Question 1

Food microbiology — Colonies of Staphylococcus aureus grown on Baird–Parker agar are typically associated with which confirmatory property?

Accepted Answer

Introduction / Context: Baird–Parker agar is a selective–differential medium commonly used to isolate presumptive Staphylococcus aureus from foods. It contains tellurite and egg yolk; S. aureus typically forms black, shiny colonies due to tellurite reduction, often with clear zones from lipolytic/lecithinase activity. However, laboratory identification requires a confirmatory test associated with this species: the coagulase test. Given Data / Assumptions: We are dealing with “suspected” colonies on Baird–Parker medium. Final confirmation relies on biochemical properties characteristic of S. aureus. Concept / Approach: Most clinically and food-borne significant S. aureus strains are coagulase-positive, differentiating them from many coagulase-negative staphylococci. While catalase is positive in all staphylococci (and thus non-discriminatory), and protease activity is not a standard confirmatory trait, coagulase is the classical confirmatory test paired with Baird–Parker isolation. Step-by-Step Solution: Use Baird–Parker for selective isolation (appearance: black colonies, clear halo).Apply tube or slide coagulase test to suspected colonies.Confirm identity: coagulase positivity supports S. aureus. Verification / Alternative check: Food microbiology standards specify coagulase testing (and often latex agglutination for clumping factor/protein A) to confirm S. aureus from Baird–Parker plates. Why Other Options Are Wrong: Protease activity: not a routine confirmatory hallmark for S. aureus. Catalase activity: shared by nearly all staphylococci; not specific. None of these: incorrect; coagulase is the classical confirmatory marker. Common Pitfalls: Assuming colony appearance alone is definitive; always run coagulase or equivalent confirmatory tests. Final Answer: coagulase activity

Question 2

Thermal resistance of bacteria — Identify the statement that is NOT true (i.e., false) about how bacterial cell traits affect heat resistance.

Accepted Answer

Introduction / Context: Heat resistance of bacteria varies with morphology, physiology, and cell envelope composition. Recognizing which features increase resistance guides process selection in food preservation, sterilization, and healthcare decontamination. Given Data / Assumptions: We compare shape (cocci vs rods), temperature adaptation, clumping/capsules, and lipid content effects. We are looking for the one statement that is NOT true. Concept / Approach: Gram-positive cocci often exhibit higher resistance than many Gram-negative rods. Thermophiles and organisms adapted to high temperatures usually have more thermostable proteins/membranes, correlating with higher resistance. Clumping or capsules impede heat transfer and protect inner cells. In contrast, high lipid content (for example, in mycobacteria) tends to increase resistance to many agents; therefore, saying “cells low in lipid content are harder to kill” is contrary to general trends and is the false statement. Step-by-Step Solution: Evaluate cocci vs rods: cocci commonly more heat-resistant → true.Evaluate growth temperature adaptation: thermophiles more resistant → true.Evaluate clumping/capsules: protective effects → true.Evaluate lipid content claim: low-lipid cells harder to kill → false. Verification / Alternative check: Food microbiology texts cite increased resistance in organisms with waxy, lipid-rich envelopes (e.g., mycobacteria) and in thermophiles; process lethality tables reflect these differences. Why Other Options Are Wrong: Cocci more resistant: consistent with many comparative studies. Higher optimal growth temperature → higher resistance: aligns with thermophile biology. Clumping/capsules: physical shielding from heat and chemicals. Common Pitfalls: Overgeneralizing across all taxa; exceptions exist, but the false statement here is the one about low lipid content increasing resistance. Final Answer: Cells low in lipid content are harder to kill than other cells

Question 3

Food preservation chemistry — At a given pH, which weak organic acid typically exerts the stronger bacteriostatic effect due to a higher fraction of undissociated molecules penetrating cells?

