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Maharashtra HSC Food Products Technology (Vocational) study materials
Maharashtra HSC Food Products Technology (Vocational) study resources

Maharashtra HSC Food Products Technology (Vocational) practice questions

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Question 1Food Science and Food Chemistry

In food science, water activity (aw) is best defined as the

  1. ability of a food to dissolve added solutes such as salt and sugar
  2. ratio of the vapour pressure of water in a food to that of pure water at the same temperature Correct answer.
  3. total mass of water present in a food, expressed as a percentage of the food's mass
  4. mass of water that can be frozen out of a food at −20 °C

Explanation

Water activity is a thermodynamic measure of how available water is in a food. It is calculated as the vapour pressure of water in the food divided by the vapour pressure of pure water at the same temperature. Moisture content is the total amount of water present and is not the same as water activity.

Question 2Food Science and Food Chemistry

Vegetables are usually blanched before freezing. The main purpose of this step is to

  1. destroy all bacterial endospores present on the surface
  2. add a chemical preservative layer to the vegetable surface
  3. raise the water activity so that ice crystals form more slowly
  4. inactivate enzymes such as polyphenol oxidase that cause browning and off-flavours Correct answer.

Explanation

Blanching uses hot water or steam to denature enzymes that remain active during frozen storage. Polyphenol oxidase and other enzymes can cause browning, off-flavours and loss of colour if they are not inactivated. Blanching does not sterilise the vegetable or make it commercially sterile.

Question 3Food Science and Food Chemistry

For canning purposes, a food is classified as low-acid when its equilibrium pH is

  1. between 3.5 and 4.0
  2. exactly 7.0
  3. below 3.5
  4. greater than 4.6 Correct answer.

Explanation

Low-acid canned foods are those with an equilibrium pH greater than 4.6. This threshold is important because Clostridium botulinum can grow and produce toxin in low-acid foods that receive insufficient heat treatment. Acidified foods are those with a pH of 4.6 or below.

Question 4Food Science and Food Chemistry

A bakery wants to limit Maillard browning on the surface of a stored biscuit. Which combination of changes is most likely to slow the reaction?

  1. Add free amino acids and keep the biscuits warm
  2. Reduce the reducing-sugar content and store the biscuits cool and dry Correct answer.
  3. Raise the water activity and expose the biscuits to light
  4. Increase the reducing-sugar content and raise the storage temperature

Explanation

Maillard browning requires reducing sugars, amino compounds and heat, and it is favoured by moderate water activity. Reducing the supply of reducing sugar and lowering the storage temperature therefore slows the reaction. Light is more relevant to oxidative reactions than to Maillard browning.

Question 5Food Science and Food Chemistry

When wheat flour is heated with milk to make a white sauce, the main reason the mixture thickens is that

  1. sucrose caramelises and increases the viscosity
  2. fat globules crystallise and trap the liquid phase
  3. starch granules absorb water, swell and release amylose, forming a gel network Correct answer.
  4. milk proteins coagulate into a continuous protein gel

Explanation

Heating flour in a liquid causes starch gelatinization: the granules absorb water, swell and release amylose, which helps form a viscous gel network. This is the principal thickening mechanism in a white sauce. Protein coagulation and fat crystallisation are not the main causes of the thickening.

Question 6Food Science and Food Chemistry

In mayonnaise, which component primarily acts as the emulsifier that keeps oil droplets dispersed in the aqueous phase?

  1. sucrose
  2. sodium chloride
  3. vinegar
  4. egg yolk Correct answer.

Explanation

Egg yolk contains lecithin and proteins that adsorb at the oil–water interface and lower interfacial tension. This stabilises the oil droplets against coalescence. Vinegar, salt and sugar affect flavour and preservation but are not the principal emulsifiers.

