Question 1

In vinegar technology, “spirit vinegar” (also called distilled white vinegar) is produced by oxidizing which starting material as the ethanol source?

Accepted Answer

Introduction / Context:Vinegar is an aqueous solution of acetic acid produced by the biological oxidation of ethanol. While cider vinegar and wine vinegar start from fermented fruit juices or wine, “spirit vinegar” specifically uses neutral ethanol as the substrate for acetic acid bacteria, producing a clear, colorless product of consistent quality. Given Data / Assumptions: Product: spirit vinegar (distilled white vinegar). Organisms: Acetic acid bacteria (Acetobacter/Gluconobacter). Process: aerobic oxidation of ethanol to acetic acid in acetators. Concept / Approach:The defining feature of spirit vinegar is its ethanol origin. Distilled or rectified spirit is diluted and fed to acetators for controlled oxidation. This differs from varietal vinegars, which derive ethanol via fermentation of fruit or grain mashes prior to acetification, often retaining flavor compounds from the original substrate. Step-by-Step Solution: Clarify product type: spirit vinegar is standardized and colorless.Identify substrate: neutral ethanol diluted to the required strength (commonly 6–12% v/v before acetification).Note acetic acid bacteria convert ethanol to acetic acid under aerobic conditions.Select ethanol as the correct starting material. Verification / Alternative check:Process descriptions for generator and submerged fermentation acetators denote ethanol feed for spirit vinegar production. Why Other Options Are Wrong: Fruit juices/malted grain/ale: those produce cider/beer/malt-based vinegars, not spirit vinegar. Wine with glycerol: not the definition of spirit vinegar and glycerol is not a substrate for acetic acid formation. Common Pitfalls:Confusing “spirit vinegar” with any vinegar; assuming all vinegars start from fruit or grain ferments. Final Answer:Ethanol (neutral spirit/rectified spirit)

Question 2

In downstream processing of citric acid fermentations, calcium hydroxide [Ca(OH)2] is added to the fermentation broth (slurry). What is the purpose of this step?

Accepted Answer

Introduction / Context:Commercial citric acid production with Aspergillus niger typically employs a precipitation and reconversion route. After fermentation, citric acid is recovered by forming an insoluble salt (calcium citrate), which is then treated with mineral acid to release citric acid and remove calcium as gypsum. Given Data / Assumptions: Broth contains citric acid and impurities. Ca(OH)2 addition is controlled by pH and stoichiometry. Subsequent step uses sulfuric acid to regenerate free citric acid. Concept / Approach:Ca(OH)2 reacts with citric acid to form sparingly soluble calcium citrate. The solid can be filtered, washed, and then “acid cracked” with H2SO4 to produce free citric acid while precipitating CaSO4, simplifying purification. Step-by-Step Solution: Add Ca(OH)2 until calcium citrate precipitates.Filter and wash the calcium citrate to remove mother liquor impurities.Treat with H2SO4: calcium citrate + H2SO4 -> citric acid (in solution) + CaSO4 (solid).Filter off gypsum and proceed to concentration/crystallization of citric acid. Verification / Alternative check:Standard flowsheets for citric acid recovery show the calcium citrate–gypsum route as a classic industrial method. Why Other Options Are Wrong: Calcium carbonate/phosphate precipitation is unrelated to citric acid capture. Direct gypsum formation from broth without the intermediate is not the practiced sequence. Alkalinity alone does not sterilize nor recover product. Common Pitfalls:Confusing the two precipitation steps (calcium citrate first, gypsum later); overliming leading to product loss. Final Answer:To precipitate calcium citrate, allowing later acid cracking to free citric acid

Question 3

During aerobic acetification, which bacteria are responsible for oxidizing ethanol to acetic acid in vinegar production?