Accepted Answer

Introduction / Context: Weak organic acids inhibit microbes primarily in their undissociated form. These neutral molecules diffuse across cell membranes; once inside the higher-pH cytoplasm, they dissociate, acidifying the cytosol and collapsing proton gradients. Food preservative effectiveness therefore depends on pH, pKa, and lipophilicity of the acid. Given Data / Assumptions: All options are organic acids used or encountered in foods. “At a given pH” means comparing relative undissociated fractions and inhibitory power under the same acidity. We focus on generally observed preservative performance in foods. Concept / Approach: For a fixed pH, the Henderson–Hasselbalch relation shows that acids with pKa closer to the pH have higher undissociated fractions. Acetic acid (pKa ~4.76) often yields a substantial undissociated fraction in the mildly acidic range common to many foods (pH ~3.5–5.5) and also exhibits favorable membrane partitioning, giving strong antimicrobial action. Polycarboxylic acids like citric and tartaric, while acidulants and chelators, usually show lower bacteriostatic potency per mole at the same pH. Maleic acid behavior is more complex; in typical food systems acetic acid remains the benchmark household preservative (vinegar) for bacteriostasis. Step-by-Step Solution: Hold pH constant and compare acids by undissociated fraction and lipophilicity.Recognize acetic acid’s track record as an effective inhibitor in pickles, sauces, and dressings.Select acetic acid as the stronger bacteriostatic agent under common food pH conditions. Verification / Alternative check: Food preservation literature consistently documents robust inhibition of many bacteria by vinegar (acetic acid) at culinary pH, outperforming citric/tartaric in comparable conditions for many spoilage organisms. Why Other Options Are Wrong: Tartaric acid: effective acidulant/chelator but generally less potent bacteriostat than acetic at equal pH. Citric acid: good flavoring/chelator; antimicrobial mainly via pH reduction and metal binding, not as bacteriostatic as acetic at the same pH. Maleic acid: not a standard food preservative; practical use is limited. Common Pitfalls: Ignoring the role of pH and assuming “stronger acid = better preservative”; what matters is undissociated fraction and membrane permeability. Final Answer: Acetic acid

Question 4

Food spoilage microbiology: At which hydrogen-ion concentration (pH range) do most spoilage bacteria exhibit optimum growth in foods?

Accepted Answer

Introduction / Context: Most bacterial food spoilers are neutrophiles. Knowing their preferred pH helps processors design effective hurdles (acidification, fermentation, pickling) to slow or prevent bacterial growth and extend shelf life. Given Data / Assumptions: Bacterial spoilers include Pseudomonas, Enterobacteriaceae, many Bacillus species. Typical optimal growth for neutrophiles is close to pH 6.5–7.5. Yeasts and moulds tolerate lower pH better than most bacteria. Concept / Approach: Bacteria maintain intracellular pH homeostasis but still show strongest growth where external pH is near neutral. Acidic foods (e.g., fruit juices, pickles) naturally select against many bacteria while allowing yeasts/moulds or acid-tolerant bacteria (e.g., lactic acid bacteria) to dominate. Alkaline conditions are uncommon in foods and also inhibitory to many microbes. Step-by-Step Solution: Identify major spoilage groups in protein-rich, fresh foods (meats, milk) and their pH optima. Recognize that these commodities are near-neutral, matching bacterial preferences. Contrast with acidic foods where bacteria are suppressed and fungi prevail. Select the option corresponding to neutral pH. Verification / Alternative check: Shelf-life extension strategies commonly lower pH below about 4.6 to inhibit pathogens and many spoilers; this aligns with neutrophile physiology and regulatory cutoffs used for acidified foods. Why Other Options Are Wrong: Acidic/alkaline/any pH – many bacteria are inhibited outside neutral ranges; “any pH” ignores physiological limits. Strongly acidic ≤ 3.0 is tolerated mostly by fungi, not routine bacterial spoilers. Common Pitfalls: Assuming all microbes prefer neutral pH; acidophiles exist but are not the dominant spoilers of most fresh foods. Final Answer: Neutral pH.

Question 5

Food hygiene methods: For detecting and enumerating coliform bacteria in foods, which culture media are preferably employed in routine microbiological examination?