Question 7Food Science and Food Chemistry

When milk is acidified to about pH 4.6, casein micelles aggregate and precipitate. The best explanation is that at this pH

  1. the net charge on casein is near zero, so electrostatic repulsion is minimised Correct answer.
  2. calcium phosphate becomes more soluble and stabilises the micelles
  3. casein carries its maximum net negative charge and repels other micelles
  4. proteolytic enzymes completely hydrolyse casein into peptides

Explanation

The isoelectric point of casein is about pH 4.6. At this pH the net charge on the casein molecules is close to zero, so electrostatic repulsion between micelles is reduced and they aggregate. Acidification also solubilises colloidal calcium phosphate, further destabilising the micelles.

Question 8Food Science and Food Chemistry

Which storage practice will most slow oxidative rancidity in a packet of fried potato chips?

  1. Store the packet exposed to bright light at room temperature
  2. Flush the packet with oxygen before sealing
  3. Store the packet in a warm place to keep the oil liquid
  4. Pack the chips in an oxygen-free atmosphere and keep them cool and dark Correct answer.

Explanation

Oxidative rancidity is promoted by oxygen, light and heat. Removing oxygen and storing the product cool and dark therefore slows oxidation of the frying oil. Warm storage and light exposure accelerate the development of rancid off-flavours.

Question 9Food Science and Food Chemistry

A dried fruit product has a water activity of 0.60. Which statement best explains its microbiological stability?

  1. Clostridium botulinum can multiply because water activity is above its minimum
  2. Vegetative bacteria and most moulds cannot multiply, although osmophilic yeasts may still grow slowly if other conditions allow Correct answer.
  3. Water activity of 0.60 is equivalent to 60% moisture, so bacterial growth is unaffected
  4. All microorganisms, including bacterial endospores, are killed by the low water activity

Explanation

Most vegetative bacteria require water activity above about 0.90, and most moulds require above about 0.80. At 0.60, bacterial growth and ordinary mould growth are inhibited, but osmophilic yeasts and some xerophilic moulds can still grow slowly. Water activity is not the same as percentage moisture.

Question 10Food Science and Food Chemistry

A heat-resistance test gives a D-value of 1.5 min at 121 °C and a z-value of 10 °C for a spoilage bacterium. The z-value of 10 °C means that

  1. the organism cannot survive for more than 10 min at any temperature above 100 °C
  2. a 12-log reduction is achieved in 10 min at 121 °C
  3. the organism needs 10 min at 121 °C for a 90% reduction in numbers
  4. raising the temperature by 10 °C shortens the D-value to one-tenth of its previous value Correct answer.

Explanation

The z-value is the temperature change needed to change the D-value by a factor of ten. A z-value of 10 °C therefore means that increasing the temperature by 10 °C reduces the D-value to one-tenth of its former value. The D-value itself is the time at a given temperature for a 90% reduction in numbers.

Question 11Food Science and Food Chemistry

A loaf of bread kept in a refrigerator for three days becomes firm and crumbly. Which change in the starch fraction is mainly responsible?

  1. Retrogradation of gelatinised starch Correct answer.
  2. Dextrinisation of starch by dry heat
  3. Denaturation of gluten protein
  4. Caramelisation of reducing sugars

Explanation

After baking, gelatinised starch chains slowly reassociate into ordered crystalline regions, and amylopectin is the main polymer involved. This retrogradation expels water from the crumb and gives the firm, crumbly texture of staling, and it is fastest at refrigeration temperature. Dextrinisation and caramelisation need dry or high heat, which are absent inside a wrapped loaf.

Question 12Food Science and Food Chemistry

Which vitamin is lost most rapidly through oxidative reactions when a fruit juice is stored in an oxygen-permeable bottle?

  1. Niacin
  2. Vitamin K
  3. Ascorbic acid Correct answer.
  4. Thiamine

Explanation

Ascorbic acid is a water-soluble reducing agent that is readily oxidised to dehydroascorbic acid and further breakdown products in the presence of dissolved oxygen, light and metal ions. Fruit juices are therefore pasteurised and packed with minimal headspace and oxygen ingress to protect it. Niacin is comparatively stable, vitamin K is fat-soluble and stable in such systems, and thiamine loss is driven mainly by heat rather than by oxygen alone.