Accepted Answer

Introduction / Context:Vinegar manufacture relies on acetic acid bacteria (AAB) to oxidize ethanol to acetic acid under aerobic conditions. Two principal genera used industrially are Acetobacter and Gluconobacter, each with differing oxidative capacities and membrane-bound dehydrogenase systems. Given Data / Assumptions: Substrate: ethanol in aqueous solution. Process: strictly aerobic, temperature- and aeration-controlled. End product: acetic acid with characteristic vinegar organoleptics. Concept / Approach:AAB perform incomplete oxidation. Ethanol is oxidized to acetaldehyde and then to acetic acid via membrane-bound alcohol and aldehyde dehydrogenases, with oxygen as the terminal electron acceptor. Both Acetobacter and Gluconobacter contribute to acetification; strain choice depends on process (surface vs submerged) and desired kinetics. Step-by-Step Solution: Identify the functional group: acetic acid bacteria (AAB).Recognize industrially relevant genera: Acetobacter and Gluconobacter.Exclude lactic acid bacteria and obligate anaerobes (Clostridium) not responsible for acetification.Select “Both Acetobacter and Gluconobacter species.” Verification / Alternative check:Textbook descriptions of Orleans, generator, and submerged processes cite both genera as key acetifiers. Why Other Options Are Wrong: Single-genus options omit the other major contributor. Lactobacillus produces lactic acid, not acetic acid from ethanol. Clostridium requires anaerobic conditions, contrary to vinegar production. Common Pitfalls:Assuming acetification is possible anaerobically; overlooking Gluconobacter’s role in incomplete oxidation pathways. Final Answer:Both Acetobacter and Gluconobacter species

Question 4

Vinegar manufacture typically involves two bioprocess stages. Which sequence correctly describes the overall production path from sugars to acetic acid?

Accepted Answer

Introduction / Context:Vinegar production couples two distinct microbial processes: yeast-driven ethanolic fermentation and bacterial oxidation. Understanding the order of these steps is fundamental to designing and operating a vinegar plant efficiently. Given Data / Assumptions: Sugars from fruit/grain are first converted to ethanol by yeasts. Acetic acid bacteria subsequently oxidize ethanol to acetic acid in the presence of oxygen. Final product is an aqueous acetic acid solution with characteristic flavor. Concept / Approach:Yeast fermentation of sugars is largely anaerobic (facultative metabolism), maximizing ethanol yield. Acetification is strictly aerobic, requiring efficient oxygen transfer for rapid conversion of ethanol to acetic acid and to avoid overoxidation losses. Step-by-Step Solution: Stage 1: Anaerobic or low-aeration fermentation by Saccharomyces to produce ethanol.Stage 2: Aerobic oxidation by Acetobacter/Gluconobacter converting ethanol to acetic acid.Therefore the correct chronological order is anaerobic then aerobic. Verification / Alternative check:All traditional (Orleans) and modern (submerged) processes follow this two-stage biology, confirming the sequence. Why Other Options Are Wrong: Aerobic only or anaerobic only omit one essential step. Aerobic followed by anaerobic reverses the real order. Pure chemical synthesis is not the standard for food-grade vinegar. Common Pitfalls:Confusing oxygen requirements of yeast versus acetic acid bacteria; overlooking that excess aeration in stage 1 lowers ethanol yield. Final Answer:Anaerobic fermentation followed by aerobic oxidation

Question 5

Citric acid via the koji (solid-state) process: which statement accurately describes this production method using Aspergillus niger?

Accepted Answer

Introduction / Context:Citric acid can be produced by Aspergillus niger using different cultivation modes. The “koji” process is a classic solid-state/surface method where the mold grows on moist solids rather than submerged liquid, historically including substrates like sweet potato or cassava. Given Data / Assumptions: Organism: Aspergillus niger. Process variant: koji (solid-state) versus submerged fermentation. Substrate examples: sweet potato, wheat bran, cassava. Concept / Approach:Koji refers to a solid-state culture where filamentous fungi develop on the surface and within a moist solid substrate with controlled humidity and aeration. This contrasts with submerged/deep-tank processes where mycelium is suspended in liquid and oxygen transfer is via sparging/agitation. Step-by-Step Solution: Define koji: solid-state growth on moist solids.Identify correct statement: A. niger on sweet potato (or similar) fits the definition.Exclude floating growth or deep-tank mycelial suspension as submerged culture, not koji.Therefore, choose the option describing solid-state cultivation. Verification / Alternative check:Historical industrial routes and textbooks consistently classify sweet potato–based A. niger cultivation as a koji/solid-state process for citric acid. Why Other Options Are Wrong: Floating surface growth in liquid is surface/submerged, not solid-state koji. Deep-tank growth describes submerged fermentation, not koji. Koji relies on fungi; bacteria are not “only” used. Common Pitfalls:Mixing up surface fermentation in shallow liquid with true solid-state koji; assuming all citric acid today is submerged (solid-state remains relevant historically and for specific contexts). Final Answer:A. niger is grown on a moist solid substrate (e.g., sweet potato) in a surface/solid-state system

Question 6

In the classic calcium citrate route for citric acid recovery, the precipitated calcium citrate is treated with which mineral acid to regenerate free citric acid and simultaneously remove calcium as an insoluble salt?