Accepted Answer

Introduction / Context: Coliform testing is a cornerstone of food quality monitoring because coliforms indicate hygiene lapses and potential fecal contamination. Several selective/differential media are validated for screening, confirmation, and enumeration. Given Data / Assumptions: Presumptive tests often use lactose broths (e.g., MacConkey) with indicators. Plate counts for coliforms commonly use VRBA or EMB. Methods depend on matrix and regulatory standard (e.g., ISO, BAM). Concept / Approach: MacConkey broth supports presumptive lactose fermentation with bile salts and indicator; VRBA is selective/differential for coliform colony counts; EMB differentiates lactose-fermenting coliforms with characteristic colonies (e.g., E. coli metallic sheen). In practice, labs may use combinations for presumptive, confirmatory, and completed tests. Step-by-Step Solution: Map each medium to stage: presumptive (broth), selective plate (VRBA), differential plate (EMB). Confirm that all are standard for coliform work. Choose the inclusive option. Verification / Alternative check: Official methods outline multiple acceptable media; selection depends on whether the goal is presence/absence, MPN, or colony counts. Why Other Options Are Wrong: Single-media choices are incomplete; “None” contradicts common laboratory practice. Common Pitfalls: Assuming a single medium suffices for all food types; fat, acids, or preservatives may require method adjustments. Final Answer: All of these.

Question 6

Aerobic Colony Count (ACC) standards: Differences in ACC limits between food categories primarily reflect which underlying considerations?

Accepted Answer

Introduction / Context: ACC (Aerobic Colony Count) targets are risk-based. Regulators and QMS teams set category-specific limits recognizing that raw materials, process lethality, and storage conditions differ widely among foods. Given Data / Assumptions: ACC is a general hygiene indicator rather than a specific pathogen test. Intrinsic (pH, aw) and extrinsic (temperature, atmosphere) factors vary by category. Shelf life expectations influence acceptable baseline counts. Concept / Approach: A high-heat, low-aw shelf-stable product can tolerate a higher initial ACC than a ready-to-eat refrigerated salad with long storage potential. Raw inputs (e.g., raw meats vs aseptic ingredients) dictate starting contamination. Growth potential during distribution further shapes limits: foods supporting growth demand lower ACCs at release. Step-by-Step Solution: Link raw material contamination to starting ACC. Assess growth potential under intended storage (chill chain, MAP, freezing). Consider shelf life and spoilage tolerance for sensory quality and safety. Select the inclusive answer recognizing all three drivers. Verification / Alternative check: Category plans in HACCP/ISO systems stratify ACC limits by product risk profile and shelf-life studies. Why Other Options Are Wrong: Choosing only one factor ignores the multivariate basis of specifications; packaging size alone is rarely determinative. Common Pitfalls: Treating ACC as a pass/fail pathogen test; it is a trendable hygiene quality metric. Final Answer: All of the above.

Question 7

Hurdle technology – effect of salt (NaCl): In food preservation, sodium chloride can behave in which ways relative to microbial inhibition, depending on concentration and context?

Accepted Answer

Introduction / Context: Salt is a classic preservation hurdle that lowers water activity and can synergize with acidity, nitrite, or refrigeration. However, at modest levels, compatible solutes accumulated by microbes may blunt its impact. Understanding these interactions explains why recipes and standards specify precise salt levels. Given Data / Assumptions: NaCl lowers aw and imposes osmotic stress. Microbes can adapt at low to moderate salt via osmoprotection. High concentrations make cytoplasmic regulation energetically costly or impossible. Concept / Approach: At “optimal” levels for product quality but below strong inhibitory thresholds, NaCl may antagonize the action of other hurdles by triggering stress responses (e.g., osmolyte uptake) that incidentally protect cells, reducing the net effect. When NaCl exceeds the optimum, osmotic pressure strongly restricts growth, often synergizing with low pH, nitrite, or chill to achieve greater-than-additive inhibition. Step-by-Step Solution: Relate NaCl concentration to aw reduction and microbial adaptation. Explain how moderate salt can induce cross-protection, appearing antagonistic. Show that higher salt pushes cells past adaptation capacity, enhancing other hurdles (synergy). Therefore select the option acknowledging both behaviors. Verification / Alternative check: Challenge studies in cured meats and brines show steeper kill or lag extension when NaCl is combined at higher levels with acid/nitrite than predicted by single hurdles alone. Why Other Options Are Wrong: “None” or “neutral only” ignores abundant evidence of salt-hurdle interactions; picking only one behavior oversimplifies concentration dependence. Common Pitfalls: Assuming any amount of salt is uniformly inhibitory; microbial adaptation can mask effects at low to moderate levels. Final Answer: Both (a) and (b).