Question 13Food Science and Food Chemistry

Sodium benzoate is permitted in acidic beverages such as squashes and ready-to-serve drinks mainly because it can

  1. Improve the emulsion stability of the drink
  2. Inhibit the growth of yeasts and moulds Correct answer.
  3. Increase the vitamin C content of the drink
  4. Give the drink a brighter, more appealing colour

Explanation

Sodium benzoate is a chemical preservative whose undissociated benzoic acid form is inhibitory to yeasts, moulds and several bacteria, and it works best in acidic products where that form predominates. It is therefore used for preservation, not for texture, colour or nutrition. Emulsifiers, nutrients and colours are separate additive classes with their own functions.

Question 14Food Science and Food Chemistry

Cloudy apple juice is treated with a pectinase preparation before filtration. The enzyme acts because it

  1. Converts starch into maltose and dextrins
  2. Oxidises polyphenols to brown pigments
  3. Hydrolyses pectin and lowers viscosity Correct answer.
  4. Breaks down proteins into peptides

Explanation

Pectin, a galacturonic acid polymer, gives cloudy juice its high viscosity and holds suspended particles in dispersion. Pectinase splits these chains, so viscosity falls and the particles aggregate and settle or filter out readily. Starch, protein and polyphenol substrates require amylase, protease and polyphenol oxidase respectively.

Question 15Food Science and Food Chemistry

Red cabbage extract is used as a natural pH indicator. When a few drops are added to lime water, the extract

  1. Turns blue-green Correct answer.
  2. Turns bright red
  3. Turns dark brown
  4. Becomes colourless

Explanation

The pigments responsible are anthocyanins, whose structure and hence colour depend on pH. In acid they exist mainly as the red flavylium cation, while in alkaline conditions such as lime water they shift to blue-green and eventually yellow forms. This reversible shift is why anthocyanin extracts are used as simple pH indicators.

Question 16Food Science and Food Chemistry

When vegetable oil is partially hydrogenated to make bakery shortening, the main chemical consequences are

  1. Saturation of fatty acids together with an increase in iodine value
  2. Oxidation of double bonds together with formation of peroxides
  3. Hydrolysis of triglycerides together with release of free fatty acids
  4. Saturation of double bonds together with an increase in melting point Correct answer.

Explanation

Hydrogenation adds hydrogen across carbon-carbon double bonds, so the degree of unsaturation falls and the structured fats become more solid, raising the melting point. Because the process is usually partial, some double bonds are isomerised and trans fatty acids are formed as a side reaction. Iodine value therefore decreases, and no hydrolysis or oxidation is intended.

Question 17Food Science and Food Chemistry

A strong, extensible gluten network develops in a wheat flour dough primarily because

  1. Fat is rubbed into the flour before water is added
  2. Starch granules gelatinise during mixing
  3. Hydrated glutenin and gliadin are worked by kneading Correct answer.
  4. Yeast ferments sugars into carbon dioxide

Explanation

Wheat gluten proteins, glutenin and gliadin, absorb water during mixing and, with continued kneading, align and cross-link through disulphide and hydrogen bonding into a viscoelastic sheet. Glutenin contributes strength and elasticity, whereas gliadin contributes extensibility. Starch gelatinisation needs heat, gas production comes later from yeast, and added fat tends to shorten the network.

Question 18Food Science and Food Chemistry

A food sample is found to contain 3.2% nitrogen by the Kjeldahl method. Using the conventional factor of 6.25, the crude protein content is

  1. 3.2%
  2. 9.4%
  3. 16.0%
  4. 20.0% Correct answer.

Explanation

Crude protein is calculated as nitrogen content multiplied by the conversion factor, so 3.2 × 6.25 = 20.0%. The factor 6.25 assumes a protein containing about 16% nitrogen and is the general value used when the food's specific factor is unknown. The other figures come from dividing or from leaving the nitrogen value unconverted.