Accepted Answer

Introduction / Context:Following precipitation of calcium citrate from the fermentation broth, an acid “cracking” step liberates citric acid and removes calcium. Choosing the correct acid determines whether the calcium leaves as an easily filterable salt and minimizes contamination of the citric acid solution. Given Data / Assumptions: Intermediate: calcium citrate solid. Objective: release citric acid and precipitate calcium as an insoluble mineral. Operational need: simple solid–liquid separation post-reaction. Concept / Approach:Calcium citrate reacts with sulfuric acid to produce free citric acid in solution and calcium sulfate (gypsum) as a sparingly soluble, easily filterable precipitate. This step is central to the traditional recovery flowsheet and yields a clear citric acid liquor suitable for decolorization and crystallization. Step-by-Step Solution: React calcium citrate with H2SO4 under controlled conditions.Form products: citric acid (aqueous) + CaSO4 (solid gypsum).Filter off gypsum and purify the citric acid solution. Verification / Alternative check:Industrial practice universally cites sulfuric acid for this conversion due to the low solubility of CaSO4 and its ease of removal. Why Other Options Are Wrong: HCl/HNO3 keep Ca2+ soluble as chlorides/nitrates, complicating removal. Carbonic acid is too weak for efficient conversion. “None of these” contradicts standard methodology. Common Pitfalls:Over-acidification leading to losses; inadequate filtration of gypsum causing carryover. Final Answer:Sulfuric acid (H2SO4)

Question 7

Feedstocks for citric acid fermentation by Aspergillus niger: which option correctly lists commonly used carbohydrate substrates?

Accepted Answer

Introduction / Context:Citric acid is a high-volume organic acid produced using carbohydrate-rich feedstocks. Process economics and availability determine whether molasses, refined sugars, or enzymatically hydrolyzed starch syrups are used. Aspergillus niger adapts well to each of these substrates under controlled conditions. Given Data / Assumptions: Organism: A. niger optimized for citric acid overflow metabolism. Potential feedstocks: beet molasses, sucrose, starch hydrolysate (glucose syrup). Process control: low metal ions, appropriate pH, and aeration. Concept / Approach:All three listed carbohydrate sources are standard industrial feedstocks. Molasses provides a low-cost, mineral-rich substrate (metals must be controlled), sucrose offers consistency, and starch hydrolysate is widely used where corn or cassava starch is abundant. Step-by-Step Solution: Confirm A. niger can utilize each listed sugar source.Recall multiple industrial routes documented in literature and practice.Select the inclusive option that recognizes all of them. Verification / Alternative check:Industrial case studies across regions employ all three feedstocks successfully for citric acid production. Why Other Options Are Wrong: Single-source options ignore the flexibility of the process. Whey/lactose is atypical for citric acid with A. niger. Common Pitfalls:Neglecting impurity control with molasses; failing to ensure adequate aeration and metal limitation for high yields. Final Answer:All of these (beet molasses, sucrose, and starch hydrolysate)

Question 8

When molasses is used as the carbohydrate substrate for citric acid production by Aspergillus niger, the fermentation medium is typically adjusted to which initial pH range for optimal production?

