YOGURT

Yogurt is defined as a product resulting from milk by fermentation with a mixed starter culture consisting of Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus.

• Nutritional Consideration

• Based on method of production and the physical structure:

a) Set yogurt: fermentation of milk is carried out in a retail container.

b) Stirred yogurt : gel structure is broken before cooling and packaging.

Fluid yogurt: stirred yogurt of low viscosity.

Based on flavorings

a) Plain or natural yogurt: typical sharp ‘nutty’ flavor.

b) Fruit yogurts: addition of fruits( in the form of fruit preserves, puree or jam.)

c) Flavored yogurts: by adding sugar and or other sweetening agents, flavorings and colorings.

Pretreatments….

• Ultrafiltration and reverse osmosis:

• Milk concentrated to 18–20% total solids to produce good quality yogurt.

• Casein is the major contributor of viscosity followed by fat and whey proteins.

• Gelatin, starch, vegetable gums, carrageenan, and pectin are used most widely as stabilizers for yogurt.

• The best yogurt texture is achieved by using gelatin at 0.3–0.8%.

Homogenization

• stabilization of the lipid phase against separation by gravity.

• specific gravity of milk fat = 0.9, skim milk= 1.036.

• D of the fat globules: milk(1 to 15 μm, average of 3–4 μm.) for Homogenized milk 1 or 2 μm.

• 4 to 6 fold increase in the surface area.

• The fat-globule membrane protects the fat globules from lipase. (after homogenization, the fat globule is vulnerable to attack by lipase so the milk is pasteurized to inactivate lipase before homogenization.) efficiency of homogenization is much better at around 65 °C

Heat Treatment

• 85 °C for 30 min.

• objectives:

(1) to destroy all pathogenic bacteria,

(2) to inactivate all the enzymes, including lipase,

(3) to destroy most of the spoilage bacteria

(4) to denature whey proteins, β-lactoglobulin (90%) and α-lactalbumin ( 60 %). (their ablity to bind water is inreased, via hydrogen bonding.)

Inoculation and Incubation

• cooled to 45 °C and inoculated ( from 0.5 to 5%.)

• 5% cause very rapid acid production, but leads to defects in aroma.

• The optimum level is 2%, 1% each of Lb. delbrueckii ssp. bulgaricus and Sc. thermophilus.

• Streptococcus thermophilus and Lactobacillus bulgaricus ( symbiotic growth)

• More L.acid than individual pure cultures more acetaldehyde than individual pure cultures

• Streptococcus thermophilus and Lactobacillus bulgaricus ( symbiotic growth)

• More L.acid and more acetaldehyde than individual pure cultures

• L delbrueckii ssp. bulgaricus : hydrolyzes proteins, , releasing essential amino acids, including valine, which stimulate the growth of S thermophilus.

• S thermophilus produces formic acid, which stimulates the growth of the lactobacillus.

• S thermophilus grows rapidly, reducing the pH to around 5.4, which stimulates the growth of Lb. delbrueckii ssp. bulgaricus, which is acid-tolerant

• The opt.temp. S thermophilus 37 °C, for Lb. delbrueckii ssp. bulgaricus is 45 °C.

• The ratio of lactobacilli and streptococci in natural yogurt for optimum flavor should be 1:1.

• 2% inoculum, at 42 °C, gel will form in 3.5–4 h.
frozen concentrated cultures: 5–8 h may be required.

• freeze-dried cultures may require longer times than fresh and active cultures.

• L.bulgaricus is responsible for excessive acid production. to prevent excessive acid production:

• -incubate at lower temp.( 37°C). L. bulgaricus can be replaced with L. casei.

•-select acid-sensitive strains

•-use strains that are sensitive to low temperature

Biochemistry of Fermentation:

• Carbohydrate metabolism:

• Lactose should be transported into cell.

• Phosphotransferase (PTS) system: lactose is phosphorylated during its dislocation (lactose-P)

• Inside the cell: lactose-P ® D- glucose +galactose-GP

• ¯ (hydrolysed by beta-D- phosphogalactosidase

• Glucose (by EMP pathway) ® ® lactic acid

• galactose-GP, D-tagatose-GP pathway ® lactic acid

• mostly glucose utilized, very little galactose utilized.

• in the presence of glucose, galactose pathway is suppressed.

Production of Lactic Acid:

• L. acid destabilise the casein micelles by converting the colloidal calcium phosphate/phosphate complex ® soluble calcium

• coagulation of casein at pH=4.6-4.7 and the formation of the yoghurt gel.

• L.acid gives sharp and acidic flavor.

COOH
HO-C – H L(+) lactic acid (S. thermophilus)
CH3
COOH
H- C – OH D(-) lactic acid (L.bulgaricus)
CH3

• S.thermophilus grows first so L(+) lactic acid is produced first followed by D(-) lactic acid.

• Normal yogurt: 45-60% L(+) and 40-55% D(-).