Question 19Food Science and Food Chemistry

In many countries, frying oil in a continuous fryer must be discarded once which quality index exceeds the statutory limit?

  1. Peroxide value
  2. Total polar compounds Correct answer.
  3. Iodine value
  4. Saponification value

Explanation

Repeated heating in the presence of water and air hydrolyses and oxidises triglycerides, producing polar compounds such as free fatty acids, mono- and diacylglycerols, polymers and oxidised products. Total polar compounds therefore accumulate steadily and are used as a regulatory discard criterion, commonly around 24–27%. Iodine, peroxide and saponification values do not reflect cumulative thermal abuse in the same reliable way.

Question 20Food Science and Food Chemistry

Spray-dried skim milk powder stored in a warm, humid room becomes sticky and cakes into hard lumps. The most probable explanation is that

  1. Milk fat melts and spreads as a continuous film
  2. Bound water evaporates from the particle surface during storage
  3. Casein micelles coagulate as the pH falls to 4.6
  4. Amorphous lactose softens above its glass transition temperature Correct answer.

Explanation

Spray drying leaves lactose in an amorphous, glassy state. When moisture uptake lowers the glass transition temperature below the storage temperature, the particles change to a rubbery state, become sticky, and the lactose later crystallises, releasing water and forming hard lumps. Skim milk powder contains little fat, and no acidification occurs during ordinary storage.

Question 21Food Science and Food Chemistry

Which of the following is a synthetic azo dye permitted for use as a food colour?

  1. Caramel
  2. Tartrazine Correct answer.
  3. Curcumin
  4. Chlorophyll

Explanation

Tartrazine (E102, also called FD&C Yellow 5) is a synthetic azo dye allowed as a food colour in many countries. Curcumin, chlorophyll and caramel are natural colourants obtained from turmeric, plant material and heated sugar respectively.

Question 22Food Science and Food Chemistry

Which of the following compounds is the primary product formed during the initial stage of lipid oxidation and is measured by the peroxide value?

  1. Hydroperoxides Correct answer.
  2. Free glycerol
  3. Fatty acid methyl esters
  4. Triglycerides

Explanation

Peroxide value measures hydroperoxides, which are the first stable products formed when unsaturated fatty acids react with oxygen. These hydroperoxides later decompose to aldehydes, ketones and acids that cause rancid off-flavours.

Question 23Food Science and Food Chemistry

Gelatin is widely used in foods such as jellies and marshmallows mainly because, on cooling, it forms a

  1. thermoreversible gel that melts again on heating Correct answer.
  2. permanent covalent gel that cannot be melted
  3. water-in-oil emulsion
  4. foam stabilised by air bubbles

Explanation

Gelatin forms a thermoreversible gel: it sets on cooling as collagen-derived chains form a three-dimensional network held by hydrogen bonds, and it melts again when reheated. This property gives the characteristic soft, melt-in-the-mouth texture of gelatin jellies.

Question 24Food Science and Food Chemistry

In a spray-dried fruit powder, which statement best explains why storage below its glass transition temperature (Tg) slows deterioration?

  1. Below Tg enzymes become more active and consume oxygen
  2. Below Tg molecular mobility is low, slowing diffusion-limited reactions Correct answer.
  3. Below Tg the powder melts and forms a protective coating
  4. Below Tg the powder has a higher water activity and is therefore more stable

Explanation

Below the glass transition temperature the amorphous matrix is in a glassy state with very low molecular mobility. Diffusion-controlled processes such as crystallisation, enzymatic reaction and oxidation are therefore slowed, extending shelf life.

Question 25Food Microbiology and Fermentation

Curd is prepared from milk at home mainly through the action of which group of microorganisms?

  1. Lactobacillus species Correct answer.
  2. Bacteriophages
  3. Saccharomyces cerevisiae
  4. Aspergillus niger

Explanation

Lactic acid bacteria, especially Lactobacillus species, ferment lactose in milk to lactic acid. The resulting fall in pH causes casein to coagulate and form a firm curd. Yeasts, moulds and bacteriophages do not perform this controlled acid fermentation.