Accepted Answer

Introduction / Context:pH control is a key lever in citric acid fermentations. Low pH not only supports product stability but also suppresses contaminants and shifts Aspergillus niger metabolism toward citric acid over biomass or by-products such as oxalic acid and gluconic acid. Given Data / Assumptions: Substrate: molasses, containing minerals and impurities. Organism: A. niger optimized for acid production. Operational target: maximize citric acid yield and minimize contamination. Concept / Approach:Industrially, initial pH is often adjusted around 2–3.5 depending on the process variant and feedstock. For molasses-based processes, pH around 3 is widely cited as a practical starting point that balances productivity and robustness against contamination and metal ion effects. Step-by-Step Solution: Recognize that citric acid fermentation favors low pH.Identify typical setpoint for molasses: approximately pH 3.Select the option that matches this operating window. Verification / Alternative check:Process manuals and case histories for molasses-based citric acid plants commonly report initial pH ~3 with subsequent natural acidification. Why Other Options Are Wrong: pH 4–6 increases contamination and alters product profile. Neutral to alkaline pH is unsuitable for A. niger citric acid production. Common Pitfalls:Allowing metal contamination to interfere at higher pH; inadequate buffering leading to pH drift. Final Answer:pH 3

Question 9

Nitrogen supplementation in citric acid fermentation: which of the following nitrogen sources are commonly added to the production medium for Aspergillus niger?

Accepted Answer

Introduction / Context:Although citric acid is a carbon-heavy product formed under conditions that limit certain nutrients, a modest supply of assimilable nitrogen is necessary to initiate growth and support the biosynthetic machinery of Aspergillus niger. The choice and level of nitrogen source influence yield and by-product formation. Given Data / Assumptions: Organism: A. niger requires nitrogen for biomass and enzyme production. Goal: high citric acid yield with minimal by-products. Common nitrogen sources: ammonium salts (e.g., (NH4)2SO4) and urea. Concept / Approach:Both ammonium salts and urea are classic inorganic/simple nitrogen sources for citric acid fermentations. Their concentrations are carefully controlled; excess nitrogen promotes biomass over product, whereas slight nitrogen limitation encourages overflow metabolism toward citric acid. Step-by-Step Solution: Identify nitrogen sources compatible with low-pH, high-carbon media.Confirm both ammonium salts and urea are widely used.Select the inclusive option acknowledging both. Verification / Alternative check:Industrial media formulations frequently include (NH4)2SO4 and sometimes urea; both are standard. Why Other Options Are Wrong: Single-source options are unnecessarily restrictive. Peptone is not typical for bulk citric acid due to cost and nitrogen complexity. Omitting nitrogen entirely impairs growth and production. Common Pitfalls:Over-supplying nitrogen; ignoring the impact of ammonium on pH and buffering. Final Answer:Both ammonium salts and urea

Question 10

Citric acid fermentation — in the traditional liquid surface culture (surface fermentation) process, which statement correctly describes how Aspergillus niger grows and how the culture is set up?  Select the most accurate description of organism pla…

Accepted Answer

Introduction / Context:Citrate production by Aspergillus niger can be performed by two classic bioprocess configurations: surface (also called liquid surface culture) and submerged fermentation. Understanding how the fungus grows in each configuration is crucial for exam questions and for scaling up industrial processes. Given Data / Assumptions: Organism: Aspergillus niger. Process type: liquid surface culture (surface fermentation), not submerged. Focus: physical location of fungal growth in relation to the liquid medium. Concept / Approach: In liquid surface culture, shallow trays or pans of nutrient solution are inoculated, and the filamentous fungus grows as a mycelial film on the air–liquid interface. This setup maximizes oxygen access without sparging, unlike deep-tank aerated systems used for submerged fermentation, where hyphae form pellets or dispersed mycelia throughout the broth. Step-by-Step Solution: Identify the process: “liquid surface culture” implies growth at the air–liquid interface.Recall morphology: A. niger forms a visible floating mat (mycelial film) on the surface.Eliminate solid-state descriptions (sweet potato substrate) because the question specifies a liquid surface system.Eliminate deep-tank growth throughout the liquid, which defines submerged fermentation.Choose the option matching surface growth: a floating mycelial mat on the liquid surface. Verification / Alternative check: Industrial microbiology texts consistently describe surface culture as shallow liquid with surface mycelium; submerged culture uses aerated, agitated tanks with fungal pellets or dispersed hyphae. Why Other Options Are Wrong: (a) Describes solid-state fermentation, not a liquid surface system. (c) Describes submerged fermentation. (d) and (e) combine incompatible modes, so they cannot both be correct for the same process. Common Pitfalls: Confusing “surface culture” with “solid-state” due to similar oxygenation logic; assuming all citric acid fermentations are submerged because that is common in modern plants. Final Answer: A. niger forms a floating mycelial mat on the surface of the liquid medium

Question 11

Industrial microbiology check: which microorganism is classically employed at scale for commercial production of citric acid?