Question 8

Water activity (aw) in food safety: In risk assessments and process design, water activity functions as which type(s) of factor influencing microbial proliferation and product stability?

Accepted Answer

Introduction / Context: Water activity (aw) is one of the most powerful levers for controlling microbial growth. It is central to product formulation, process choice, and storage environment design. Given Data / Assumptions: aw is the effective availability of water for microbial use. Food formulation and processing can reduce aw. Storage conditions (ambient relative humidity) can change aw over time. Concept / Approach: As an intrinsic factor, aw reflects ingredients and structure (solutes, humectants, binding). As a processing factor, operations like drying, salting, or adding sugars adjust aw to target levels. As an extrinsic factor, the surrounding RH determines moisture exchange; equilibrated aw tracks environmental RH, so packaging and storage climate are critical. Step-by-Step Solution: Define aw and link to microbial minimums (e.g., most bacteria require aw ≥ ~0.90; moulds can grow lower). Show how processing deliberately sets aw (drying, concentration, curing). Explain environmental coupling: aw changes toward RH equilibrium if poorly packaged. Therefore choose the inclusive option: all of the above. Verification / Alternative check: Shelf-life failures often trace to moisture pickup or loss during storage, confirming the extrinsic dimension of aw control. Why Other Options Are Wrong: Limiting aw to a single category overlooks its cross-cutting role from formulation through distribution. Common Pitfalls: Confusing moisture content with aw; equal moisture does not mean equal microbial risk if solute binding differs. Final Answer: All of the above.

Question 9

Enumeration of microorganisms in foods: What does the term “enumeration” refer to in standard food microbiology plate-count methods?

Accepted Answer

Introduction / Context: Enumeration yields quantitative results (CFU/g or CFU/ml). Different plating techniques are chosen based on expected counts, matrix type, and whether selectivity is required to suppress background flora. Given Data / Assumptions: Validated techniques include pour plate, spread plate, and spiral plating. Medium choice (selective vs non-selective) depends on the analyte (e.g., total aerobic count vs coliforms). Serial dilutions and replicates improve accuracy. Concept / Approach: For total aerobic counts, non-selective media (e.g., Plate Count Agar) are used. For target groups (e.g., coliforms, Staphylococcus aureus), selective/differential media are applied. Spiral plating automates precise deposition for high-throughput enumeration with reduced agar usage. Step-by-Step Solution: Define enumeration as CFU-based quantitation via culture. List core plating methods used to enumerate. Tie medium selectivity to test purpose. Select the comprehensive option reflecting both method and media choice. Verification / Alternative check: Regulatory compendia describe acceptable variants for enumeration with equivalent performance criteria (countable range, incubation conditions). Why Other Options Are Wrong: Selective-only or non-selective-only definitions are too narrow; microscopy without culture is not standard “enumeration” in this context. Common Pitfalls: Counting plates outside 25–250 CFU range; inaccurate dilutions compromise results. Final Answer: Spiral plating, pour plate, or spread plate of a food suspension onto a suitable selective or non-selective agar, depending on the test objective.

Question 10

Presence/absence testing in foods: In qualitative assays, “examination of the presence/absence of organisms” most correctly refers to which workflow?

Accepted Answer

Introduction / Context: Presence/absence tests are qualitative. They aim to detect very low levels of target pathogens or indicators by first enriching them to detectable levels, then isolating on selective/differential media for confirmation. Given Data / Assumptions: Food matrices often contain low pathogen numbers and high background flora. Enrichment increases target concentration. Selective media suppress competitors and reveal characteristic colonies. Concept / Approach: A typical workflow is sample homogenization → non-selective or selective enrichment → selective plating → confirmation (biochemical/serological/molecular). This yields a qualitative result (Detected/Not detected) per sample size, aiding release decisions and corrective actions. Step-by-Step Solution: State the objective: detection at low levels, not quantitation. Justify enrichment to resuscitate injured cells and amplify targets. Apply selective plating to differentiate targets from background. Select the option that captures enrichment followed by selective isolation. Verification / Alternative check: Standards for Salmonella, Listeria, and E. coli O157 routinely include primary/secondary enrichments prior to selective plating and confirmatory tests. Why Other Options Are Wrong: Non-selective-only plates permit overgrowth; enrichment without isolation is incomplete; “none” and direct PCR omit the culture-based definition asked here. Common Pitfalls: Skipping selective plating after enrichment can yield ambiguous flora and false negatives/positives. Final Answer: Incubating a food suspension in an enrichment medium followed by inoculation onto a suitable selective medium.