Question 26Food Microbiology and Fermentation

During bread dough fermentation, the gas that mainly stretches the gluten network and raises the dough is:

  1. Oxygen
  2. Carbon dioxide Correct answer.
  3. Hydrogen sulphide
  4. Nitrogen

Explanation

Yeast in bread dough ferments available sugars to ethanol and carbon dioxide. The carbon dioxide is trapped in the gluten network, stretching the dough and making it rise. Oxygen, nitrogen and hydrogen sulphide are not the main raising gas.

Question 27Food Microbiology and Fermentation

The natural fermentation of idli batter depends mainly on which combination of microorganisms?

  1. Bacteriophages and coliforms
  2. Acetic acid bacteria and moulds
  3. Lactic acid bacteria and yeasts Correct answer.
  4. Coliform bacteria and moulds

Explanation

Idli batter undergoes natural lactic fermentation in which lactic acid bacteria produce acid and yeasts produce carbon dioxide. The acid gives the characteristic sour taste, while the gas makes the batter porous. Coliforms, acetic acid bacteria and bacteriophages are not the main leavening combination.

Question 28Food Microbiology and Fermentation

Which process is used to make low-acid canned foods such as green peas commercially sterile and safe from Clostridium botulinum?

  1. Refrigeration at 4°C for 24 hours
  2. Drying to a moisture content below 10%
  3. Retort sterilization at about 121°C Correct answer.
  4. Pasteurization at 63°C for 30 minutes

Explanation

Clostridium botulinum forms heat-resistant spores that can germinate in low-acid canned foods and produce a deadly neurotoxin. Commercial sterilization in a retort at about 121°C is required to inactivate these spores. Pasteurization, refrigeration and drying are not sufficient for commercially sterile low-acid cans.

Question 29Food Microbiology and Fermentation

Which organism is commonly added to fermented milk to provide a probiotic effect?

  1. Aspergillus flavus conidia
  2. Lactobacillus acidophilus Correct answer.
  3. Clostridium botulinum spores
  4. Salmonella Typhimurium cells

Explanation

Lactobacillus acidophilus is a lactic acid bacterium used in fermented milks because it survives passage through the gut and may contribute to gut health. Pathogens such as Clostridium botulinum and Salmonella Typhimurium are not used as probiotics. Aspergillus flavus is a mould associated with aflatoxin, not a probiotic.

Question 30Food Microbiology and Fermentation

Tempeh is a fermented food prepared from soybeans using which mould?

  1. Penicillium roqueforti
  2. Rhizopus oligosporus Correct answer.
  3. Saccharomyces cerevisiae
  4. Acetobacter aceti

Explanation

Tempeh is made by fermenting cooked soybeans with the mould Rhizopus oligosporus. Its white mycelium binds the beans into a compact cake and its enzymes modify the proteins and lipids. Penicillium, yeast and acetic acid bacteria are used in other fermented foods, not tempeh.

Question 31Food Microbiology and Fermentation

In the production of vinegar from alcoholic beverages, acetic acid bacteria mainly convert:

  1. Proteins into amino acids
  2. Glucose directly into lactic acid
  3. Starch into maltose
  4. Ethanol into acetic acid Correct answer.

Explanation

Vinegar is produced when acetic acid bacteria such as Acetobacter oxidise ethanol to acetic acid in the presence of oxygen. This is why alcoholic liquids are the usual substrate for vinegar fermentation. Conversion of glucose to lactic acid, proteins to amino acids, or starch to maltose describes other processes.

Question 32Food Microbiology and Fermentation

A batch of cooked chicken is left at room temperature for several hours. Which foodborne pathogen is most likely to multiply and cause illness?

  1. Lactobacillus plantarum
  2. Bacillus subtilis
  3. Salmonella enterica Correct answer.
  4. Saccharomyces cerevisiae

Explanation

Salmonella enterica is commonly associated with poultry and can multiply rapidly when cooked chicken is held at room temperature. Proper cooking, rapid cooling and refrigerated storage are needed to control it. Bacillus subtilis, Lactobacillus plantarum and Saccharomyces cerevisiae are not the typical poultry-borne pathogen in this situation.