Accepted Answer

Introduction / Context:Citric acid is a high-volume organic acid used in foods, beverages, pharmaceuticals, and cleaning products. The organism used for its production must tolerate low pH, high sugar, and produce citric acid at high yield. Given Data / Assumptions: The question asks for the canonical organism. Process is aerobic and favors acid accumulation. Product inhibition and metal ion balance are relevant but secondary here. Concept / Approach: Aspergillus niger is the historical and current workhorse for citric acid thanks to its robustness, high flux through the tricarboxylic acid pathway under specific nutrient limitations, and tolerance to acidic conditions (broth pH often near 2–3 during production). Yeasts and lactic streptococci listed do not match the classical industrial process for citrate. Step-by-Step Solution: Identify which candidate is a filamentous fungus known for organic acid production.Recall A. niger’s documented dominance in citric acid manufacturing.Exclude dairy lactic acid bacteria and baker’s yeast that are not standard for citrate at scale.Select Aspergillus niger. Verification / Alternative check: Industrial bioprocess references and FAO/WHO reports cite A. niger as the organism of choice for citric acid, with optimized strains and fed strategies. Why Other Options Are Wrong: (a) and (d) are lactic fermenters used for dairy fermentations. (b) Saccharomyces cerevisiae is used for ethanol, not citrate. (e) Candida utilis is used for SCP (single-cell protein), not bulk citric acid. Common Pitfalls: Confusing any acid-producing microbe with the specific production organism; assuming yeasts dominate all food-related fermentations. Final Answer: Aspergillus niger

Question 12

Why vinegar (acetic acid) fermentations are often run in fed-batch mode: identify the key process control objective achieved by the feeding strategy.

Accepted Answer

Introduction / Context:Vinegar production relies on aerobic oxidation of ethanol to acetic acid by acetic acid bacteria. Process mode influences productivity and cell health. Fed-batch is favored to control substrate levels and heat release while allowing high oxygen transfer. Given Data / Assumptions: Ethanol is both substrate and potential inhibitor. Acetic acid and heat can also inhibit, but ethanol spikes are particularly harmful. Aerobic conditions are essential; oxygen demand is high. Concept / Approach: Feeding ethanol gradually prevents substrate inhibition and excessive exothermic heat from rapid oxidation. This stabilizes the culture, avoids ethanol toxicity, and helps keep dissolved oxygen within range. While acetic acid must also be managed, the principal control variable in fed-batch is ethanol concentration to protect cells and improve yield. Step-by-Step Solution: Recognize ethanol as the key inhibitory substrate when present at high concentration.Understand fed-batch purpose: limit substrate spikes by controlled addition.Link to aerobic oxidation: reducing heat peaks and maintaining DO.Select the option emphasizing low ethanol maintenance. Verification / Alternative check: Process descriptions of Frings or packed-generator acetators emphasize ethanol feed control (usually 0.5–1.5% v/v range) to avoid inhibition and maintain safe temperature. Why Other Options Are Wrong: (a) While acetic acid affects cells, fed-batch primarily targets ethanol; final titers rise by removing acid or buffering, not just by feed control. (c) The statement is false; high ethanol inhibits acetic acid bacteria. (d) Cannot be true because (c) is incorrect. (e) Aeration remains essential. Common Pitfalls: Assuming “fed-batch” always targets product inhibition rather than substrate inhibition; overlooking temperature rise from rapid ethanol oxidation. Final Answer: To maintain low ethanol concentrations that would otherwise inhibit or cause overheating

Question 13

Definition check: “malt vinegar” is produced from which primary feedstock base?