Question 11

Pasteurization target organism: The HTST condition of 71.1°C for 15 seconds (high-temperature short-time) is established primarily on the heat resistance of which organism?

Accepted Answer

Introduction / Context: Milk pasteurization time–temperature combinations are chosen to inactivate the most heat-resistant non-spore-forming pathogen reasonably expected in milk. This ensures safety while preserving quality. Given Data / Assumptions: HTST: 71.1°C for 15 s (also written 72°C for 15 s in many jurisdictions). Target is the most heat-resistant vegetative pathogen in raw milk. Spore-formers are not completely inactivated by pasteurization. Concept / Approach: Coxiella burnetii (agent of Q fever) exhibits relatively high thermal resistance among vegetative pathogens found in milk. Designing the HTST process against C. burnetii ensures a wide margin against other vegetative pathogens such as Listeria, Salmonella, and pathogenic E. coli, which are less heat resistant at these conditions. Step-by-Step Solution: Identify the heat-resistance benchmark organism for milk. Differentiate vegetative cells from spores (e.g., Bacillus, Clostridium). Confirm that pasteurization aims at vegetative pathogens, not spores. Select Coxiella burnetii as the basis. Verification / Alternative check: Thermal death data show D-values for C. burnetii higher than other common milkborne vegetative pathogens, supporting its use as the design target. Why Other Options Are Wrong: E. coli and Listeria are less heat resistant; Bacillus/Clostridium spores survive pasteurization and require sterilization for destruction. Common Pitfalls: Assuming pasteurization equals sterilization; shelf-stable UHT uses higher temperatures specifically to inactivate spores. Final Answer: Coxiella burnetii.

Question 12

ISO 2002 Salmonella method updates: Which statements correctly reflect changes in the ISO 2002 method for Salmonella detection in foods?

Accepted Answer

Introduction / Context: Standard methods evolve to improve recovery of stressed Salmonella and reduce background growth. The ISO 2002 revisions modernized enrichment and isolation steps to increase sensitivity and specificity across diverse foods. Given Data / Assumptions: Pre-enrichment and selective enrichment broths are critical for resuscitating injured cells. Selective agars differentiate Salmonella colonies from competing flora. The question lists three procedural updates. Concept / Approach: MKTTn provides improved selectivity over SC while recovering stressed cells; RVS refines the RV formulation with soya peptone to enhance growth of sublethally injured Salmonella; XLD as the primary isolation agar offers clearer differentiation (hydrogen sulfide production on a contrasting background) compared to BGA in many matrices. Step-by-Step Solution: Evaluate each statement versus ISO 2002 method details. Confirm replacements: SC → MKTTn and RV → RVS to enhance selectivity/recovery. Confirm XLD as first-choice isolation medium. Select “All of these.” Verification / Alternative check: Validation studies show improved detection rates using MKTTn/RVS and XLD for first isolation, with confirmatory testing on secondary media. Why Other Options Are Wrong: Single statements miss the holistic change set; “None” contradicts documented method updates. Common Pitfalls: Using legacy SC/RV/BGA alone may under-recover stressed cells from processed foods. Final Answer: All of these.

Question 13

Yeast and mould enumeration: Which culture medium is specifically recommended for determining yeast and mould counts in foods (after suitable dilution)?