Question 33Food Microbiology and Fermentation

Aflatoxin contamination in stored groundnuts and maize is produced mainly by which mould?

  1. Lactobacillus plantarum
  2. Saccharomyces cerevisiae
  3. Rhizopus stolonifer
  4. Aspergillus flavus Correct answer.

Explanation

Aflatoxins are produced mainly by Aspergillus flavus in warm, humid storage of groundnuts, maize and other oilseeds. The toxins are heat-stable and can damage the liver, so prevention depends on rapid drying and low-humidity storage. Rhizopus, yeast and lactic acid bacteria do not produce aflatoxin.

Question 34Food Microbiology and Fermentation

A canned low-acid vegetable shows a sour taste but the can remains flat, not swollen. This flat sour spoilage is most commonly caused by:

  1. Clostridium botulinum
  2. Saccharomyces cerevisiae
  3. Aspergillus niger
  4. Bacillus coagulans Correct answer.

Explanation

Flat sour spoilage is caused by Bacillus coagulans, a thermophilic spore-former that produces acid without gas in underprocessed low-acid canned foods. The can therefore stays flat while the contents become sour. Clostridium botulinum usually causes swelling and toxin risk, while yeasts and moulds are not the typical flat sour agents.

Question 35Food Microbiology and Fermentation

The traditional holder method (LTLT) of pasteurizing milk uses which time–temperature combination?

  1. 63 °C for 30 minutes Correct answer.
  2. 100 °C for 5 minutes
  3. 72 °C for 15 seconds
  4. 121 °C for 15 minutes

Explanation

The holder, or low-temperature long-time (LTLT), method of milk pasteurization is defined as 63 °C for 30 minutes. 72 °C for 15 seconds is the high-temperature short-time (HTST) method, while 121 °C for 15 minutes is a sterilization cycle used for low-acid canned foods. Boiling at 100 °C for 5 minutes is not a standard pasteurization treatment for milk.

Question 36Food Microbiology and Fermentation

In sauerkraut production, the salt added to shredded cabbage mainly:

  1. raises the pH so that moulds can grow
  2. raises the water activity of the cabbage tissue
  3. provides a nitrogen source for lactic acid bacteria
  4. favours lactic acid bacteria and inhibits spoilage Correct answer.

Explanation

Salt draws water out of the cabbage by osmosis, lowering water activity and restricting many spoilage bacteria and moulds. The lactic acid bacteria naturally present tolerate this salt level and dominate the fermentation, producing acid that further preserves the product. Salt is therefore a selective agent rather than a nutrient.

Question 37Food Microbiology and Fermentation

The traditional starter culture used for setting dahi (yogurt) contains mainly:

  1. Lactobacillus bulgaricus and Streptococcus thermophilus Correct answer.
  2. Acetobacter aceti and Gluconobacter oxydans
  3. Saccharomyces cerevisiae and Rhizopus oligosporus
  4. Escherichia coli and Enterobacter aerogenes

Explanation

Dahi and yogurt are set by a symbiotic starter in which Lactobacillus delbrueckii subsp. bulgaricus produces acid and Streptococcus thermophilus contributes flavour and texture. These two species grow together and rapidly convert lactose to lactic acid. Coliforms, acetic acid bacteria and yeasts are not the traditional yogurt starter.

Question 38Food Microbiology and Fermentation

In cheese manufacture, the main function of rennet is to:

  1. convert lactic acid into acetic acid
  2. kill starter bacteria after curd formation
  3. coagulate casein and form the curd Correct answer.
  4. hydrolyse lactose into glucose and galactose

Explanation

Rennet contains chymosin, which cleaves kappa-casein and destabilises the casein micelles so that they coagulate into a curd. Lactose fermentation by starter bacteria produces acid, but that is a separate step from rennet coagulation. Rennet does not act on lactose or lactic acid.