Accepted Answer

Introduction / Context:Vinegar types are named for their substrate origins. Understanding these distinctions helps in food science, labeling regulations, and sensory profiling. Given Data / Assumptions: Malt vinegar derives from cereal starches converted to malt sugars, then fermented to alcohol and oxidized to acetic acid. Other vinegars (e.g., cider, wine, spirit, alegar) have different bases. The question asks for the base specific to malt vinegar. Concept / Approach: Malt vinegar begins with malted barley or other malted grains. Starches are saccharified by malt enzymes, fermented to beer-like wort, then acetic acid bacteria oxidize the ethanol. This contrasts with cider vinegar (apple juice), wine vinegar (grape wine), spirit vinegar (neutral alcohol), and alegar (ale). Step-by-Step Solution: Identify “malt” as referring to malted cereals.Connect to process: saccharification → alcoholic fermentation → acetification.Choose “Malted grain.” Verification / Alternative check: Food standards and culinary texts define malt vinegar by cereal malt origin, often barley malt. Why Other Options Are Wrong: (a) Fruit juices yield cider or fruit vinegars. (c) Neutral spirit yields “spirit vinegar.” (d) Ale-based vinegar is called “alegar.” (e) Distillers’ wash is not the regulated name for malt vinegar origin. Common Pitfalls: Confusing ale-based vinegar with malt vinegar; although related, alegar is specifically from ale, not directly from malted grain mash. Final Answer: Malted grain

Question 14

Terminology: “alegar” refers to a vinegar made from which alcoholic base?

Accepted Answer

Introduction / Context:Nomenclature of vinegars reflects their fermentative origin. Alegar is a traditional British term often encountered in food technology exams. Given Data / Assumptions: “Ale” is a finished beer made from malt. Acetification converts ethanol in beer to acetic acid. We must map the named vinegar to its base. Concept / Approach: Alegar is specifically vinegar derived from ale. While malt vinegar originates from malted grains via wort and beer production, its naming emphasizes the malted grain origin; alegar emphasizes the ale base itself as the immediate substrate undergoing acetic fermentation. Step-by-Step Solution: Recognize the root word “ale” in “alegar.”Recall that acetification of ale yields alegar.Select “Ale.” Verification / Alternative check: Food processing references list alegar as ale-based vinegar distinct from malt, cider, wine, and spirit vinegars. Why Other Options Are Wrong: (a) Leads to cider or fruit vinegar. (b) Refers to mash, not ale. (c) Refers to spirit vinegar. (e) Leads to wine vinegar. Common Pitfalls: Equating alegar with malt vinegar because both start from cereal; the naming distinction depends on the immediate alcoholic substrate. Final Answer: Ale

Question 15

Citric acid medium design: typical sugar (carbon source) concentration maintained during production is approximately:

Accepted Answer

Introduction / Context:Citric acid accumulation by Aspergillus niger requires a high initial sugar concentration to drive flux through central carbon metabolism under conditions that limit biomass but favor product formation. The common exam ask is the typical range used industrially. Given Data / Assumptions: Carbon sources include sucrose, glucose, or refined molasses. High sugar is tolerated at low pH by A. niger strains. Metal ion limitation and nitrogen/phosphate balance are adjusted for citrate overproduction. Concept / Approach: Operationally, many surface and submerged processes maintain sugar at roughly 150–200 g/L (15–20 % w/v) to sustain production without frequent feeding, while controlling viscosity and oxygen transfer. Concentrations above this can impair oxygen transfer and create osmotic stress; below this may reduce yields. Step-by-Step Solution: Recall the standard design range reported in industrial practice.Balance osmotic pressure vs. productivity: 15–20 % is a workable compromise.Select the option that matches this range. Verification / Alternative check: Classic fermentation handbooks cite initial sugars often between 140 and 200 g/L for efficient citric acid accumulation, with pH control and trace metal management. Why Other Options Are Wrong: (a) 10–15 % may be used in some cases but is below the typical exam standard. (c)–(d) 20–30 % often causes mass transfer issues. (e) 5–8 % is too low for industrial citrate runs. Common Pitfalls: Confusing w/v with w/w; ignoring that fed strategies can alter “maintained” concentration while initial charge still targets ~15–20 %. Final Answer: 15–20 % (w/v)

Question 16

Definition check: “cider vinegar” is produced from which base material?

