United racked up six goals in the second half after being held to a 1-1 scoreline by Spartans in the opening 45 minutes.
The Reds were soon in front when John Cofie shot in off a post but the plucky hosts cancelled it out just before the interval when Davide Petrucci fouled Jack Beesley and Alex King calmly beat Tomasz Kuszczak from the penalty spot.
In energy-sapping conditions, the introduction of fresh legs at half-time aided Warren Joyce's side and the floodgates opened once Jesse Lingard cracked a 25-yard drive into Chris Flockhart's top corner. Oliver Norwood's run led to a spot-kick that Lingard converted for 3-1 and the 18-year-old midfielder then lobbed home after latching onto a Norwood pass to complete a quickfire hat-trick within nine minutes.
The impressive Larnell Cole dusted himself down to bag the third penalty of the game after being felled by Daniel O'Donnell before setting up Michael Keane's angled drive to make it six with three minutes remaining.
Cole, enjoying a more advanced role after the break, added his second of the afternoon in stoppage time by pouncing on a rebound when a Danny Drinkwater shot was pushed out.
Trialist Max Lonsdale started the match in midfield to make his second appearance for the Reserves in the space of a few days, with Darron Gibson skippering the team at centre-back, in front of an impressive attendance of 3,127.
The Reds are remaining in Scotland for Monday night's friendly with Ayr United as the countdown to the new campaign continues.
United: Kuszczak (Johnstone 46); Cole, Giverin, Thorpe (Fryers 46), Gibson (Fornasier 46); Lingard, Leao (McGinty 56), Lonsdale (Norwood 56), Petrucci (Drinkwater 56); Cofie (Pogba 56), W.Keane (M.Keane 46). Unused sub: Jacob.
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