• if yogurt contains >70% of L(+) lactic acid

• Probable reasons ?????

• starter culture was predominantly S.thermophilus

• fermentation temperature •product is cooled at low acidity(<0.8% L. Acid)

• final acidity of yogurt is 0.9–1%.

• US standard: titratable acidity of 0.9% or higher.

• IDF: minimum acidity of yogurt should be 0.7%.

Cooling

• At desired acidity cool to •The viscosity of yogurt improves during storage for 1–2 days

• During cold storage: minimize rough mechanical handling of the packaged yogurt, and to maintain the temperature of •During transport shaking of yogurt can lead to a reduction in viscosity and syneresis.

Continous Stirred Yoghurt Production:

• A two stage process;

• Milk ® pasteurized ® homogenized ® a pH-stat prefermentation and plug flow

• pH-stat fermentor: prefermentation of milk until pH decreases to 5.7 ( at 45 C)

• Plug flow fermentor: formation of the yoghurt texture due to further acidification to pH 4.5 (37 C)

• Three problems to establish and maintain plug flow;

1- Transferring milk without disturbing coagulating milk

2- Prevent from adhering to the wall

3- Remove from the tank without disturbing the plug flow.

• Solution to these problems:

• Solutions to problems:

1- Milk is fed through a centrifugal distributor on to metal screen in to plugflow fermentor to minimize disturbance of coagulating milk

2- Soya lecithin and ethanol mixture sprayed on the wall before process.

3- A horizontal plate with holes move up and down (coagulum is sheared through the holes in the plate and downward movement pushes the yoghurt out)

Advantages of continuous fermentation of yoghurt:

• Smaller size of equipment

• Reduction of yoghurt losses in tanks and pipelines

• Smaller cooling and filling sections

• No need for all the milk to be in stock at the start of production

• Uniformity of product quality and characteristics

• Better control over acid development

SPECIAL YOGURT PRODUCTS

Low-lactose or Lactose Hydrolysed Yogurt (LHY):

Lactose intolarant people do not produce sufficient lactase (β–galactosidase) :cannot hydrolyze the ingested lactose completely, gastrointestinal discomfort, stomach upset, diarrhea.

• Low-lactose or lactose-free milk: by the physical removal of lactose by ultrafiltration or by hydrolysis of lactose into the corresponding monosaccharides, glucose, and galactose.

• One can increase sweetness of yoghurt without increasing calorific value by hydrolysing lactose (0.4 as sweetness) by beta-D-galactosidase
Lactose ® galactose + glucose

• Sweetness 0.4 0.6 0.7

• (sweetnes of sucrose is 1)

• desirable for manufacture of fruit/flavored yoghurt

•cheaper to add sweetining agents.

Carbonated Yoghurt:

Flavored : orange, lemon, cherry or apple flavors

• Liquid or dry form ® gradually releases the CO2 when it is reconstituted with water.

• Ca-carbonate is more advantageous than Na-carbonate because ,it releases CO2 much slowly

Soy-milk Yoghurt:

• soybean is inexpensive and rich in protein

• Main problems : beany flavor and flatulence (CO2 production, hydrogen and methane production by intenstinal flora from oligosaccharides)

• L.bulgaricus : acceptable yoghurt-like product

• Lactic acid (optimum) = nearly 1.15 %

• Flavor of fermented soy milk was directly related to the levels of n-pentonal (produced by S.thermophilus) and n-hexonal (naturally present in soy milk)

• Color problem is another problem for soy-milk yoghurt.

Heated or Pasteurized Yogurt

• at 4 °C, acidity increases due to activity of the starter bacteria ( ‘post-acidification)

• So the shelf-life of yogurt is few weeks at 4 °C.

• Heating destroys most of the starter bacteria and yeast and molds: the shelf-life of the product can be extended to 8 weeks.

• by heat-treating yogurt in the package at about 55 °C for 30 min, followed by cooling.

• The main problems: loss of flavor and syneresis. Stabilizers can be used to overcome the latter problem.

• yogurt contains very few or no viable yogurt bacteria after heating ( raises questions about the definition of yogurt)

Prebiotics, Probiotics, and Synbiotics

Probiotic: from two Greek words meaning for life.

•‘they are live microorganisms : The intake of these bacteria is reported to help restore the balance in the intestinal microflora, which may have been lost due to stress, antibiotic use, or illness.

• Compounds that are either partially degraded or not degraded by the host and are preferentially utilized by bifidobacteria as carbon and energy sources are defined as ‘prebiotics.’

• The major strains of bacteria used in probiotics, L acidophilus and Bifidobacterium spp., are dominant organisms of human large intestines.

• They inhibit the growth of pathogenic organisms by producing organic acids and bacteriocins.

• Other benefits: suppression of potentially harmful enzymes, increased immune response, reduction in serum cholesterol, and antimutagenic effects.

• Resistant starch and nonstarch polysaccharides are classified as colonic foods but not as prebiotics ( not metabolized by certain beneficial bacteria.)