Accepted Answer

Introduction / Context: Yeasts and moulds are significant spoilers in acidic, low-aw, or refrigerated foods. Enumeration guides shelf-life predictions, sanitation decisions, and ingredient acceptance. Given Data / Assumptions: Fungal counts require media that favor yeasts/moulds and suppress bacteria. Acidification (e.g., to pH ~3.5–4.0) inhibits many bacteria while allowing fungi to grow. Potato-based media support robust fungal growth. Concept / Approach: Acidified potato glucose/dextrose agar (APGA/APDA) or other acidified fungal media are standard for YM counts. The potato infusion provides nutrients, glucose supports growth, and low pH suppresses bacterial background. Some protocols add antibiotics as further bacterial inhibitors when needed. Step-by-Step Solution: Select a medium formulated for fungi and acidified to inhibit bacteria. Recognize APGA/APDA as widely used for YM counts. Exclude bacterial-selective media (MacConkey, VRBA, XLD) and general nutrient agar. Choose acidified potato glucose agar. Verification / Alternative check: Compendial methods list acidified PDA or Dichloran Rose Bengal Chloramphenicol agar for YM enumeration, both designed to suppress bacterial growth and limit mould spread. Why Other Options Are Wrong: Nutrient agar favors bacteria; MacConkey/VRBA/XLD are for enteric bacteria and coliforms, not fungi. Common Pitfalls: Failing to acidify or add antibiotics leads to bacterial overgrowth and underestimation of fungal counts. Final Answer: Acidified potato glucose (dextrose) agar.

Question 14

Environmental microbiology classification: A microbe described as a psychrophilic halophile would preferentially grow under which combined set of conditions?

Accepted Answer

Introduction / Context: Microbes are often classified by their preferred physical and chemical environments. Two important ecological labels are temperature preference (psychrophile, mesophile, thermophile) and osmotic tolerance (halophile for salt-loving). The term “psychrophilic halophile” combines both concepts and is frequently tested in food microbiology, marine microbiology, and extremophile ecology. Given Data / Assumptions: “Psychro-” indicates cold-loving, with optimal growth typically at or below about 15 °C and the ability to grow near 0 °C. “Halophile” indicates salt-loving, requiring elevated NaCl compared with typical freshwater environments. We must choose the option that pairs low temperature with high salinity. Concept / Approach: Psychrophiles possess membrane lipids and enzymes adapted to function at low temperatures, preventing loss of membrane fluidity and kinetic slowdown. Halophiles accumulate compatible solutes or use “salt-in” strategies to balance osmotic pressure. A microbe that is both psychrophilic and halophilic prefers simultaneously cold conditions and elevated salt concentrations, such as those found in polar saline lakes, sea ice brines, or refrigerated salted foods. Step-by-Step Solution: Parse the compound term: “psychrophilic” = cold; “halophile” = salt-loving. Match descriptors to environmental factors: temperature and salinity. Identify the option that correctly couples low temperature with high salt. Select “cold temperatures and increased amounts of salt.” Verification / Alternative check: Ecology texts and food safety guidance list halophilic–psychrophilic spoilers in salted, chilled products; growth at refrigeration temperatures with high NaCl confirms the definition. Why Other Options Are Wrong: Warm temperatures & pressure/acid: Describe thermophiles or piezophiles/acidophiles, not psychrophilic halophiles. Cold & absence of oxygen: That indicates psychrophilic anaerobes, not necessarily halophiles. Moderate temperature & neutral salinity: Typical mesophilic, non-halophilic conditions. Common Pitfalls: Confusing “psychro-” with “psycho-” or assuming halophiles automatically prefer heat; temperature and salinity preferences are independent traits. Final Answer: Cold temperatures and increased amounts of salt.

Question 15

Food microbiology methods: For standard plate counts of bacteria in foods, the plating medium generally consists of which basic composition?

Accepted Answer

Introduction / Context: Plate count methods estimate viable bacterial loads in foods. The medium used must be non-selective and nutritive enough for a wide range of heterotrophs while avoiding unnecessary inhibitors. A classic choice is Plate Count Agar (PCA), also called Standard Methods Agar, widely used in dairy, meat, and ready-to-eat food testing. Given Data / Assumptions: We need the typical base composition for plate count media used in foods. Core components are a digest (peptone/tryptone), a vitamin source (yeast extract), a fermentable carbohydrate (glucose/dextrose), a solidifying agent (agar), and water. Added sodium chloride is not essential in the standard formula. Concept / Approach: Standard PCA contains peptone (or tryptone) for amino nitrogen, yeast extract for growth factors, and glucose as an energy source, with agar for solidification. Sodium chloride is not a required ingredient and may alter osmotic balance or selectivity unnecessarily for routine total counts. Therefore, the correct composition is the formula that includes peptone, yeast extract, glucose, agar, and water only. Step-by-Step Solution: Identify essential nutrients: peptone + yeast extract + glucose. Include agar to make plates; distilled water is the solvent. Exclude nonessential NaCl for standard PCA to avoid unintended selectivity. Choose the option listing peptone, yeast extract, glucose, agar, and water only. Verification / Alternative check: Common laboratory manuals list PCA as tryptone/peptone + yeast extract + glucose + agar + water; NaCl appears in many selective media but is not a staple of PCA. Why Other Options Are Wrong: Options a–d: Either add NaCl unnecessarily, omit a key nutrient (peptone or yeast extract), or remove agar, which is essential for plating. Common Pitfalls: Confusing PCA with Nutrient Agar variants or selective media that do include NaCl; for broad plate counts in foods, the simpler PCA base is preferred. Final Answer: Peptone, yeast extract, glucose (dextrose), agar, and distilled water (no added sodium chloride).

