Understanding the basics
Lactose is the principal milk sugar which imparts characteristic sweetness to milk. Lactose occurs as a true solution in the milk of all species. The highest percentage of lactose is found in human milk. The galactose moiety of lactose is part as well as parcel of cerebrosides of brain and myelin sheath of nerves. On account of this, scientists consider humans have progressed in all directions. Chemically, it is a disaccharide which on hydrolysis or treatment with beta-galactosidase gives a molecule of glucose and galactose.
Commonly, it occurs in three forms amorphous or glassy form, alpha hydrate and beta-anhydrous forms. However, the most common form is alpha hydrate (C12H22O11.H2O) which occurs when a supersaturated solution of lactose below 93.5°C is kept for crystallization. Above 93.5°C results in the formation of beta-anhydrous crystals.
Lactose as antagonist
The major concern is its solubility, which apparently is less than other sugars. The situation seems to be critical when milk is concentrated 3:1, at this stage lactose concentration approaches its ultimate solubility. When such a concentrated product is subjected to low temperatures or sucrose is added in it, crystals of alpha-hydrate develop. These crystals when kept undisturbed for a longer time transformed into larger crystals consequently causing ‘Sandiness’ or ‘Grittyness’ defect. In this defect, the product has a grainy texture and due to this, it appears that there is sand in the product. The large size crystals are of prism or tomahawk shape. On account of this reason, the amount of milk solids is kept low during the formulation of ice cream mix.
When lactose rich solutions are dried quickly; amorphous or glassy lactose is formed. This fact is kept in mind during the processing of dried milk. This glassy lactose is extremely hygroscopic. When the moisture content of the product reaches 8%, the lactose crystals recrystallize to form alpha hydrate crystals which result in ‘lumpy’ product.
On account of the above discussed problems and in order to promote slow crystallization; seeding is done. The process of seeding comprises introduction of finely ground crystals into a concentrated product which encourages the formation of uniform small sized crystals.
Role in Fermented Dairy Products
The importance, utility and necessity of milk and its products was recognised by mankind since civilization. Aim to preserve milk for longer days lead to the accidental discovery of fermented dairy products. The fermentation of lactose produces sweet aroma and flavour compounds. As per modern biochemical approach, lactose remains as the precursor for lactic acid in fermented dairy products. It was also observed that lactose was not the only product of fermentation rather there occurs other by-products too which contribute to the rich and unique flavour of fermented dairy products.
There exist two kinds of lactose fermenting microorganisms viz homofermentative and heterofermentative. Homofermentative bacteria produced lactic acid as the sole end product in anaerobic conditions by following Embden Meyerhoff Parnas (EMP) Pathway. Heterofermentative bacteria ferment lactose by a pathway which has close resemblance with Pentose Phosphate Pathway in the initial few steps of fermentation. This results in the production of CO2, ethanol and lactic acid in the presence of air whereas in the presence of extraneous oxygen; acetic acid, lactic acid and CO2 are produced. The sweet aroma of fermented dairy products is due to utilisation of citric acid by some specific aroma producing microorganisms. In such a pathway, diacetyl and acetoin (acetyl methyl carbinol) are major end products. In the production of the above two compounds, acetaldehyde plays a pivotal role which arises from excess pyruvate formed from the breakdown of citric acid.
Role in Browning Reactions
Maillard reactions occur between reducing sugars and amino compounds. In the case of milk and milk products, there occurs a reaction between lactose and milk proteins. It was reported that low lactose values are reported in samples of lactose wherein there exists interactions between lactose and milk proteins. The heat-induced processing was involved in the products having lower lactose values. Further, it was established that heat desiccated milk and milk products gave low lactose values. The browning reactions are the ultimate effect of heat-induced acidification along with storage conditions, relative humidity, moisture content and processing temperature. Apart from Maillard browning; non-amino-sugar browning i.e. caramelisation is also seen in dairy products. It occurs on lactose content of Dairy products due to severe heat treatment and change in pH. This browning is oxidative enzymatic or non-enzymatic. The brown pigment that forms as a result of Maillard browning reactions is ‘melanoidin’. Usually, the browning flavour is described as a nutty, bready and cracky aroma.
Lactitol is a sugar alcohol which is derived from lactose. It has the potential to act as a non-nutritive sweetener. It can be easily esterified to yield a family of food emulsifiers which are analogous to sorbitans derived from sorbitol.
Technically it is produced by hydrogenation of lactose in the presence of Nickel as a catalyst. Under special conditions, it exists as lactitol monohydrate which is non-hygroscopic. It possesses good solubility in water. At 25°C, 149 grams lactitol monohydrate will dissolve in 100 grams of water. Since lactitol has no reducing group, therefore, it is more chemically stable than lactose. It is highly resistant to pH change and high temperatures. On account of this, it is extensively used in manufacturing if hard-boiled sweets. Commercially available types of lactitol are lactitol monohydrate and lactitol solution with 54% dry matter.
Lactulose is sweeter than lactose and 48-62% as sweet as sucrose. It is a disaccharide produced from alpha-lactose-monohydrate by isomerisation with calcium hydroxide. Small amounts of lactulose have been detected in heated human milk. Under normal conditions, it hasn’t been isolated in its genuine form. Commercially, it is available as fluid hydroscopic concentration with 65% dry matter. Lactulose is made up of galactose and fructose. Neither it is broken down in small intestine nor absorbed there. The reason is that there are no adequate enzymes to split the disaccharides into two readily absorbable units (galactose and fructose). Acidophilus bacteria and Bifidobacter bifidus can utilise lactulose by giving lactic acid as the end product. Acidophilus bacteria create a typical acidic environment by breaking down lactulose which suppresses alclophilic intestinal bacteria. The above formed lactic acid stimulates intestinal peristalsis. It is commonly added to infant foods to stimulate bifidus factor. It is also reported to suppress the growth of certain tumour cells. It is not metabolised by oral bacteria and hence is not carcinogenic in nature. It is also used as a 50% syrup.
Presence of lactose in food can considerably improve calcium absorption. This is due to formation of acidic pH in digestive tract which increases solubility and availability of calcium for absorption. Lactose is broken down into glucose and galactose by enzymatic action resulting in acidic environment in intestine. This acidic environment aids in the suppression of putrefactive germs. Lactose fraction present in milk and milk products has no carcinogenic effect. Further, it doesn’t form dental plaque. After consumption of lactose blood, sugar levels rise by only 74 mg/100 ml which is less than that of glucose thus lactose can be consumed by diabetic patients.
The uses of lactose are plenty. It is used in infant formula, pharmaceutical industry, confectionery industry etc. However, lactose has two major undesirable consequences viz, lactose intolerance and galactosaemia. Lactose intolerance is a condition in which lactase activity in the mucosa of the small intestine is reduced. Galactosemia is a condition in which ingested foods or liquids containing galactose leads to undigested sugars build up in the blood.
- Mathur, Datta Roy and Dinakar (1999), Textbook of Dairy Chemistry
- Fox and McSweeney (1998), Dairy Chemistry and Biochemistry
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