• Products that contain both prebiotics and probiotics are referred to as ‘synbiotics.’

• (ie SymBalance yogurt, contains inulin as prebiotic and L reuteri, Lb. acidophilus, and L casei as probiotics. )

• Inulins are a group of naturally occurring oligosaccharides produced by many types of plants

• Inulin is indigestible by the human enzymes ptyalin and amylase, which are designed to digest starch.

• It is only in the colon that bacteria metabolise inulin, with the release of significant quantities of CO2 and/or methane.

• (rich in chicory(hindiba), garlic,leek( pırasa)

Concentrated-Strained yogurt ( Süzme yogurt )

• popular in the Middle East region

• Traditional Process

• cold plain yogurt is stirred and emptied into cloth bags of about 25 kg. That are stacked on top of each other

• Pressure is applied ( 12–18 h) the concentrated product is emptied into a mixing bowl to obtain a uniform texture prior to packaging.

long, horizontal filter cloths: gently oscillated up and down, while slight lateral pressure is applied. ‘Berge’ system( developed in France 1960s) product that contains ≥ 30 g 100 g−1 total solids, and is known as ‘yogurt cheese.’

Frozen yogurt

• resembles icecream, in that the fresh stirred yogurt is stabilized, fortified with fruit base (syrup or pieces), whipped, and frozen.

• soft, hard, or mousse.

• a longer shelf-life product can be achieved by using nitrogen rather than air.

Drying

• Spray dried:

• The concentration stage, before drying, at a low temperature (about 50–60 °C) to minimize scorching on to the surfaces of the evaporator, or discoloration of the final powder,
• The concentrated buttermilk, which is highly viscous, is pumped to the spray drier using an inlet air drying temperature between 175 and 190 °C.

Kefir

• First produced in the Caucasus ( fermented naturally in bags made of animal hides.)

• Kefir is a self-carbonated, fermented beverage containing about 0.8–1% lactic acid and 1–2% alcohol.

• Cow’s and goat’s milk are usually preferred.

• Some proteolysis occurs in the milk, and a yeasty aroma develops.

Microbiology of Kefir Grains

• composition 89–90% water, 0.2% lipid, 3% protein, 6% sugar (mainly polysaccharide) and 0.7% ash.

• stored wet at 4°C, or should be dried at room temperature for 36–48 h.

• Dried kefir grains retain their activity for 12–18 months, whereas wet grains retain their activity for only 8–10 days.

• Mo’s in the kefir grains are embedded in a slimy polysaccharide material named ‘kefiran’.

• The appearance of kefir grains is similar to that of the tiny florets of cauliflower.

Freezing Kefir Grains

• storing kefir grains up to 2 months, by freezing the grains. dry milk powder is added as a protective agent.

Drying Kefir Grains

• Kefir grains may be dehydrated for long term storage of up to 12 to 18 months.

Koumiss

• Koumiss is a drink with ancient origins, and is common in eastern Europe and central Asia.

• Produced from mare’s milk by a combined fermentation to lactic acid and alcohol, and its highly nutritive and curative characteristics are well known.

• Fermentation takes some 3–8 h

Tarhana

• Tarhana is a fermented food resulting from the combined fermentation of yoghurt with cracked wheat or flour.

• Tarhana is a traditional Turkish food

• ‘trahana’ (Bulgaria), ‘trahanas’ (Greece), ‘taron’ (Macedonia), ‘tarhonya’ (Hungary), ‘kisk’ (Iraq), ‘kishk’ (Egypt, Syria and Lebanon) and ‘goce’ (Turkmenistan).

Production

• The method of manufacture of tarhana does not vary significantly from one region to another

• Slight variations in the microbiological and compositional characteristics may occur depending on the locality and its traditions.

• Production is based on a yoghurt fermentation, but concentrated yoghurt or sour milk can also be used.

• Usually, equal quantities of yoghurt and cracked wheat or flour are used.

• S. cerevisiae is generally added to the mixture, to give the characteristic aroma of tarhana.

• L. bulgaricus and S. thermophilus, from the yoghurt, are responsible for the production of lactic acid and the yeast produces ethanol and CO2.

• L. casei, L. plantarum and L. brevis can also be added in order to improve the aroma and flavour.

• After fermentation, the mixture is spread onto a large cloth (traditional method) or a pulsating tray (industrialized method), and is dried at 40°C for 20–30 h (traditional method) or 60°C for 5–8 h (industrialized method).

• The dried mixture is milled and stored at room temperature.

Composition

• Turkish standards: protein (min. 14%), moisture (max. 10%) and salt (max. 5%).

• A low level of lactose and a high level of hydrolysed starch facilitate the digestion of the product.

• It is rich in protein, calcium, iron and zinc.
it is an important source of group B vitamins –

Soy tarhana

• Wheat flour is replaced ( totally or partialy with soy flour)

• No color problem

• Better amino acid profile

• Reduced off flavor

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