Question 39Food Microbiology and Fermentation

A standard plate count of a ready-to-eat food is very high, but no pathogens are detected. What is the most appropriate interpretation?

  1. The food is safe because no pathogens were detected
  2. The high count proves that fermentation occurred
  3. The food is definitely unsafe and must be discarded
  4. It points to poor hygiene or temperature abuse Correct answer.

Explanation

A standard plate count measures the total viable aerobic mesophilic population, not the presence of pathogens. A high count signals general hygiene failure, temperature abuse or a short remaining shelf life, so the product should be investigated even though no pathogen was detected.

Question 40Food Microbiology and Fermentation

Acid foods with pH below 4.6 can be heat-processed at 100 °C rather than 121 °C mainly because:

  1. only yeasts and moulds can survive in such foods
  2. Clostridium botulinum spores cannot germinate at that pH Correct answer.
  3. the low pH destroys every bacterial spore during boiling
  4. water activity is always below 0.60 in acid foods

Explanation

Clostridium botulinum spores cannot germinate, grow and form toxin in foods with pH below 4.6, so a boiling heat treatment is sufficient for acid foods. Low-acid foods need 121 °C to inactivate those spores. The safety of acid foods depends on the pH restriction, not on complete spore destruction.

Question 41Food Microbiology and Fermentation

In industrial production of dahi, a starter culture is added to boiled, cooled milk mainly to:

  1. provide colour and flavour compounds directly
  2. produce lactic acid rapidly and inhibit spoilage Correct answer.
  3. increase the fat content of the product
  4. prevent any change in the milk during storage

Explanation

The starter converts lactose to lactic acid, lowering pH quickly; this acidity and the resulting gel suppress spoilage and pathogenic bacteria. Rapid acidification is also what gives dahi its characteristic set and sour taste. The culture does not increase fat or prevent all changes in the milk.

Question 42Food Microbiology and Fermentation

Fruit jam with pH 3.5 and 65% soluble solids is stable at room temperature mainly because:

  1. moulds are unable to grow at any pH below 7
  2. hot filling makes the jam commercially sterile
  3. all microorganisms are killed by added sugar
  4. low pH and high sugar lower water activity Correct answer.

Explanation

High sugar concentration and low pH act as hurdles: sugar binds water and lowers water activity, while acid prevents many bacteria from growing. The combined effect, not sterility, is what keeps the jam stable. Moulds can still spoil jam if it is exposed to air or diluted.

Question 43Food Microbiology and Fermentation

A fermented milk batch fails to acidify even though fresh starter was added, and the same starter works normally in another vat. The most likely cause is:

  1. bacteriophage attack on the starter culture Correct answer.
  2. addition of too much rennet to the milk
  3. an excessively high sugar level in the milk
  4. the milk was already fully fermented before filling

Explanation

Bacteriophages are viruses that lyse specific starter bacteria, so acid production suddenly stops while the same culture performs normally in a vat without the phage. Sanitation and rotation of starter strains are used to control this problem. A phage attack is a classic cause of slow or failed fermentation in dairy plants.

Question 44Food Microbiology and Fermentation

The presence of coliform bacteria in a pasteurized milk sample is used mainly as an indicator of:

  1. poor hygiene after pasteurization Correct answer.
  2. the fat and solids content of the milk
  3. adequate heating during pasteurization
  4. the vitamin and mineral content of the milk

Explanation

Coliforms are killed by proper pasteurization and are easy to detect, so their presence in pasteurized milk points to contamination after heating, such as poor cleaning of equipment or handling. They are not used to assess fat, vitamins or the adequacy of heating. Their detection therefore signals a hygiene failure in processing or packaging.

Question 45Food Microbiology and Fermentation

In industrial fermentation, citric acid is produced mainly by which microorganism?

