange juice mixture using Persian gum: Efficiency and mechanism. Food Bioscience, 2: 53–60.

[3]

Azarikia, F., and Abbasi, S. (2010). On the stabilization mechanism of Doogh (Iranian yoghurt drink) by gum tragacanth.

Food Hydrocolloids, 24: 358–363.

[4]

Cajigas, S. (1981). Instant yogurt drink composition. US patent, US004289789A.

[5]

Cajigas, S. (1992). Instant yogurt composition and process. US patent, US005145697A.

[6]

Everett, D. W., and Mcleod, R. E. (2005). Interactions of polysaccharide stabilizers with casein aggregates in stirred skim‐ milk yoghurt. International Dairy Journal, 15: 1175–1183.

[7]

deMan, J.M. (2008). Principles of Food Chemistry. Aspen publishers.

[8]

Ghaderi, A., Abbasi, S., and Hamidi, Z. (2010). Yoghurt powder production using a microwave‐vacuum drier. Iranian Food Science and Technology Research Journal, 6 (3): 210–222.

[9]

Kim, S. S., Shin, S. G., Chang, K. S., Kim, S. Y., Noh, B. S. and Bhowmik, S. R. (1997). Survival of lactic acid bacteria during microwave vacuum‐drying of plain yoghurt. Lebensm.‐Wiss. U.‐ Technol., 30: 573–577.

[10] Kim, S.S., and Bhowmik, S. R. (1994). Moisture sorption isotherms of concentrated yogurt and microwave vacuum dried yogurt powder. Journal of food Engineering, 21: 157–175.

[11] Kim, S.S., and Bhowmik, S. R. (1995). Effective moisture diffusivity of plain pogurt undergoing microwave vacuum drying.

Journal of Food Engineering, 24: 137–148.

[12] Kumar, P. and Mishra, H. N. (2004). Yoghurt powder—A review of process technology, storage and utilization. Food and Bioproducts Processing, 82 (C2): 133–142.

[13] Sharma, M. K., and Arora, C. P. (1995). Influence of product thickness, chamber and heating conditions on production rate of freeze‐dried yoghurt. International Journal of Refrigeration, 18: 297–307.

[14] Sharma, M. K., Arora, C. P., and Mital, B. K. (1992). Influence of concentration of milk solids on freeze drying rate of yoghurt and its quality. Journal of Food Engineering, 15(3): 187–198.

[15] Tamime, A. Y. and Robinson, R. K. (2007). Yoghurt: Science and Technology. 3rd ed. Cambridge: Woodhead Publishing Limited and CRC Press LLC, England.

[16] Unal, B., Metin, S. and Isikli, N. D. (2003). Use of response surface methodology to describe the combined effect of storage time, locust bean gum and dry matter of milk on the physical properties of low‐fat set yoghurt. International Dairy Journal, 13: 909–916.

[17] Walstra, P., Wouters, J. T. M., and Geurts, T. J. (2006). Dairy Science and Technology, 2nd ed. CRC press, USA.

15