Question 16

Food substrate properties: Which of the following are intrinsic (food-inherent) factors that govern microbial growth in foods?

Accepted Answer

Introduction / Context: In food microbiology, microbial growth depends on both intrinsic (food-inherent) and extrinsic (environmental) factors. Recognizing intrinsic factors is essential for designing preservation strategies and predicting spoilage risk for different commodities. Given Data / Assumptions: Intrinsic factors are those inherent to the food matrix itself. Candidate factors include pH, moisture (water activity), and redox potential (Eh). We must decide whether all listed features qualify as intrinsic. Concept / Approach: Key intrinsic factors include pH (acid–base status), water activity (available water for metabolism), redox potential (electron availability), nutrient content, natural antimicrobials, and biological structures (peels, rinds). All three listed factors—pH, moisture as aw, and Eh—are classic intrinsic determinants of which organisms can grow and at what rate within a given food. Step-by-Step Solution: Identify the nature of each factor: pH, aw, and Eh are properties of the food, not the storage environment. Relate each factor to microbial physiology: acid tolerance, osmotic stress, and respiratory metabolism. Since all are intrinsic, select the inclusive option. Verification / Alternative check: Textbook models such as the “hurdle” concept explicitly list pH, aw, and Eh among intrinsic hurdles that suppress or permit growth, often combined with extrinsic controls like temperature and modified atmospheres. Why Other Options Are Wrong: Single-factor options are incomplete; “None of these” contradicts established theory. Common Pitfalls: Confusing extrinsic factors (e.g., storage temperature, relative humidity, gas atmosphere) with intrinsic ones; here, only food-inherent properties are considered. Final Answer: All of these are intrinsic factors.

Question 17

Indicator counts in foods: The Aerobic Colony Count (ACC) is also commonly referred to by which other names?

Accepted Answer

Introduction / Context: Routine hygiene monitoring in foods uses general indicator counts to estimate the overall microbial load. The Aerobic Colony Count (ACC) is widely used and appears in literature under multiple synonymous names, which can be confusing for students and practitioners. Given Data / Assumptions: ACC is performed by plating serial dilutions on non-selective media and incubating aerobically. Synonyms include TVC, APC, and SPC in various standards or regions. We must select the set of names that match ACC. Concept / Approach: ACC, APC (Aerobic Plate Count), TVC (Total Viable Count), and SPC (Standard Plate Count) are often used interchangeably to describe counts of aerobic, culturable bacteria under specified conditions. While minor methodological differences can exist among standards, the terms broadly refer to the same general microbiological index of product hygiene and handling history. Step-by-Step Solution: Match each acronym to ACC usage in food standards. Recognize that all three are conventional synonyms for generic aerobic counts. Select the inclusive option “All of these.” Verification / Alternative check: Regulatory documents and compendia (e.g., ISO/ICMSF methods) routinely cross-reference these terms for general aerobic counts in foods and ingredients. Why Other Options Are Wrong: Coliform index only: Coliform counts are specific indicator tests, not synonyms for ACC. Common Pitfalls: Assuming that TVC always includes strict anaerobes; by method, ACC/APC typically measures aerobically cultivable flora under set incubation conditions. Final Answer: All of these (TVC, APC, SPC) are synonymous with ACC in common usage.

Microbiology of Foods Questions