Accepted Answer

Introduction / Context:Vinegar names reflect substrate origin. Cider vinegar is widely used in culinary applications and health products, and its name reveals the starting material. Given Data / Assumptions: Cider is fermented apple juice. Acetic acid bacteria oxidize ethanol in cider to acetic acid. We must match the name to the substrate. Concept / Approach: Apple juice is first converted to cider by yeast fermentation; then acetic acid bacteria convert ethanol to acetic acid, producing cider vinegar. This distinguishes it from malt, wine, spirit, or ale-derived vinegars. Step-by-Step Solution: Recognize “cider” as alcoholic apple juice.Map to vinegar via acetification of cider.Choose the fruit juice (apple) option. Verification / Alternative check: Food standards define cider vinegar by apple origin, sometimes labeled apple cider vinegar (ACV). Why Other Options Are Wrong: (b) Malted grain → malt vinegar. (c) Neutral spirit → spirit vinegar. (d) Ale → alegar. (e) Molasses → not cider vinegar; could be used for other fermentations. Common Pitfalls: Assuming any fruit vinegar qualifies as cider vinegar; “cider” specifically denotes apple. Final Answer: Fruit juices (specifically apple juice/cider)

Question 17

Citric acid from sucrose: what initial medium pH is commonly set when sucrose is the substrate for Aspergillus niger production runs?

Accepted Answer

Introduction / Context:Citric acid accumulation depends on carbon source, trace metals, and pH strategy. With sucrose as substrate, classical practice sets the initial pH higher than the final pH observed during production, which later drops as acid accumulates. Given Data / Assumptions: Organism: Aspergillus niger. Substrate: sucrose (refined sugar). Question focuses on initial set pH of the medium. Concept / Approach: Traditional guidance: when using sucrose or glucose solutions, the initial pH is typically adjusted around 5–6 to optimize spore germination and early growth. During fermentation, pH progressively decreases (often to near 2) as citric acid accumulates; low pH also minimizes contamination and by-products. Molasses media may be set somewhat lower due to impurities and buffering differences, but refined sucrose media commonly start near pH 5. Step-by-Step Solution: Recall initial vs. final pH distinction in citric acid runs.Match sucrose media with typical set pH ≈ 5–6 before inoculation.Select the provided value closest to standard practice: 5. Verification / Alternative check: Process descriptions in fermentation manuals cite initial pH 5–6 for sucrose media; the broth later acidifies substantially to near pH 2–3 during peak citrate formation. Why Other Options Are Wrong: (a) and (b) are more typical of later stages or specific molasses protocols; (d) 6 can be used but 5 is the more commonly cited single value; (e) 2 is a typical late fermentation pH, not an initial set-point. Common Pitfalls: Confusing initial set pH with the final pH after acid accumulation; assuming the same pH target for all carbon sources regardless of buffering and impurity profiles. Final Answer: 5

Question 18

Citric acid fermentation — in the submerged (deep-tank) process, which statement best describes the growth habit of Aspergillus niger?

Accepted Answer

Introduction / Context:Modern citric acid production predominantly uses submerged, aerated, stirred tanks. Recognizing how filamentous fungi grow under these conditions distinguishes this method from surface or solid-state processes. Given Data / Assumptions: Organism: Aspergillus niger (filamentous fungus). System: submerged, deep-tank bioreactor with aeration/agitation. Focus: spatial distribution of biomass. Concept / Approach: In submerged systems, A. niger forms pellets or dispersed hyphal clumps throughout the broth. Oxygen transfer, shear, and nutrient gradients influence pellet size and morphology, directly affecting productivity and viscosity. This is distinct from surface culture, where mycelium forms a film at the air–liquid interface, and from solid-state systems using moist solid substrates. Step-by-Step Solution: Identify “submerged” as growth within a deep, aerated liquid volume.Recall pellet/dispersed morphologies characteristic of filamentous fungi in stirred tanks.Select the option that explicitly states growth throughout the broth. Verification / Alternative check: Bioprocess engineering texts describe pellet control (by spore inoculum size, shear, metals) as a key lever in submerged citrate production. Why Other Options Are Wrong: (a) Solid-state setup; (b) surface film matches surface culture; (d) mixes incompatible modes; (e) is nonsensical. Common Pitfalls: Assuming that surface mat formation occurs in submerged systems; ignoring the impact of morphology on oxygen transfer and viscosity. Final Answer: Fungal mycelium grows throughout the aerated broth in a deep tank (pellets or dispersed mycelia)

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