  1. Aspergillus niger Correct answer.
  2. Saccharomyces cerevisiae
  3. Lactobacillus delbrueckii
  4. Acetobacter aceti

Explanation

Citric acid is produced industrially by submerged or solid-state fermentation using the filamentous fungus Aspergillus niger. The fungus secretes citric acid into the medium when nitrogen is limited and sugar is in excess. Yeasts, acetic acid bacteria and lactobacilli do not accumulate citric acid as their main fermentation product.

Question 46Food Microbiology and Fermentation

Traditional soy sauce fermentation proceeds through two main microbial stages. Which sequence correctly describes them?

  1. Aspergillus oryzae followed by yeasts and lactic acid bacteria Correct answer.
  2. Acetobacter aceti followed by Propionibacterium freudenreichii
  3. Penicillium camemberti followed by Brevibacterium linens
  4. Saccharomyces cerevisiae followed by Streptococcus thermophilus

Explanation

In traditional soy sauce making, steamed soybeans and wheat are first colonised by Aspergillus oryzae in the koji stage, which produces proteases and amylases. The koji is then mixed with brine to form moromi, where halophilic yeasts such as Zygosaccharomyces rouxii and lactic acid bacteria such as Tetragenococcus halophilus carry out the second fermentation. This two-stage process creates the amino acid-rich, salty, savoury liquid.

Question 47Food Microbiology and Fermentation

During ripening of Swiss cheese, the carbon dioxide that forms the characteristic eyes is produced mainly when Propionibacterium freudenreichii ferments which substrate?

  1. Citrate
  2. Lactose
  3. Lactate Correct answer.
  4. Butterfat

Explanation

Propionibacterium freudenreichii ferments lactate during the warm ripening phase of Swiss cheese, producing propionic acid, acetic acid and carbon dioxide. The carbon dioxide slowly diffuses and collects as bubbles, forming the characteristic eyes. Lactose is already largely consumed by the starter lactic acid bacteria before this ripening stage.

Question 48Food Microbiology and Fermentation

A bread dough made with a very high level of sugar ferments more slowly than a low-sugar dough using the same yeast and temperature. What is the most likely explanation?

  1. The sugar lowers dough pH and directly kills the yeast cells
  2. The dough has lost salt, which is needed for yeast growth
  3. Osmotic stress from high sugar reduces yeast activity Correct answer.
  4. The yeast switches from alcoholic fermentation to lactic acid fermentation

Explanation

A high concentration of sugar lowers water activity and draws water out of yeast cells, creating osmotic stress. This slows metabolism and gas production even when the yeast is viable. The effect is a common cause of sluggish fermentation in sweet doughs, not a change in the fermentation pathway.

Question 49Principles of Food Preservation and Processing

Which statement correctly describes water activity (aw) as used in food preservation?

  1. Ratio of the food's vapour pressure to that of pure water at the same temperature Correct answer.
  2. Ratio of bound water to free water present in the food matrix
  3. Mass of water that can be squeezed out of the food under standard pressure
  4. Total moisture content of the food, expressed as grams of water per 100 g of the product

Explanation

Water activity is an equilibrium property: it equals the vapour pressure of water in the food divided by that of pure water at the same temperature, so pure water has aw = 1.00. It measures the free water available for microbial growth and chemical reactions, which is why it predicts shelf life better than total moisture content. A food can be high in moisture yet have low aw, as in a sugar syrup.

Question 50Principles of Food Preservation and Processing

Before freezing green peas, the vegetable is usually blanched in hot water. The main purpose of this blanching step is to:

  1. Increase the water activity of the peas so that they freeze faster
  2. Destroy enzymes that cause off-flavours and colour loss in frozen storage Correct answer.
  3. Sterilise the peas by destroying all bacterial spores present on the surface
  4. Increase the salt content of the peas to improve their flavour after thawing

Explanation

Blanching exposes the vegetable to hot water or steam for a short time to denature enzymes such as peroxidase and polyphenol oxidase. If these enzymes remain active during frozen storage, they cause off-flavours, browning and loss of vitamins. Blanching is not a sterilisation step and does not destroy bacterial spores.